MENDELIAN INHERITANCE- INTRODUCTION

INTRODUCTION:

• In Classification – individuals showing similarity in their external features or close resemblance are grouped together.
• Resemblance will be still close if they have common ancestors. e.g. Parents and their offspring's.
• Every living organism exhibits a set of characters by means of which it can be identified as a members of particular species.
• At the same time we also know that no two individuals are alike.
• The offspring's show slight but definite differences from one another.
• The basic questions regarding set of characters possessed by species and the way they are maintained from one generation to the next generation, cause and effects of variations can well be answered by Genetics.

INHERITANCE : HERIDITY AND VARIATIONS

• Genetics – It is the branch of biology that deals with the heredity and variations.
• Study of genetics includes the study of chemical foundations of heredity and its relation with variation.
• Sexual reproduction is the important mode of reproduction. It creates new individuals.
• During sexual reproduction the characters of parents are transmitted to the offspring's.
• Thus, heredity can be defined as the transmission of characters from parents to the offspring or from one generation to the successive generation of the living things.
• Therefore, there exists a lot of similarity among the members of family and between the parents and offspring's. Childrens tend to resemble their parents.
• Such similarities are expressed in terms of characteristics or traits. e.g. in human beings, colour of the eyes, shape of nose, skin colour, height etc. are described as traits of characteristics.

• In fruit fly Drosophila melanogaster the body colour, eye colour, skin colour, shape of tail in dog, skin colour and hair length in cat are hereditary characters.
• In plants, height, colour of flower, shape, size of seeds and fruit are hereditary characters.
• In sunflower, branching habit, in cotton, colour of lint are hereditary characters.
• The fact that, children resemble their parents is due to heredity.
• At the same time they also differ slightly but definitely because of the phenomenon of variation. The diversity arises because of variations in combination of characters.
• Experience tells us that “like begets like” and “chips of old block” whenever, we come across a situation to compare parents and children. Thus, human being produce human babies and mango seeds produce mango tress.
• Before- 1900 it was not known how the characters are transmitted from one generation to other. In the world, the individuals show many differences from one to another. It was of great importance for the earlier biologists to trace the path in which characteristics were inherited. The first efforts were taken by the famous Austrian Monk Gregor Johann Mendel in a systematic and scientific way.

GENETIC TERMS AND SYMBOLS

1) Factor or determiner : Mendel presumed that there is something in the organism which is responsible for the expression of a particular character. To this “something” Mendel used the term factor or determiner. Mendel also presumed that the factors pass through gametes to the next generation and act as functional units of heredity.

(2)Gene: Mendel's factor is now known as gene. The term gene was coined by Johannsen (1909). Gene is particular segment of DNA molecule which determines the inheritance and expression of a particular character.

(3)Alleles or Allelomorphs : Two or more alternative forms of a gene are called alleles or allelomorphs of each other. For example : In pea, gene for the position of flower occurs in two alternative forms : Axial (A) and terminal (a). The axial and terminal position of a flower are allelic characters of each other. The genes A and a are alleles of allelomorphs of each other. The allelomorphs occupy the same locus on homologous chromosomes.

(4)Dominant : When parents differing in a pair of contrasting characters are crossed, then only one character appears in a F1 generation is called a dominant character. For example, “Tall” is the dominant character.

(5) Recessive : When parents differing in a pair of contrasting characters are crossed, then the characters which gets suppressed in F1 generation is called recessive character. For example dwarf is a recessive character.

(6) Homozygous : Individuals having the similar genes for the particular trait are called homozygous or pure individuals. For example : AA, aa, TT, tt are the pure or homozygous individuals for Axial position of flower, terminal position of flower, Tallness, dwarfness etc.

(7) Heterozygous : An individual having the dissimilar genes for the particular trait is called heterozygous or hybrid. For example : Aa, Tt, are the heterozygous or hybrid individuals for axial position of flower, height respectively.

(8) Phenotype : The morphological or external appearance of an individual for a trait is called its phenotype. e.g. Tall, dwarf, red flower colour, white flower colour etc. are all phenotypes.

(9) Genotype : The genetic make up or genetic constitution of an individual for a particular trait is called the genotype. For example : Pure red flowered genotype [RR], Hybrid or Heterozygous red flowered genotype [Rr]. Hence [RR], [Rr], [Aa], [aa] etc. are all the genotypes.

(10) Parent generation : The parents used for the first cross represent the parent generation of P1 generation.

(11) Inbreeding : It is the breeding between closely related organisms. Selfing is also a type of inbreeding.

(12) F1 generation : The progeny obtained by crossing two parents with one pair of contrasting character is represented as F1 generation (first filial).

(13) F2 generation : The progeny obtained by crossing two F1 organisms or by performing self pollination in bisexual organism of F1 generation gives the generation called F2 generation (Second filial).

(14) Monohybrid cross : It is the cross taken between two pure parents differing in one pair of contrasting characters. The F1 generation of this cross is monohybrid.

(15) Monohybrid ratio : It is the ratio that is obtained in the F2 generation of monohybrid cross. The monohybrid phenotypic ratio is 3 dominants : 1 recessive.

(16) Dihybrid cross : It is the cross between taken two pure parents differing in two pairs of contrasting characters. The F1 generation of this cross is dihybrid.

(17) Dihybrid ratio : It is the ratio that is obtained in the F2 generation of the dihybrid cross. The dihybrid phenotypic ratio is 9:3:3:1.

(18) Unit character : It is well defined morphological or physiological trait of an organism which is inherited independent of other characters.

(19) Checker Board/ Punnet square : (Punnet, 1927). It is square divided into smaller squares which shows the mathematical or probable result of a cross, both phenotypic and genotypic. It is of three types gametic, phenotypic and genotypic. Forked line or branching system is used to know phenotypic and genotypic probabilities.

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Prior to Mendel's discovery of laws of inheritance, many thinkers, philosophers and scientists tried to explain the mechanism of inheritance. But all of them were wrong.

He carried out the first quantitative study of inheritance in garden peas. Therefore, he is aptly known as father of genetics.

Gregor Mendel (1822 - 1894)

• Mendel was born in an area which is now a part of Czechoslovakia. He was ordinated as priest of Augustinian monastery at an early age. After some years he was sent to University of Vienna to get a training in Mathematics, Physics and natural sciences.
• Returned to the monastery and become a teacher in one of the schools.
• During the period 1856 – 1865 he developed interest in phenomenon of inheritance. He carried out some experiments on garden pea plant Pisum sativum.
• He cross- pollinated the flowers of Pea plants. To his surprise he came across many uncommon results. He carried out crosses for seven contrasting characters and calculated the results.
• He published his results in 1866 in the proceedings of the scientific society at Brunn (“Annual proceedings of the Natural History Society” of Brunn).
• Mendel's thoughts were so much ahead of time that his colleague scientists could not appreciate them. They ignored Mendel completely.
• In, 1868, Mendel was appointed as abbot of monastery and this put as end to his scientific research. His research reveals the mind of a genius. He died in 1884, highly respected as priest, but unrecognized as a scientist.
• The actual value of research findings was recognized in the 19^th century when three botanists considered the Mendel's conclusion. They were Hugo de Vries, Karl Correns and Eric Von Tschermark . They pointed out that Mendel's laws applied not only to garden pea but to other plants and animals. They named his conclusions as laws of heredity after Mendel as original experimenter.

