DNA Molecule

12B06

Molecular Basis of Inheritance

Learning Objectives

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The DNA

The Search of Genetic Material

RNA World

Replication

Transcription

Genetic Code

Translation

Regulation of Gene Expression

Human Genome Project

DNA Fingerprinting

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12B06 Molecular Basis of Inheritance

12B06.1

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The DNA

12B06.1 The DNA

DNA is known as Deoxyribonucleic acid.

RNA is known as Ribo nucleic acid.

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12B06.1 The DNA

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N

N

N

N

N

N

N

N

N

Deoxyribonucleotide

1

8

7

6

5

4

3

2

Length of DNA= 8 Deoxyribonucleotides

Haploid content of human DNA is 3.3 × 10⁹ base pairs

Bacteriophage known as φ ×174 has 5386 nucleotides

Escherichia coli has 4.6 × 10⁶ base pairs

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12B06.1 The DNA

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Bacteriophage

Escherichia coli

12B06.1

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CV1

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Structure of Polynucleotide Chain

A Nucleotide has Three components

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Structure of Polynucleotide Chain

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N-Glycosidic Linkage

NUCLEOSIDE

Pentose Sugar

Nitrogenous Base

Phosphate

A Nucleotide has Three components

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Structure of Polynucleotide Chain

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N-Glycosidic Linkage

Pentose Sugar

Nitrogenous Base

Phosphate

N-glycosidic linkage

Phosphoester Linkage

Nucleoside

Nitrogeneous Base

Purines

Adenine

Guanine

Pyrimidines

Thymine (DNA)

Uracil (RNA)

Cytosine

T

U

C

A

G

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Structure of Polynucleotide Chain

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N

N

N

N

N

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Structure of Polynucleotide Chain

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A

Phosphate

Pentose sugar

T

G

C

Adenine

Thymine

Guanine

Cytosine

1

3

OH’

5 C

4

2

N

Ph

P

A

A

Phosphate

Pentose sugar

T

G

C

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Structure of Polynucleotide Chain

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Ph

P

A

T

G

Ph

P

Ph

P

Adenine

Thymine

Guanine

Cytosine

1

3

OH’

5 C

2

4

5’ end

3’ end

3’-5’ Phosphodeister bond

3’-5’ Phosphodeister bond

Backbone of polynucleotide

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Structure of Polynucleotide Chain

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OH’

DNA Nucleotide

RNA Nucleotide

Helix

Hydrogen Bonds

Nitrogenous Bases

Chargaff rule

A

C

T

G

U

RNA

James Watson and Francis Crick in 1953 proposed:

Double Helix Structure of DNA

Hydrogen Bonds

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Structure of Polynucleotide Chain

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0.34nm

Each turn= 10 bp

3.4nm

Ph

T

G

C

Ph

Ph

Ph

Ph

A

G

C

Ph

3’

3’

5’

5’

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Structure of Polynucleotide Chain: Features of DNA

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Double stranded Polypeptide chain

  Ready for challenge

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What is the nature of the strands of the DNA duplex?

1. Anti-parallel and Complementary
2. Identical and Complementary
3. Anti-parallel and Non-complementary
4. Non-indentical and Complementary

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Answer: A

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Explanation: The strands of DNA are antiparallel with the polarity of 5’ to 3’ and 3’ to 5’ with base complementarity.

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Concept Test

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Structure of Polynucleotide Chain

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Replication

Central dogma proposed by Francis crick

Central dogma

DNA

RNA

virus

RNA

Protein

Occur in all organisms

DNA

Transcription

Translation

12B06.1

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CV2

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Packaging Of DNA Helix

Packaging Of DNA Helix

Do not have well defined nucleus

Nucleus without Nuclear Membrane

Prokaryotes (Bacteria)

or

Nucleoid

Negatively Charged DNA

+

Positively Charged Proteins

Packaging Of DNA Helix

Well Defined nucleus

DNA + Histone Proteins

Negatively Charged

Positively Charged

Arginine

Lysine

Basic Amino acids residues

Eukaryotic Cell ( Animal cell)