MENDELS EXPERIMENT ON GARDEN PEA

• Mendel selected garden pea as his experimental material. These plants exhibited immense variety of floral and vegetative characters. Garden peas are cultivated where in self pollination was possible. In this method the young buds are to be covered by bags. After self pollination seeds were collected. In case of cross pollination where pollen from one specific plant is to pollinate another specific plant, certain measures are to be taken in order to avoid self pollination, as these plants have bisexual flowers.
• The plants designated as female parent in a cross- pollination were the ones whose anthers were removed from flowers before maturity. The pollen from the required parent (i.e. male parent) were dusted on the surface of stigma. This flower was then covered by bag.
• For his experiments he selected seven different traits (Table). Large number of offspring's were obtained by crossing experiments.
• He took a single character of a pure variety and crossed them. He used self pollination technique for obtaining pure variety (i.e. homozygous). For this purpose he allowed several generations of peas to self – pollinate. Thus, seeds of tall plants produce tall plants.

• Seven pairs of contrasting characters in pea plants were taken into consideration by Mendel for his experimental purpose.

• Similarly, he obtained from pure (homozygous) dwarf plants, generations of dwarf plants. He then cross pollinated pure tall plants with pure dwarf plants and recorded next generations.
• In the first filial generation (F1) he found only tall plants. These plants were self pollinated to obtain second filial generation (F2). He found three tall plants and one dwarf plant in the ratio 3:1. He repeated similar experiments with the remaining six pairs.
• Mendel on self pollination in the F2 generation found that 1/3 tall plants of F2 produced only tall plants in F3 while 2/3 tall plants of F2 produced tall and dwarf plants in the ratio of 3:1.
• On the basis of his experiments and recording, Mendel postulated the fundamental laws of heredity. The reasons for Mendel's success can be described as follows.
• (i) Mendel’s choice of pea plant was perfect because of its life cycle of short duration, availability of its pure forms and presence of many contrasting characters in it.
• (ii) The ease with which the plants can be self-pollinated and cross pollinated.
• (iii) The hybrids were fertile.
• (iv) He always started his experiments with pure breeding variety which bred true to species at least for last three generations.
• (v) Mendel considered only one or two pairs of contrasting characters at a time.
• (vi) He maintained up-to-date pedigree record.
• On the basis of the results of his experiments Mendel formulated following three laws :

Mendel's laws of Inheritance�(i) Law of Dominance

• Mendel gave convincing interpretation of the results of his experiments. He presumed that the various characters such as tall and dwarf, red and white (colour of flower) etc. are determined by something present in the gametes which is transformed to the progeny. This something is called ‘determiner’ or ‘factor’. These factors come together in individuals. They never fuse or lost but segregate at the time of gamete formation. These determiners are now called genes. Thus each character in the individual is governed by pair of factors. This is known as law of paired unit characters.
• Law of Dominance : This law states that from a pair of contrasting factors one dominates completely over the other, while the other remains supposed or dormant.
• Thus, in garden pea plant, in stem length, tallness is dominant character and dwarfness is recessive character. Therefore, when pure tall and pure dwarf are crossed the plants of F1 generation showed only tall plants. But in F2 generation dwarf plants appear. The ratio of tall plants to dwarf plants is always 3:1. In animals e.g. Guinea pig, the black colour coat is dominant (B) over white colour coat (b). In human beings, brown eye character (B) is dominant over blue eye character (b) which is recessive one. In sunflower plant, branched habit is dominant character over unbranched habit and so on

• E.g. If pure tall pea plant is crossed with pure dwarf pea plant, in F1 all plants were tall because tallness is dominant character while dwarfness does not express because it is a recessive character.

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(ii)Law of Segregation

• Mendel stated that, “ a pair of factors (alleles) is segregated or separated during the formation of gametes.
• Thus, gamete contain only one factor (character) of a pair, the other having gone to another gamete. The composition of one factor is not altered by the presence of another factor in pair. e.g. the recessive factor (y) in hybrid (Yy) is not altered by the presence of dominant factor. If in an offspring of hybrid the recessive factor is paired with another recessive factor (yy), the recessive character (green colour of seed) will reappear.
• Thus, hybrids of F1 generation have two contrasting characters or allelomorphs of dominant and recessive nature. Consider the cross between tall (TT) and dwarf (tt) parents. The F1 hybrids have (Tt), two allelomorphs of dominant and recessive nature. These characters though remain together, do not intermingle with each other and will separate during gamete formation. Thus, each gamete will have one character i. e. dominant or recessive.
• In this example, the gametes will be either ‘T’ (tall) or ‘t’ (dwarf). The law of segregation is also called law of purity of gametes.

• E.g. When pure tall pea plant is crossed with pure dwarf pea plant in F1, T and t remain together without mixing but during gametes formation these factors get separated and enter in different gametes, hence gametes contain either T or t.

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(iii)Law of Independent Assortment

• This law states that, the two characters though inherited together behave independently of each other while segregating in F2 generation. or
• Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another.
• This law can be explained by Mendel’s experiment in which he crossed tall plants having coloured flowers and dwarf plants having white flowers (‘T’ for tallness, ‘C’ for Coloured flower, ‘t’ dwarfness and ‘c’ for white flowers). In this cross both parents (P1) were homozygous. Thus, we have pea plant pure for tallness with coloured flowers [TTCC] crossed with another pea plant pure for dwarfness with white flowers [ttcc].
• During meiosis gamete formation takes place and one plant produces ‘TC’ gametes and the other plant produces ‘tr’ gametes. These gametes will unite to produce the plants of F1 generation, which are all tall with coloured flowers. They are hybrids with ‘TtCc’ as their genotypes. These plants produce not only two types of gametes from which they themselves are formed but two more types of gametes. This is because every chromosome is independent to go with any other non- homologous chromosome in a gamete.
• Therefore in addition to TC and tc gametes Tc and tC are also formed.
• When these gametes unite, there are four types of plants in F2 generation in the ratio of 9:3:3:1.Thus, we have 9 tall plants with coloured flowers, 3 tall plants with white flowers, three dwarf plants coloured flowers and one dwarf plant with white flowers. As a result of independent assortment two new combinations appear in F2, i.e. tall with white flowers and dwarf with coloured flowers. They are the products of recombination. This is possible because each allele (e.g. yellow or green ) combines with equal probabilities with alleles of other characters say smooth or wrinkled in the above example. The independent assortment takes place at the time of gamete formation of hybrid F1 generation. Random union between these gametes (four types) makes independent assortment clear among F2 generation.
• It is clear that dihybrid ratio actually contains monohybrid ratios of both the characters. Thus, out of 16 combinations, 12 yield yellow and 4 green. Similarly, 12 yield round and 4 wrinkled.

MONOHYBRID CROSS AND MONOHYBRID RATIO

• In early stages of work, Mendel studied the inheritance of one pair of contrasting characters. In crossing two pure individuals if one pair of contrasting characters is concerned, the phenotypic ratio which one gets in F2 generation is known as monohybrid ratio. It is always 3:1.
• Mendel performed the cross with pea plants considering a seed colour as a character. Thus, he used pure line parents producing yellow seeds and green seeds. He crossed parents bearing yellow seeds with parent having green seeds. This was done by method of artificial cross-pollination. The seeds produced as a result of this cross were the seeds of F1 generation. All F1 seeds were yellow in colour.
• He then allowed this F1 generation plants to be self pollinated and produced the seeds. It was seen that the colour of seeds in F2 generation was both yellow and green. The approximate ratio between the two was 3 Yellow : 1 Green.
• Homozygous yellow denoted as YY and homozygous green is denoted as yy. The monohybrid cross between yellow and green coloured seeded forms is symbolized as follows.

• The visual diagram is called Punnet square after R. C. Punnet, who devised it. It is special chart resembling checker board which is used to calculate or determine the possible results of various crosses.

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DIHYBRID RATIO or DIHYBRID CROSS

• In these crosses, he took he took two pairs of contrasting characters at a time and noted the relation of these characters during inheritance.
• If a pea plant pure for round, yellow seeds is crossed with another pea plant pure for wrinkled green seeds, all the seeds produced in F1 generation were round yellow.
• Thus, recessive character of wrinkled and green seed coat were masked by two dominant characters of yellow and round seed coat. ( If ‘R’- stands for round seed coat, ‘r’ – wrinkeled seed coat, ‘Y’ – Yellow colour and ‘y’ – green colour, the F1 hybrids will be RrYy (Round yellow). When F1 hybrids are selfed they gave four types of offspring's : yellow round, yellow wrinkled, green round, green wrinkled in the ratio of 9:3:3:1. The ratio in F2 generation of is known as dihybrid ratio. The progeny developed F2 generation by selfing the F1 individuals can be represented by checker board method (Fig.).
• Thus, the phenotypic dihybrid ratio is :
• 9 – Yellow round (Number 1, 2, 3, 4, 5, 7, 9, 10, 13).
• 3 – Yellow wrinkled (Number 6, 8, 14).
• 3 – Green wrinkled (Number 11, 12, 15).
• 1 – Green Wrinkled (Number 16).

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TEST CROSS and BACK CROSS

• When an F1 organism is crossed with any one of the parents, then the cross is called a back cross. For example, if [Tt] hybrid is crossed with either [TT] or [tt], then it is called a back cross.
• But, if is crossed with recessive parent, then the cross is called test cross. It is called test cross because the cross is useful to find out the genotype of a parent having dominant character.
• Mendel found this to be true, both in monohybrid and dihybrid crosses.
• When a back cross is taken with dominant parent then, no phenotypic separation takes place in the progeny, but when a back cross is taken with recessive parent, (test cross), phenotypic separation takes place in the progeny.
• Test cross is used in hybridization experiments because it yields a simple ratio of 1:1 or 1:1:1:1 (in monohybrid test cross or dihybrid test cross respectively) and this needs the analysis of fewer types in progeny.

Chromosomal theory of inheritance

• What are Chromosomes?
• Chromosomes are a thread like structure of nucleic acids and protein located within the nucleus of the living cells and are mainly involved in carrying genetic information the form of genes.
• The chromosomal theory of inheritance:
• This theory was given by Boveri and Sutton in the early 1900s. It is the first fundamental theory of genetics. According to this theory, genes are the units of heredity and are found in the chromosomes. or
• It is the fundamental theory of genetics which recognizes chromosomes as the carriers of genetic material.
• Chromosomal theory of inheritance came into existence long after Mendelian genetics. During Mendel's experimentation, the society was not acceptable to such drastic changes in their scientific ideas. They could not believe the existence of such discrete factors such as genes which would segregate without mixing as this did not support their idea of the constant changes leading to evolution. Moreover, the means of communication was poor at that time as a result of which, the information could not be conveyed to the masses. Also, Mendel's mathematical approach to prove biological laws unacceptable.
• As time passes scientists Vries, Correns and Tschermak discovered chromosomes which existed inside the nucleus. Sutton and Boveri discovered observed the behaviour of the chromosomes when the cells were divided. With the advancement in the microscope, this task became easier. Hence, they proved Mendel’s laws with the help of chromosomal movement. They showed the segregation of the chromosomes during the anaphase of the cell division. The idea of chromosomal segregation combined with the Mendelian principles gave rise to the chromosomal theory of inheritance. The work was further carried forward and proved by T.H. Morgan to show how sexual reproduction gave rise to variations.

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• Evidence for chromosomal theory of inheritance :
• In 1910, Morgan presented a more direct evidence supporting the chromosomal theory of heredity. He reported the peculiar inheritance pattern of white eye (w) gene of Drosophila. The pattern of transmission of white eye gene was identical with that of the X chromosome of Drosophila. This prompted Morgan to postulate that gene w was located in X chromosomes. This was the first demonstration of an association between a specific gene and a specific chromosome.

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• Observations of Chromosomal Theory of Inheritance :
• The chromosomal theory of inheritance support Mendel’s laws. Listed below are the observations of this theory.
• During the process of cell division- meiosis, the pairs of homozygous chromosome move as discrete structures, which are independent of other pairs of chromosomes.
• There is random distribution of chromosomes into the pre-gametes from each homologous pair.
• Each parent synthesizes gametes, which constitute only half of their chromosomal complement.
• Even though female (egg) and male (sperm) gametes differ in their size and morphology, they have the same number of chromosomes, submitting equal genetic contributions from each parent.
• The gametic chromosomes fuse during fertilization to produce offspring with the same number of a chromosome as their parents.

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Linkage : Introduction, Types and Significance

• Linkage :- The tendency of two or more genes to stay together during inheritance is known as linkage.
• :Introduction:
• it is well established that genes are located in chromosomes. During cell division, each chromosome appears to behave as a unit. It may, therefore be expected that genes located in the same chromosome would move to the same pole during cell division. As a consequences, such genes would fail to show independent segregation and would tend to be inherited together. This expectation was expressed by Walter Sutton in 1903 while propounding the chromosomal theory of inheritance. A significant deviation from independent segregation was first reported by Bateson and Punnet in 1905 in the case of flower colour and pollen shape in pea. However, Bateson and Punnet failed to advance a suitable explanation for their findings. But the terms coupling phase and repulsion phase, coined by them to describe the results, are widely used to denote linkages between two (or more) dominant genes, and between one (or more) recessive genes respectively.
• The concept of linkage has its origin in the demonstration by Morgan in 1910 that the peculiar inheritance pattern of white fly (w) gene of Drosophila can be easily explained by assuming that w is located in X chromosome (sex - linkage). Subsequently, he studied the inheritance of several other sex linked genes. In 1911, he published the following conclusions from these studies. (1) Genes located in the same chromosomes tend to stay together during inheritance; this tendency is called linkage. (2) Genes are arranged in a linear fashion in the chromosomes. (3) The intensity of linkage between two genes is inversely related to the distance between them in the chromosome. (4) He also proposed that coupling and repulsion phases are two aspects of the same phenomenon, i.e. linkage. Later studies have confirmed the above conclusions of Morgan.
• Linkage is consequence of the concerned genes being located in the same chromosome. Linked genes do not show independent segregation; as a consequence, the ratios obtained in F2 and test- cross generations are significantly different from the expected ratios of 9:3:3:1, respectively. This effect of linkage is more clearly noticeable in a test-cross generation: the frequencies of parental character combinations are markedly more than expected, while those of new character combinations are considerably lower.