Packaging Of DNA Helix

Histone Octamer

DNA

Nucleosome

H4

H3

H2B

H2A

H3

H2B

H4

H4

H2B

H2A

H3

H2A

H1

Histone Subunits

Histone Octamer

DNA molecule

H1

Packaging Of DNA Helix

Histone Octamer

DNA

Nucleosome

Repeating Unit of Nucleosomes

Chromatin Thread

H1

Packaging Of DNA Helix

Nucleosomes

Nucleosomes in chromatin as ‘beads-on-string’

Chromatin gets condensed

Nucleosomes: Beads on String

Packaging Of DNA Helix

Packaging of Chromatin

Additional set of Proteins

Non-Histone Chromosomal Proteins

Chromatin

Region which are loosely packed (and stains light)

Region that is more densely packed and stainsdark

Euchromatin

Heterochromatin

Did You Know?

The length of DNA Helix can be calculated as:

Total Number of bp X

Distance between two consecutive bp

6.6 X 10⁹ bp X 0.34 X 10^-9

Single Nucleosome contains 200 base pairs

= 2.2 metres

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• DNA and RNA are the genetic material
• DNA is the polymer of nucleotides. A nucleotide consists of Sugar, Nitrogenous base and Phosphate group.
• DNA is double stranded structure consists of antiparallel polynucleotide chains.
• The two chains have anti-parallel polarity. It means, if one chain has the polarity 5‘ 3', the other has 3’ 5'.
• The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp).

A T

G C

• DNA is negatively charged and it is associated with positively charged Proteins.
• In Eukaryotes DNA is packaged as Nucleosomes with Histone proteins.
• Histone Protein are rich in Basic Amino acid residues of Arginine and Lysine.

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Notemaking/Summary

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PSV 01

Question: If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.

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Answer: Given,

Cytosine= 20%

According to Chargaff Rule

A= T

G= C

Guanine= 20%

Now, G= 20% and C=20%

G+C= 40%

100-40= 60%

Therefore,

A=30%

T=30%

The percent of Adenine in DNA is 30%

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PSV 02

Question: If the sequence of one strand of DNA is written as follows:

5' -ATGCATGCATGCATGCATGCATGCATGC-3‘

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Answer: According to base complementarity:

A pairs with T

G pairs with C

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3’- TACGTACGTACGTACGTACGTACGTACG-5’

Reference Question

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NCERT: 1,2,4

Work Book: 5

12B06.2

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The Search of Genetic Material

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12B06.2 The Search of Genetic Material

Transforming Principle

Performed by Frederick Griffith in 1928

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Rough strain or R strain: Non virulent

Streptococcus pneumoniae (pneumococcus) bacteria are grown on a culture plate

Smooth strain or S strain: Virulent

Mucous (polysaccharide) coat

12B06.2

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CV1

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Transforming Principle - Griffith’s Experiment

Transforming Principle - Griffith’s Experiment

Lives

Injected

Die from pneumonia infection

Injected

R strain

S strain

Transforming Principle - Griffith’s Experiment

Injected

Die from pneumonia infection

Injected

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Die from pneumonia infection

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Heat killed S strain

R strain with Heat killed S strain

Live S strain recovered

Transforming Principle - Griffith’s Experiment

Biochemical Characterisation of Transforming Principle

The genetic material was thought to be a protein

1933-44

Oswald Avery

Colin MacLeod

Maclyn McCarty

Purified biochemicals

Protein

DNA

RNA

Heat killed S strain

Transformed R strain

Transformed

DNA

12B06.2

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CV2

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The Search Of Genetic Material- Hershey And Chase Experiment

The Genetic Material is DNA- Hershey And Chase Experiment

1952

Alfred Hershey and Martha Chase

DNA as Genetic material

Proved

Bacteriophage

The Genetic Material is DNA- Hershey And Chase Experiment

Radioactive Phosphorus

Bacteriophage

Radioactive Sulfur

Bacteriophage

Bacteriophage with Radioactive DNA( Phosphorus)

Bacteriophage with Radioactive Protein Coat(Sulfur)

Culture Plate

Culture Plate

Protein Coat with Radioactive Sulfur

The Genetic Material is DNA- Hershey And Chase Experiment

Sulphur labelled Protein coat

1. Infection

2. Blending

3. Centrifugation

After Centrifugation no Radioactive Sulphur detected in the cell

Bacteriophage

E.Coli Bacteria

Phosphorus labelled DNA

The Genetic Material is DNA- Hershey And Chase Experiment

1. Infection

2. Blending

3. Centrifugation

After Centrifugation Radioactive Phosphorus detected in the cell

Bacteriophage

E.Coli Bacteria

Concept Test

 Ready for challenge

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Hershey and Chase’s experiment was based on the principle:

1. Transformation
2. Transduction
3. Translation
4. Transcription

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Answer: B

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Explanation: Transduction is the process by which a virus transfers genetic material from one bacterium to another. Viruses called bacteriophages are able to infect bacterial cells.

12B06.2

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CV3

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Properties Of Genetic Material (DNA Versus RNA)

Properties Of Genetic Material (DNA Versus RNA)

Why RNA is not Predominant Genetic Material?

RNA

HIV Virus

Tobacco Mosaic Virus

Genetic Material

Properties Of Genetic Material (DNA Versus RNA)

RNA

Messenger

Adapter

Properties Of Genetic Material (DNA Versus RNA)

OH’

RNA Nucleotide

Reactive group

Catalytic

Reactive

Unstable

DNA Nucleotide

More stable

Less reactive

Properties Of Genetic Material (DNA Versus RNA)

Criteria to be fulfilled by the genetic material

Replication

Chemically and structurally be stable

Mutation

Mendelian Characters

Summary/Notemaking

• Transforming principle performed by Griffith in the year 1928 in the search of genetic material.
• Oswald Avery, Colin MacLeod and Maclyn McCarty suggested DNA as genetic material.
• Hershey and Chase confirmed DNA as genetic material.
• Adenine, Guanine and Cytosine are common in RNA and DNA.
• Uracil is present in RNA and in DNA in place of Uracil, Thymine is present.
• In RNA, Pentose sugar is ribose and in DaNA, it is Deoxyribose.
• The 2'-OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable.
• The genetic material should be stable enough, the two strands of DNA are complementary to each other, and come together if separated by heat.

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NCERT QUESTION

Q. How did Hershey and Chase differentiate between DNA and protein in

their experiment while proving that DNA is the genetic material?

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Answer: Hershey and Chase worked with bacteriophage and E.coli to prove that DNA is the genetic material. They used different radioactive isotopes to label DNA and protein coat of the bacteriophage.

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They grew some bacteriophages on a medium containing radioactive phosphorus (32P) to identify DNA and some on a medium containing radioactive sulphur (35S) to identify protein.

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Then, these radioactive labelled phages were allowed to infect E.coli bacteria. After infecting, the protein coat of the bacteriophage was separated from the bacterial cell by blending and then subjected to the process of centrifugation.

HOMEWORK

1. Give few points of difference between DNA and RNA in their structure chemistry and function.
2. State the 4 criteria which a molecule must fulfill to act as a genetic material.

12B06.3

RNA WORLD

RNA WORLD

Which is the first genetic material? RNA or DNA?

RNA was the first genetic material.

RNA is catalytic and unstable, Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.

12B06.4

REPLICATION

REPLICATION

Watson and Crick had immediately proposed a scheme for replication of DNA.

Watson-Crick model for semiconservative DNA replication

REPLICATION

The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands.

After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand.