Types of Linkage

• Complete linkage : is a type of linkage where two or more traits are inherited and are visible in two or more further generations. These types of linkages are found together in the chromosomes of the same type. In this type of linkage only parental character combinations are recovered in test – cross progeny.
• Incomplete linkage : Incomplete linkages produce some portion of non- parental combinations. These types of linkages found at a distance and result in occasional destruction of chromosomal segments while crossing over. In this linkage, linked genes do not always move to the same pole; sometimes their alleles recombine to produce recombinant genes.
• Linkage is also classified as (1) Coupling and (2) Repulsion phase linkage depending on whether all dominant or some dominant and some recessive genes are linked together. In coupling phase, dominant alleles of the linked genes are present in the same chromosome. In contrast, repulsion phase linkage denotes that dominant alleles of some genes are present with recessive alleles of the other genes in the same chromosome.

Significance of Linkage :

• Linkage helps to keep parental features together by reducing the probability of gene recombination. As a result, it aids the organism in maintaining parental, racial, and other characteristics.
• It lessens the chances of recombination of genes.
• It helps in maintaining the valuable traits of a newly developed variety.
• It disallows the plant and animal breeders to combine all the desirable traits in single variety.
• It helps in ascertain exact position of genes and distances between them in chromosome.

Crossing over - Introduction

• Crossing over : Crossing over is the exchange of strictly homologous segments between non-sister chromatids of homologous chromosomes.
• Crossing over takes place during pachytene, when each bivalent has four chromatids or strands (Four-strand stage). Generally, one chromatid from each of the two homologues is involved in crossing over. In this process, a segment of one of the chromatids becomes attached in place of the homologous segment of the nonsister chromatid and vice versa. This produces a cross (x)-like figure at point of exchange of the chromatid segments; this figure is called chiasma.
• Obviously, each event of crossing over produces two recombinant chromatids (involved in the crossing over) called crossover chromatids, and two original chromatids (not involved in the crossing over) referred to as noncrossover chromatids. The crossover chromatids will have new combinations of the linked genes, i.e. will be recombinant; gametes carrying them will produce the recombinant phenotypes in test-crosses. Similarly, the noncrossover chromatids will contain the parental gene combinations, and the gametes carrying them will give rise to the noncrossover types.
• Crossing over or recombination actually results from exchange of chromosomes segments which is believed to take place during prophase I of meiosis. During prophase I of meiosis, homologous chromosomes are brought together during synapsis at zygotene. Each chromosome also divides longitudinally into two chromatids at this stage. At pachytene, chromatids exchange segments. Homologous centromeres then start separating away from one another (terminalization) and figures of chiasmata are the result and not the cause of crossing over.

Significance of Crossing over

• The exchange of chromosome material results in variation in the offspring.
• It plays crucial role in the evolution process.
• The creation of genetic maps is aided by crossing over frequency.
• Crossing over is a evidence of a chromosomes linear organization of connected genes.

Variation in Chromosome Number and Structure

• Structure of Chromosome :-
• Chromosomes are thread-like structures present in the nucleus, which carries genetic information from one generation to another.
• They play a vital role in cell division, heredity, variation, mutation, repair and regeneration.
• In eukaryotic cells, genetic material is present in the nucleus in chromosomes, which is made up of highly organized DNA molecules with histone proteins supporting its structure.
• Chromosome meaning and discovery:-
• Chromosome means ‘coloured body’, that refers to its staining ability by certain dyes.
• Karl Nägeli in 1842, first observed the rod-like structure present in the nucleus of the plant cell.
• W. Waldeyer in 1888 coined the term chromosome.
• Walter Sutton and Theodor Boveri in 1902 suggested that chromosomes are the physical carrier of genes in the eukaryotic cells.
• The number of chromosomes in any species is constant for all the cells. The number of chromosomes in gametes (e.g. sperms, egg) is half of the somatic cell and known as haploid set of chromosomes, which is the result of meiosis during sexual reproduction.
• Chromosome number is preserved in the mitotic division of somatic cells, which is required for an organism to grow, repair and regenerate.
• Chromosome number varies in different species. A nematode species contains only 2 chromosomes in a cell, whereas a protozoan species contains as much as 1600 chromosomes in the cell. Most of the plant and animal species contain 8 to 50 number of chromosomes in its somatic cell. The number of chromosomes does not reflect the complexity of species. A human cell contains total 23 pair of chromosomes (2n, total 23 × 2 = 46), of which 22 are autosomes and 1 sex chromosome.

Chromosome Structure

• The chemical composition of a chromosome is histone proteins and DNA. Each cell has a pair of each kind of chromosome known as a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome contains hundreds and thousands of genes that can precisely code for several proteins in the cell. Structure of a chromosome can be best seen during cell division.
• Main parts of chromosomes are :
• Chromatid : Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase.
• Each chromatid contains a single DNA molecule.
• At the anaphase of mitotic cell division, sister chromatids separate and migrate to opposite poles.
• Centromere and Kinetochore : Sister chromatids are joined by the centromere.
• Spindle fibres during cell division are attached at the centromere.
• The number and position of the centromere differs in different chromosomes.
• The centromere is called primary constriction.
• Centromere divides the chromosome into two parts, the shorter arm is known as ‘p’ arm and the longer arm is known as ‘q’ arm.
• The centromere contains a disc shaped kinetochore, which has specific DNA sequence with special proteins bound to them.
• The kinetochore provides the centre for polymerization of tubulin proteins and assembly of microtubules.

• Secondary constriction and nucleolar organisers: Other than centromere, chromosome possess secondary constrictions.
• Secondary constrictions can be identified from centromere at anaphase because there is bending only at centromere (primary constriction).
• Secondary constrictions which contain genes to form nucleoli are known as the nucleolar organiser.
• Telomere : Terminal part of chromosome is known as telomere.
• Telomeres are polar, which prevents the fusion of chromosomal segments.
• Satellite : It is an elongated segment that is sometimes present on a chromosome at the secondary constriction.
• The chromosomes with satellite are known as sat-chromosome.
• Chromatin : Chromosome is made up of chromatin. Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cell division, the chromatin fibres condense and chromosomes are visible with distinct features.
• The darky stained, condensed region of chromatin is known as heterochromatin. It contains tightly packed DNA, which is genetically inactive.
• The light stained, diffused region of chromatin is known as euchromatin. It contains genetically active and loosely packed DNA.
• At prophase, the chromosomal material is visible as thin filaments known as chromonemata.
• At interphase, bead-like structures are visible, while are an accumulation of chromatin material called chromomere. Chromatin with chromomere looks like a necklace with beads.

Types of Chromosome based on position of Centromere

• [1] Telocentric : Rod-like chromosome with centromere present on the proximal end. No ‘p’ arm or short arm of chromosome, is present. Telocentric chromosomes are not found in humans.
• [2] Acrocentric : Rod-like, centromere present at one end giving rise to one very short arm and an exceptionally long arm.
• [3] Submetacentric : L-shaped or J-shaped, with centromere near the centre of the chromosome giving rise to two unequal arms.
• [4] Metacentric : V- shaped chromosomes with centromere in the middle giving rise to two equal arms.