This scheme was termed as semiconservative DNA replication

12B06.4

CV1 REPLICATION- THE EXPERIMENTAL PROOF

REPLICATION: THE EXPERIMENTAL PROOF

• The DNA replicates conservatively was first shown in Escherichia coli and subsequently in higher organisms.
• Matthew Meselson and Franklin Stahl performed the following experiment in 1958

REPLICATION: THE EXPERIMENTAL PROOF

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They grew E.coli in a medium containing heavy isotope of N¹⁵ , new E.coli cells were obtained contain N¹⁵ isotope.

This heavy DNA molecule of DNA is separated by normal DNA by Centrifugation in a cesium chloride solution.

Cells with N¹⁵ is then tranferred in a medium containing N¹⁴

New DNA molecules were synthesized contains DNA with N¹⁴ and N¹⁵

12B06.4

CV2 REPLICATION- THE MACHINERY AND THE ENZYMES

REPLICATION: THE MACHINERY AND THE ENZYMES

DNA Polymerase

DNA Replication in E.coli requires a set of enzymes, The main enzyme is DNA dependent DNA Polymerase.

DNA dependent DNA Polymerase uses DNA as Template.

DNA Polymerase have to be fase and synthesize polymers of nucleotides with high accuracy.

Deoxyribonucleoside triphosphates used as substrates, and also provide energy for the polymerisation( the two terminal phosphates are high energy phospahtes)

REPLICATION: THE MACHINERY AND THE ENZYMES

The DNA-dependent DNA polymerases catalyse polymerisation only in one direction, that is 5’ 3'.

On one strand (the template with polarity 3’ 5'), the replication is continuous, while on the other (the template with polarity 5’ 3'), it is discontinuous.

REPLICATION: THE MACHINERY AND THE ENZYMES

DNA replication starts at a specific region, this region is called as Origin of replication

The discontinuously synthesised fragments are later joined by the enzyme DNA ligase

CONCEPT TEST

Enzyme that synthesize polunucleotides during replication:

1. DNA depenedent DNA polymerase
2. RNA dependent DNA polymerase
3. RNA polymerase
4. None of the above

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Answer: A

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Explanation: DNA Replication in E.coli requires a set of enzymes, The main enzyme is DNA dependent DNA Polymerase.

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12B06.5

TRANSCRIPTION

TRANSCRIPTION

The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.

Transcription Unit in DNA

A Promoter

The Structural gene

A Terminator

12B06.5

CV1 TRANSCRIPTION UNIT IN DNA

TRANSCRIPTION UNIT IN DNA

The promoter , the structural gene the terminator flank are present in a transcription unit.

The promoter is said to be located towards 5' end (upstream) of the structural gene

Promoter

Structural Gene

Terminator

3’ 5’

It is a DNA sequence that provides binding site for RNA polymerase

The terminator is located towards 3' end (downstream) of the coding strand and it usually defines the end of the process of transcription

TRANSCRIPTION: TEMPLATE AND CODING STRAND

The two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction.

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5’ 3’ and 3’ 5’

If a 3’ 5’ act as template than new coding strand is formed with polarity 5’ 3’

The other strand which has the polarity (5'→3') has the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription.

5' -TACGTACGTACGTACGTACGTACG-3' Coding Strand

The Template strand of DNA is transcribed the strand of RNA , with base complementarity.

3' -ATGCATGCATGCATGCATGCATGC-5' Template Strand

12B06.5

CV2 TRANSCRIPTION UNIT AND THE GENE

TRANSCRIPTION UNIT AND THE GENE

Gene is defined as the functional segment of DNA

The DNA sequence coding for tRNA or rRNA molecule also define a gene.

TRANSCRIPTION UNIT AND THE GENE

Cistron

Segment of DNA coding for a polypeptide

Monocistronic

Polycistronic

Consists of One Cistron(mostly in eukaryotes)

Consists of more than One Cistron(mostly in bacteria or prokaryotes).

Exons are coding sequence and Introns are intervening sequence

12B06.5

CV3 TYPES OF RNA AND THE PROCESS OF TRANCRIPTION IN BACTERIA

TYPES OF RNA

All three RNAs are needed to synthesise a protein in a cell.