Structural Variations in Chromosome

• The somatic (2n) and gametic (n) chromosome numbers of a species ordinarily remain constant due to the extremely precise mitotic and meiotic cell divisions. Somatic cells of a diploid species contain two copies of each chromosome, which are known as homologous chromosomes. Gametes of diploid organisms, therefore, contain only one copy of each chromosome, that is, one chromosome complement or genome. The chromosome of a genome are distinct from one another in their morphology and/or gene content; numbers of a single genome do not show a tendency of pairing with each other. Occasionally, spontaneous variation in chromosome number or structure do arise in nature; these are called chromosomal aberrations. Chromosomal aberrations are of two types: (1) Structural, and (2) numerical aberrations.

Structural chromosomal aberrations/ variations

• Structural chromosomal aberrations alter the chromosome structure, i.e., the number, the sequence or the kind of genes present in chromosome(s). There are four common types of structural aberrations: (1) Deletion or deficiency (2) Duplication or Repeat (3) Inversion, and (4) translocation. Deletions (a decrease) and duplications (an increase) alters the number of genes present, inversions change the gene sequence, while translocations affect the kind of genes located in the affected chromosomes.
• (1)Deletion:
• Loss of chromosome segment is known as deletion or deficiency. When the missing segment contains a telomere of the affected chromosome, it is called terminal deletion. Such deletions are believed to be uncommon. In most deficiencies, however, the missing segment does not contain a telomere, such deletions are known as interstitial or intercalary deletions. In 1938, Muller postulated that the loss of telomere makes a chromosome end unstable so that it tends to unite with a similar damaged end.

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• (2)Duplication : The presence of an additional chromosome segment, as compared to that normally present in a nucleus is known as duplication. In a diploid organism, presence of a chromosome segment is more than two copies is called duplication.
• The extra chromosome segment may be located immediately after the normal segment in precisely the same orientation, producing tandem duplication. When gene sequence in the extra segment of a tandem duplication is in the reverse order known as reverse tandem duplication: In some cases, the extra segment may be located in the same chromosome, but away from the normal segment; sauch cases are termed as displaced duplications. In translocation, the additional chromosome segment is located into a nonhomologous chromosome.

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• (3)Inversion: When a segment of a chromosome is oriented in the reverse direction, such a segment is said to be inverted, and the phenomenon is termed as inversion.
• Obviously, the gene sequence is an inverted segment is exactly the opposite of that in its normal homologous (noninverted) segment.
• Inversions may be classified into two groups on the basis of presence or absence of centromere within the inverted segment.
• (I) Paracentric inversion: The centromere is located outside the inverted segment.
• (II) Pericentric inversion: contains the centromere within the inverted segment.
• The inversion loops produced during synapsis in inversion heterozygous are similar in both the cases, but the consequences of crossing over within the inversion loops are quite different. Further, paracentric inversions do not alter the morphology of affected chromosomes, while pericentric inversions may alter the centromeric position.

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• (4) Translocation: Integration of a chromosome segment into a nonhomologous chromosome is known as translocation.
• In a simple translocation, terminal segment of a chromosome is integrated at one end of a nonhomologous chromosome, but simple translocations are rare.
• In the case of a shift, an intercalary or internal segment of a chromosome is integrated at an internal position of a nonhomologous chromosome.
• The most common are reciprocal translocations; a reciprocal translocation is produced when two nonhomologous chromosomes exchange segment, that is, a segment from one chromosome is translocated into a nonhomologous chromosome, from which a segment is reciprocally transferred to the first chromosome.

Numerical Variations : Aneuploidy and Euploidy

• In somatic cells of a diploid organism, two copies of the same genome are present (i.e. 2n=2x), while their gametes contain a single genome (that is, n=x). A deviation from the diploid (2n=2x) state represents a numerical chromosomal aberrations which often is referred to as heteroploidy; individuals possessing the variant chromosome numbers are known as heteroploids. The various heteroploidy states may be grouped into two categories: (1) aneuploidy, and (2) Euploidy.

:Aneuploidy:

• The loss or gain of one or few chromosomes as compared to the somatic chromosome number of a species is known as aneuploidy. It may be stressed that aneuploid changes in chromosome number do not involve the whole genome; they only one or few chromosome of the genome.
• In somatic cells of all species, each chromosome of a genome is represented by two homologous chromosomes; therefore this is called disomic condition. The terminology for aneuploid variation has been developed with reference to the disomic state. Thus the deficit of a chromosome from the somatic number (2n-1) is known as monosomic condition, while the absence of a chromosome pair (2n-2) is termed as nullisomic state. However, if the two missing chromosomes are nonhomologous (2n-1-1), they do not represent the loss of a chromosomes pair; such a condition is known as double monosomic state. Individuals possessing an extra chromosome (2n+1) are termed as trisomics, while those having additional chromosome pair (2n+2) are referred to as tetrasomics. But if the two additional chromosomes are nonhomologous (2n+1+1), such individuals are known as double trisomics. These are the most common situations encountered.

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• Monosomy:
• In monosomic condition (2n-1), one chromosome is missing from the somatic chromosome complement. The loss of one whole chromosome creates major genetic imbalance, which is generally not tolerated by diploid species. Monosomics of polyploid species, e.g. wheat, cotton, oats and tobacco, however they are fully viable, and a complete set of monosomics is available in these crops.
• Nullisomy:
• In nullisomy (2n-2), one pair of chromosomes is missing from the somatic chromosome complement. The absence of one chromosome pair has a strong deleterious effect on the organism so that nullisomics are viable only in highly polyploid species, e.g. hexaploidy or bred wheat (Triticum aestivum) and oats (A. sativa). But nullisomics do not survive in tobacco (Nicotiana tabacum and N. rutica), which is allotetraploid. In wheat, nullisomics are markedly weaker and show reduced size and fertility as compared to normal plants. Nullisomy for different chromosomes produces distinct morphological effects. Many of the wheat nullisomies cannot be propagated, while other nullisomics can be propagated but only with difficulty. Nullisomics occur in a low frequency (approximately 3%) in the selfed progeny of monosomics.

• Trisomy :
• When somatic cells of an organism contain three copies of any one chromosome of the haploid compliment, the condition is known as trisomy (2n+1). e.g. Klinefelter’s syndrome.
• Tetrasomy :
• In tetrasomic (2n+2) individuals, somatic cells contain one pair of a chromosome in excess of the normal somatic complement, i.e. chromosome of the complement is present in four copies. In general, gain of a chromosome pair is more deleterious than that of one chromosome. Therefore, few tetrasomics of diploid species can be recovered and maintained. Tetrasomics are viable in polyploid species, e.g. Wheat, where they have been used in genetic studies, particularly in conjugation with genetic studies.

:Euploidy:

• Euploids have one or more complete genomes, which may be identical with or distinct from each other. The most common condition of euploidy is the diploid state in which two copies of a single genome are present in the cell; it is represented as 2x. Euploid variations are designated with reference to the diploid (2x) state and not to the disomic condition (2n). These variations may be grouped into two broad categories: (1) Monoploids, including haploids, and (2) polyploids.
• Monoploidy and Haploidy :
• Monoploidy denotes the presence of a single copy a single genome, and is represented by x. on the other hand, haploidy represents the gametic chromosome number of a species irrespective of whether it is diploid or polyploid. Thus, monoploids are, in essence haploids of diploid species. Haploids have been subdivided into a number of categories. Euhaploids have the exact haploid number of the species. Euhaploids derived from diploid species known as monoploids, while those obtained from polyploid species are called polyhaploids. If the concerned species is allopolyploid, the haploids derived from it are referred to as allopolyhaploids, while haploids extracted from an autopolyploid species are termed as autopolyploids. A dihaploid is haploid from a tetraploid species, while a double haploid is obtained through chromosome doubling of haploid. Aneuploids, in contrast, generally have one additional or missing chromosome as compared to the haploid complement of a species; aneuploids are viable in the case of polyploid species only.

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• Polyploidy :
• Presence of more than two genomes in an individual is known as polyploidy. If all the genomes present in an individual are identical, it is called autopolyploidy. But in allopolyploidy, two or more distinct genomes are present. Naturally, occurring allopolyploids ordinarily contain two copies of each of the genomes present, and they show normal bivalent formation; therefore, they are known as amphidiploids.
• (1)Autopolyploidy:
• Autopolyploids may have three (triploid), four (tetraploid), five (Pentaploid), six (hexaploid), seven (heptaploid), eight (octaploid) or more copies of the same genome. Of these, triploids and tetraploids have been studied in considerable detail due to their importance in crop improvement. Autopolyploids are produced through chromosome doubling of a species. (1) Occasionally unreduced (2n) gametes are produced spontaneously; union between such gametes would yield tetraploid zygotes. (2) Sometimes, chromosome doubling may occur in somatic cells giving rise to tetraploid buds. (3) Some of the adventitious shoots produced following decapitation may be polyploid; in Solanaceae 6-36% of such shoots are reported to the tetraploid. Heat or Cold treatments and irradiation of seeds/plants with X-ray or gamma-rays may produce polyploid progeny in low frequencies. (5) A variable frequency of plants regenerated from tissue culture of many species, e.g. Nicotiana, Datura, rice etc, are polyploid. (6) Some chemicals such as Acenaphthene, 8 hydroxyquinoline and nitrous oxide, induce chromosome doubling.
• But the most common method of chromosome doubling is (7) the treatment of seeds, seedlings or shoots-tips with colchicine. Colchicine is an alkaloid extracted from the bulbs of Colchium autumnale. it has the chemical formula of C22H25O6N. It interferes with the development of spindle apparatus as a consequence of which the sister chromatids of chromosomes are unable to migrate to the opposite poles during anaphase. Therefore, all the chromatids (=4n) are included in the same restitution nucleus leading to chromosome doubling. Colchicine is generally applied as a 0.2% aqueous solution or as a mixture with lanolin paste. The specific concentration may vary considerably with plant species. e.g. in trees, 1% solution is preferred. The duration of treatment may vary from 3 to 24 hrs in the case of seed and seedlings, while in the case of trees it may extend upto 2-3 weeks. It is advisable to prepare fresh aqueous solution of colchicine of each occasion since it is unstable in water. The chromosome doubling effect of colchicine was first described by Blakeslee and Avery, and by Nebel independently in 1937.

• Allopolyploidy :
• Allopolyploids contain two or more distinct genomes, obviously, derived from different species; almost all natural allopolyploids have two copies of each of the genomes they contain. In view of these facts they are known as amphidiploids.
• Natural allopolyploids most likely originated through chromosome doubling of F1 hybrids produced through chance natural hybridization between two distinct species of the same genus or from different genera.
• Synthesis:
• Experimental production of allopolyploids is achieved through chromosome doubling of distant hybrids (hybrids between two distant species) with the help of colchicine (or some other agent); such allopolyploids are often known as synthetic allopolyploids. Thus synthesis of allopolyploids involves two steps: (1) Production of F1 hybrids by crossing two distinct species and (2) chromosome doubling of such F1 hybrids. For example, Triticale, a synthetic allopolyploid, is generally produced by crossing tetraploid wheat, Triticum turgidum (AABB) with rye (Secale cereale, RR). The F1 from this cross has 21 chromosomes (ABR) an is completely sterile due to a lack of pairing among the chromosome of A, B and R genomes. Doubling of the chromosome number of this hybrid produces Triticale hexaploide (AABBRR), which is highly fertile.

Nucleic Acids Carriers of Genetic Information

• Nucleic acids – DNA and RNA :
• Nucleic acids are long-chain polymeric molecules. The monomer or the repeating unit is known as the nucleotides and hence sometimes nucleic acids are referred to as polynucleotides. Nucleic acids can be defined as organic molecules present in living cells. It play a key factor in transferring genetic information from one generation to the next. Nucleic acids are composed of DNA- deoxyribonucleic acid and RNA –ribonucleic acid that form the polymers of nucleotides.
• In the nucleus, nucleotide monomers are linked together comprising of distinct components namely a phosphate group, nitrogenous bases and Ribose and Deoxyribose. Pyrimidines and Purines are two types of nitrogenous bases. Pyrimidines are composed of cytosine and thymine. Purines are composed of guanine and adenine. Thymine is replaced by Uracil in ribonucleic acid whereas deoxyribonucleic acid comprises of all four bases.

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DNA Structure

RNA Structure

DNA Structure- Watson and Crick’s Model

• James Watson and Francis Crick proposed that DNA must be formed like a double helix in 1953. This is called Watson-Crick Structure of DNA.
• DNA is a helical, double stranded molecule with two strands. On the surface, it has two backbones (with alternating glycosol and phosphate groups) that are linked together on the inside by hydrogen bonds between pairs of nitrogenous bases.
• The bases are divided into four categories (A, C, G and T), with A and T and C and G always matching.
• James Watson and Francis Crick realised that these pairing principles indicated that either strand held all the information needed to build a new copy of the complete molecule and that the aperiodic sequence of bases might offer a “genetic code”.

• Watson- Crick Structure of DNA:
• Watson and Crick presented a model for the DNA’s double-helix structure.
• A nucleotide polymer makes up the DNA molecule.
• A nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group are found in each nucleotide.
• There are two purines (adenine and guanine) and two pyrimidines in the DNA (cytosine and thymine).
• Two strand of DNA make up a DNA molecule.
• Each strand is made up of nucleotides that are covalently linked by their phosphate groups and deoxyribose sugars.
• The bases grow out of this backbone.
• Hydrogen bonds connect the bases of strands.
• Adenine is always linked with thymine, while cytosine is always linked with guanine.
• The two strands wrap around each other in a shape termed the ‘double helix’ because of the bonding.
• The second nucleic acid found in the cells is ribonucleic acid (RNA). RNA is a nucleotide polymer with a single strand.
• It also contains sugar ribose instead of deoxyribose and nucleotide uracil instead of thymine.
• RNA molecules play a role in the formation of proteins using DNA’s genetic code.
• Prokaryotes have a single circular chromosomes with double strands.
• Eukaryotes have chromosomes that include double-stranded linear DNA molecules.
• Nucleosomes are formed when the DNA helix wraps around proteins.
• Protein coils get progressively more coiled, and the chromosomes become even more coiled throughout mitosis and meiosis to make movement easier.