• The mRNA provides the template.
• tRNA brings aminoacids and reads the genetic code.
• rRNAs play structural and catalytic role during translation.

The process involves three steps

PROCESS OF TRANSCRIPTION IN BACTERIA

Initiation

Elongation

Termination

• In intiation RNA polymerase binds to promoter and initiates transcription.
• RNA polymerase faciliates opening of the helix and continues elongation.
• The polymerases reaches the terminator region, the nascent RNA falls off, so also the RNA polymerase. This results in termination of transcription.

PROCESS OF TRANSCRIPTION IN BACTERIA

12B06.5

CV4 TRANSCRIPTION IN EUKARYOTES

TRANSCRIPTION IN EUKARYOTES

In Bacteria the transcription and translation can be coupled as there is no such complexity

In eukaryotes, there are three RNA Polymerases.

RNA Polymerase I – rRNAs (28S, 18S, 5.8 S)

RNA Polymerase III– tRNAs, 5sRNA and SnRNA (small nuclear RNA)

RNA Polymerase II– mRNAs, hnRNA ( heterogenous RNA)

TRANSCRIPTION IN EUKARYOTES

The Primary mRNA transcripts in Eukaryotes

Contains both Exons and Introns

Non Functional

Primary transcript thus subjected to following process

Capping

Tailing

Splicing

TRANSCRIPTION IN EUKARYOTES

In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA.

In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner.

Introns are removed from hn RNA and Exons are joined after capping and tailing, now it is fully processed mRNA, that is transported out of the nucleus for translation.

SUMMARY

• DNA replication is semi-conservative.
• Transcription unit in DNA consists of a Promoter region, Structural gene and a Terminator region.
• RNA polymerases facilitates the transcription of RNAs.
• There are mainly three types of RNAs: mRNA, tRNA and rRNA.
• Post transcriptional process involves: Splicing. Capping and Tailing of mRNA transcript.

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NCERT QUESTIONS

Q. Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain.

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Answer: Watson and Crick observed that the two strands of DNA are anti-parallel and complementary to each other with respect to their base sequences. This type of arrangement in DNA molecule led to the hypothesis that DNA replication is semi-conservative.

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Q. Difference between mRNA and tRNA

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Answer: The difference among mRNA and tRNA  is that mRNA carries the coding instructions of an amino acid sequence of a protein while tRNA carries specific amino acids to the ribosome to form the polypeptide chain.

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12B06.6

GENETIC CODE

GENETIC CODE

Genetic code direct the sequence of amino acids during synthesis of proteins.

There are 4 bases code for 20 amino acid, code for 64 codons, that means a codon is triplet.

4³ = 64

GENETIC CODE

Salient features of genetic code are:

• The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.
• One codon codes for only one amino acid, hence, it is unambiguous and specific.
• Some amino acids are coded by more than one codon, hence the code is degenerate.
• The codon is read in mRNA in a contiguous fashion. There are no punctuations.
• The code is nearly universal.
• AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon.

Term genetic code was given by George Gamow in 1954

GENETIC CODE

• Lets predict the sequence of amino acid coded by mRNA

-AUG UUU UUC UUC UUU UUU UUC-

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Met-Phe-Phe-Phe-Phe-Phe-Phe

12B06.6

CV1 MUTATION AND GENETIC CODE

MUTATION AND GENETIC CODE

• POINT MUTATION: In this mutation a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence.
• Example: In Sickle Cell Anemia, the Beta globin chain glutamate changes Valine amino acid residue due to change in single base pair in the gene.

MUTATION AND GENETIC CODE

Lets take an example to understand more clearly:

• Considering a statement having three letters like a genetic code:

RAM HAS RED CAP

• If we insert a letter B in between HAS and RED and rearrange the statement, it would read as follows:

RAM HAS BRE DCA P

• Similarly, if we now insert to letters at the same place, say BI'. Now it would read:

RAM HAS BIR EDC AP

• Now we insert three letters together, say BIG, the statement would read

RAM HAS BIG RED CAP

MUTATION AND GENETIC CODE

• The same exercise can be repeated, by deleting the letters R, E and D one by one and rearranging the statement to make a triplet word.