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• Features of the Watson-Crick Structure of DNA :
• In a DNA molecule, a right-handed double-helix is made up of a series or strands of two polynucleotides spirally wrapped around each other and twisted along a common path.
• The two strand run in opposite directions, with the fifth ending of one chain facing the third ending of the other. In other words, they are antiparallel.
• The sugar phosphate backbones remain on the exterior, while the purines and pyrimidines bases are located at the centre of the helix.
• The two series are kept together by hydrogen bonds formed between the purines and pyrimidine bases of the opposite strands.
• Adenine (A) always couple with thymine (T) through two hydrogen interactions, while guanine (G) will always pair with cytosine (C) via three hydrogen bonds. This complementary nature is known as the rule of the base pair. As a result, the two chains are complementary to each other.
• The base sequence varies throughout a polynucleotide chain. A certain sequence of bases carries genetic information.
• The constitution of the bases of the DNA follows Chargaff’s assumptions (E. E. Chargoff, 1950), according to which A=T and G=C. As a corollary, ∑ purines (A+G) = ∑ pyrimidines (C+T). The rules also put forth the ratio of (A+T) and (G+C) as constants for a species (range 0.4 to 1.9).
• The DNA has a diameter of 20 nm, or 20A°. The adjoining bases are 0.34 nm or 3.4A° along the axis apart. The length of a full helix turn is 3.4 nm or 34A°, implying that there are 10b/turning.
• A small groove, called the minor groove (1.2 nm), and a deep groove, known as the major groove (2.2 nm), run across the DNA helix.

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Forms of DNA

• The right-handed double-helical Watson-Crick Model for B-form DNA is the most commonly DNA structure.
• In addition to this classic structure, several other forms of DNA have been observed.
• The helical structure of DNA is thus variable and depends on the sequence as well as the environment.
• The different forms of DNA are (i) A- form (ii) B- form and (c) Z- form.
• B-form DNA:
• B-DNA is the Watson-Crick form of the double helix that most people are familiar with.
• They proposed two strands of DNA – each in right- hand-helix-wound around the same axis. The two strands are held together by H-bonding between the bases (in anti-conformation).
• The two strand of the duplex are antiparallel and plectonemically coiled. The nucleotides arrayed in a 5’ to 3’ orientation on one strand align with complementary nucleotides in the 3’ to 5’ orientation of he opposite strand.
• Bases fit in the double helical mode if pyrimidine on one strand is always paired with purine on the other. From Chargoff’s rules, the two strands will pair A with T and G with C. This pairs a keto base pair with a amino base, a purine with a pyrimidine. Two H-bonds can form between A and T, and three can form between G and C.
• These are the complementary base pairs. The base-pairing scheme immediately suggests a way to replicate and copy the genetic information.
• 34 nm between bp, 3.4 nm per turn, about 10bp per turn.
• 9 nm (about 2.0 nm or 20 angstroms) in diameter.
• 34° helix pitch; -6° base-pair tilt; 36° twist angle.

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• A- form DNA :
• The major difference between A- form and B-form nucleic acid is the confirmation of the deoxyribose sugar ring. It is the C2’ endoconformation form for B-form, whereas it is the C3’ endoconfirmation in A-form.
• A second major difference between A-form and B-form nucleic acid is the placement of base-pairs within the duplex.
• In B-form, the base pairs are almost centered over the helical axis but in A- form, they are displaced away from central axis and closer to the major groove. The result is a ribbon like helix with a more open cylindrical core in A-form.
• Right handed helix.
• 11 bp per turn; 0.26 nm axial rise; 28° helix pitch; 20° base pair tilt.
• 33° twist angle; 2.3 nm helix diameter.

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• Z- form DNA :
• Z- DNA is a radically different duplex structure, with the two strands coiling in left-handed helices and a pronounced zig-zag (hence the name) pattern in the phosphodiester backbone.
• Z-DNA can form when the DNA is in an alternating purine-pyrimidine sequence such as GCGCGC, and indeed the G and C nucleotides are in different confirmations, leading to the zig-zag pattern.
• The big difference is at the G nucleotide.
• It has the sugar in the C3’ endoconfirmation (like A-form nucleic acid, and in contrast to B-form DNA) and guanine base is in the synconfirmation.
• The places the guanine back over the sugar-ring, in contrast to the usual anticonfirmation seen in A-form and B-form nucleic acid.
• The duplex in Z-DNA has to accommodate the distortion of this G nucleotide in the synconfirmation. The cytosine in the adjacent nucleotide of Z-DNA is in the “normal” C2’ endo, aniticofirmation.
• Discovered by Rich, Nordheim and Wanf in 1984.
• It has antiparallel strands as B-DNA.
• It is long and thin as compared to B-DNA.
• 12 bp per turn; 0.45 nm axial rise; 45° helix pitch. 7° base pair tilt.
• -30° twist angle; 1.8 nm helix diameter.

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Types of RNA :

• Three main types of RNA are involved in protein synthesis. They are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
• mRNA:
• Messenger RNA (abbreviated as mRNA) is a type of single stranded RNA involved in protein synthesis. mRNA is made from a DNA template during the process of transcription. The role of mRNA is to carry protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm (watery interior), whereas the protein making machinery reads the mRNA sequence and translates each three-base codon into its corresponding amino acid in a growing protein chain.
• So mRNA really is a form of nucleic acid, which helps the human genome which coded in DNA to be read by the cellular machinery. So we have DNA in our nuclei. And then we have ribosomes and other organelles which translate DNA. But between the DNA code itself, and the machinery that uses DNA to make proteins, there has to be translator. And mRNA is actually the translated form of DNA that the machinery can recognize and use to assemble amino acids into proteins.

• tRNA :
• tRNA is also known as transfer RNA is a subtype of RNA, tRNA help in the protein synthesis process. tRNA carries amino acid to the ribosome, which is the molecular machine that assembles the protein, and ensures that the amino acid is incorporated into the growing protein chain in the correct order. Different types of tRNA present, each with a different sequence and structure allow it to recognize a specific amino acid.
• t-RNA stands for transfer RNA. As its name suggests, it transfers the specific amino acid from the amino acid pool to the mRNA to form a polypeptide to make proteins.
• it is also known as adapter molecule as it connects the messenger RNA (mRNA) molecule and polypeptide chain. You can relate to your mobile adapter which connects your charger cable to the switchboard. So, here your adapter has an analogy with tRNA, cable with mRNA , and switchboard with polypeptide chain. Each tRNA carries a particular amino acid.