RAM HAS RED CAP

RAM HAS EDC AP

RAM HAS DCA P

RAM HAS CAP

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• Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion.Such mutations are referred to as frame-shift insertion or deletion mutations.

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12B06.6

CV2 tRNA-THE ADAPTER MOLECULE

tRNA- THE ADAPTER MOLECULE

tRNA has an anticodon loop that has bases complementary to the code on mRNA

It also has an amino acid accepter end to which it binds to amino acids.

For initiation, there is another specific tRNA that is referred to as initiator tRNA.

tRNAs are specific for each amino acid and there are no tRNAs for stop codon

CONCEPT TEST

In Sickle Cell Anemia, the Beta globin chain glutamate changes to:

1. Tyrosine
2. Valine
3. Tryptophan
4. Lysine

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Answer: B

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Explanation: In Sickle Cell Anemia, the Beta globin chain glutamate changes Valine amino acid residue due to change in single base pair in the gene.

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12B06.7

TRANSLATION

TRANSLATION

Translation refers to the process of polymerisation of amino acids to form a polypeptide

The order and sequence of amino acids are defined by the sequence of bases in the mRNA.

The amino acids are joined by a bond which is known as a peptide bond.

• First tRNA is charged by its specific amino acid.
• The charged tRNA binds to Ribosome and to complementary base pair present on mRNA.
• When Two charged tRNA are brought closer a pepide bond is formed energetically.

TRANSLATION

The ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond.

An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR).

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The UTRs are present at both 5' -end (before start codon) and at 3' -end (after stop codon).

• The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into Polypeptide sequences.
• At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.

TRANSLATION

12B06.8

REGULATION OF GENE EXPRESSION

REGULATION OF GENE EXPRESSION

Regulation of gene expression occur at various levels.

In eukaryotes, the regulation could be exerted at

Transcriptional level (formation of primary transcript),

Processing level (regulation of splicing),

Transport of mRNA from nucleus to the cytoplasm,

Translational level.

12B06.8

CV1 LAC OPERON

THE LAC OPERON

First elucidated by geneticistFrancois Jacob and a biochemist, Jacque Monod.

In lac operon (here lac referes to lactose), a polycistronic structural gene is regulated by a common promoter and regulatory genes. Such arrangement is very common in bacteria and is referred to as operon.

THE LAC OPERON

Lactose is inducer as it is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon.

if lactose is provided in the growth medium of the bacteria, the lactose is transported into the cells through the action of permease

The repressor of the operon is synthesised (all-the-time – constitutively) from the i gene.

The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon.

Essentially, regulation of lac operon can also be visualised as regulation of enzyme synthesis by its substrate.

In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer.

This allows RNA polymerase access to the promoter and transcription proceeds

12B06.9

HUMAN GENOME PROJECT

HUMAN GENOME PROJECT

Human Genome Project (HGP) was called a mega project launched in 1990 and completed in 2003

Human genome is said to have approximately 3 x 10⁹ bp,

3300 books would be required to store the information of DNA sequence from a single human cell.

HGP was closely associated with the rapid development of a new area in biology called as Bioinformatics.

Goals of Human Genome Project (HGP)

• Identify all the approximately 20,000-25,000 genes in human DNA.
• Determine the sequences of the 3 billion chemical base pairs that make up human DNA.
• Store this information in databases.
• Improve tools for data analysis.
• Transfer related technologies to other sectors, such as industries.
• Address the ethical, legal, and social issues (ELSI) that may arise from the project.

HUMAN GENOME PROJECT

HUMAN GENOME PROJECT

Methodologies

Expressed Sequence Tags (ESTs)

Sequence Annotation

Identifying all the genes that

expressed as RNA

Sequencing the whole set of genome that contained all the coding and non-coding sequence, and later assigning different regions in the sequence with functions

The DNA which is to be squenced is cloned in suitable vectors for example: as BAC (bacterial artificial chromosomes), and YAC (yeast artificial chromosomes).