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• T-RNA Structure :
• t-RNA molecule has two ends 5’ and 3’ ends. The 5’ end consists of a phosphate group which is attached to the 5^th carbon atom of ribose sugar while the other 3’ end has a free OH group on the 3-carbon atom. So there are two types of structure of tRNA - 2D cover leaf model and 3D, L shape model.
• Cloverleaf model :
• tRNA molecule consist of 70-80 nucleotides that fold like a clover leaf. A 2D t-RNA molecule appears like a cloverleaf. As clove has following ends, the t-RNA molecule also has four ends:
• (1) Acceptor end : It is called the acceptor end as it accepts the specific amino acid. It is made up of 7-9 nucleotides. It has two ends- 3’ and 5’. It is the 3’ end that consists of a base triplet CCA with OH group whereas the 5’ end consists of a phosphate group. At this OH at the 3’ end, the COOH group of amino acids joins.
• (2)Anti-codon end : It is made up of 5 base pairs and consists of codons that are complementary to the codon present on mRNA, therefore known as anticodon. Anticodon is the triplet base sequence in tRNA which binds with the codon at the time of translation. There is a specific tRNA for a particular amino acid. Base pairing between the codon and anticodon helps in the synthesis of proteins. There are no tRNAs for stop codons but have specific tRNA for initiative codons.
• (3)Enzyme site/DHU end : it is present on the lateral side of molecule that recognizes aminoacyl-tRNA synthetase that activates the amino acids and catalyses the binding of specific amino acids to a tRNA molecule. It consists of 3-4 base pairs and is called a D loop because it consists of a modified nucleotide called dihydrouridine.

• Ribosome recognition end : it is on the other lateral side of the molecule. It is meant for attachment to a ribosome.
• TΨC loop end : it is known as T arm. It consists of 4-5 base pairs and a loop consisting of modified uridine called pseudouridine.
• Variable loop : This loop is of variable size and ranges from 3-21 base pairs. It is present between the anticodon and TΨC loop. It recognizes tRNA molecules.

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• L – shaped model :
• 3D tRNA looks like an L-shaped molecule that has two functional ends:
• The acceptor stem : Site of attachment of specific amino acid (3’ end of the molecule).
• Anticodon loop : Site where codons are read on the mRNA (5’ end of the molecule).

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• r-RNA Structure :
• Ribosomal ribonucleic acid (rRNA) is the RNA component of ribosomes, the molecular machines that catalyse protein synthesis.
• Ribosomal RNA constitute over sixty percent of the ribosome by weight and are crucial for all its functions – from binding to mRNA and recruiting tRNA to catalyzing the formation of peptide bond between two amino acids.
• Even the structure of a ribosome is determined by the three-dimensional shape of its rRNA core. Proteins present in the ribosome serve to stabilize this structure through interactions with the core.
• Ribosomal RNA are transcribed in the nucleus, at specific structures called nucleoli. These are dense, spherical shapes that form around genetic loci coding for rRNA. Nucleoli are also crucial for the eventual biogenesis of ribosomes, through sequestration of ribosomal proteins.

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• Types of Ribosomal RNA :
• Both prokaryotic and eukaryotic ribosomes are made of a larger and smaller subunit and these two units come together during mRNA translation.
• The smallest subunit in prokaryotes is made of an RNA molecule about 1500 nucleotides in length with a Svedberg coefficient of 16S. Together with ribosomal proteins, the smaller subunit has a sedimentation rate of 30S. This is paired with the largest subunit, having two RNA molecules – one that is nearly 3000 nucleotides (23S) in length and other is a short sequence of 120 nucleotides (5S). These RNA molecules are accompanied by proteins that gives rise to the larger 50S subunit.
• The eukaryotic ribosome is made of a 60S and 40S subunit. There are two short rRNA molecules less that two hundred nucleotides in length (5S and 5.8S), and two RNA molecules that are much longer – one that has over five kilobases (28S), and another nearly two kilobases (18S). In all, the eukaryotic ribosome has a Svedberg coefficient of 80S. In addition, eukaryotic cells also have rRNA in mitochondria and chloroplasts. Ribosomes can be associated with the endoplasmic reticulum or be present as free floating complexes in the cytoplasm.

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• Functions of rRNA :
• The primary function of rRNA is in protein synthesis – in binding to messenger RNA and transfer RNA to ensure that the codon sequence of the mRNA is translated accurately into amino acid sequence in proteins.
• Ribosomal RNA are also expressed in every cell of all extant species. The sequence of the core catalytic sites are also highly conserved making rRNA an excellent tool for the study of taxonomy and phylogenetics.
• Many antibiotics target prokaryotic rRNA and recently the binding sites for antibiotics such as streptomycin and tetracycline on rRNA have been indicated.

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Significance of Nucleic acids :

• Nucleic acids are responsible for the transmission of inherent characters from parent to offspring.
• They are responsible for the protein synthesis in our body.
• DNA and RNA are molecular repositories of genetic information.
• Apart from DNA and RNA, nucleotides are the structural components of metabolic intermediates and enzyme cofactors.
• DNA fingerprinting is a method used by forensic experts to determine paternity. It is also used for the identification of criminals. It has also played a major role in studies regarding biological evolution and genetics.

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DNA as a genetic material – Griffith’s Experiment

• The search for genetic material started during the mid-nineteenth century. The principle of inheritance was discovered by Mendel. Based on his investigation, Mendel concluded that some ‘factors’ are transferred from one generation to another. Mendel's laws of inheritance was the basis for the researchers on genetic material. Keeping his conclusions in mind, scientists who came after him, focused on chromosomes in search of genetic material. Even though chromosomal components were identified, the material which is responsible for inheritance remained unanswered. It took a long time for the acceptance of DNA as the genetic transformation.
• Griffith’s experiment and transforming principles :
• Griffith’s experiment was a stepping stone for the discovery of genetic material. Frederick Griffith experiments were conducted with Streptococcus pneumoniae.
• During the experiment, Griffith cultured Streptococcus pneumoniae bacteria which showed the patterns of growth. One culture plate consisted of smooth shiny colonies (S) while other consisted of rough colonies (R). The difference was due to the presence of mucous coat in S strain bacteria, whereas the R strain bacteria lacked them.

• Experiment :
• Griffith injected both S and R strains to mice. The one which was infected with the S strain developed pneumonia and died while that infected with the R strain stayed alive.
• In the second stage, Griffith heat-killed the S strain bacteria and injected into mice, but the mice stayed alive. Then, he mixed the heat-killed S and R strains. This mixture was injected into mice and they died. In addition, he found living S strain bacteria in dead mice.
• Conclusion :
• Based on the observation, Griffith concluded that R strain bacteria had been transformed by S strain bacteria. The R strain inherited some ‘transforming principle’ from the heat-killed S strain bacteria which made them virulent. And he assumed this transforming principle as genetic material.

• DNA as genetic material :
• Griffith experiment was a turning point towards the discovery of hereditary material. However, it failed to explain the biochemistry of genetic material. Hence, a group of scientists, Oswald Avery, Colin MacLeod and Maclyn McCarty continued the Griffith’s experiment in search of biochemical nature of the hereditary material. Their discovery revise the concept of protein as genetic material to DNA as genetic material.
• Avery and his team extracted and purified proteins, DNA, RNA and other biomolecules from the heat-killed S strain bacteria. They discovered that protein-digesting enzymes (Proteases) and RNA-digesting enzymes (RNase) didn’t inhibit transformation but Dnase did. Although it was not accepted by all, they concluded DNA as genetic material.