• The human genome contains 3164.7 million nucleotide bases.

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• The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.

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• The total number of genes is estimated at 30,000–much lower than previous estimates of 80,000 to 1,40,000 genes. Almost all (99.9 per cent) nucleotide bases are exactly the same in all people.

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• The functions are unknown for over 50 per cent of discovered genes.

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SALIENT FEATURE OF HUMAN GENOME

• Repeated sequences make up very large portion of the human genome.

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• Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes hundred to thousand times.

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• Chromosome 1 has most genes (2968), and the Y has the fewest (231).

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• Scientists have identified about 1.4 million locations where single- base DNA differences (SNPs – single nucleotide polymorphism, pronounced as ‘snips’) occur in humans.

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SALIENT FEATURE OF HUMAN GENOME

12B06.10

DNA FINGERPRINTING

DNA FINGERPRINTING

Thedifferences in sequence of DNA which make every individual unique in their

phenotypic appearance.

Imagine trying to compare two sets of 3 × 10⁹base pairs. DNA fingerprinting is a very quick way to compare the DNA sequences of any two individuals.

DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA.

A small stretch of DNA is repeated many times.

DNA FINGERPRINTING

These repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation.

The bulk DNA forms a major peak and the other small peaks are referred to as Satellite DNA.

• These sequences normally do not code for any proteins.
• These sequence show high degree of polymorphism (variation at genetic level) and form the basis of DNA fingerprinting.
• An individual show the same degree of polymorphism, they become very useful identification tool in forensic applications.

If an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism.

DNA FINGERPRINTING

The technique of DNA Fingerprinting was initially developed by Alec Jeffreys.

He used a satellite DNA as probe that shows very high degree of polymorphism.

It was called as Variable Number of Tandem Repeats (VNTR).

It has immense applications in the field of forensic science, genetic biodiversity and

evolutionary biology.

DNA FINGERPRINTING

Steps that are involved in DNA Fingerprnting:

Isolation of DNA

Digestion of DNA by restriction endonucleases

Separation of DNA fragments by electrophoresis

Transferring (blotting) of separated DNA fragments to synthetic membranes, such as nitrocellulose or nylon.

Hybridisation using labelled VNTR probe.

Detection of hybridised DNA fragments by autoradiography

DNA FINGERPRINTING

Schematic representation of DNA Fingerprinting

SUMMARY

• Term genetic code was given by George Gamow (1954). He was the first to propose the triplet code (one codon consists of three nitrogen bases).
• A codon is composed of three adjacent nitrogen bases which specify one amino acid in polypeptide chain.
• Translation involves ribosomes, mRNA and tRNA.
• Regulation of transcription is the primary step for regulation of gene expression.
• The lac operon consists of three genes: LacZ, LacY and LacA encoding proteins involved in lactose metabolism.
• Human genome project was a mega project that aimed to sequence every base in human genome.
• DNA Fingerprinting is a technique to find out variations in individuals of a population at DNA level. It works on the principle of polymorphism in DNA sequences.

NCERT QUESTIONS

Q. Explain (in one or two lines) the function of the followings:

(a) Promoter

(b) tRNA

(c) Exons

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Answer:

1. Promoter is a sequence in DNA that provides binding site for RNA polymerase.
2. tRNA is the  type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein.
3. Exons are coding sections of an RNA transcript, or the DNA encoding it, that are translated into protein.

NCERT QUESTIONS

Q. What is DNA fingerprinting? Mention its application.

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Answer: DNA fingerprinting is a technique used to identify and analyze the variations in various individuals at the level of DNA. It is based on variability and polymorphism in DNA sequences.

(1) It is used in forensic science to identify potential crime suspects.

(2) It is used to establish paternity and family relationships.

HOMEWORK

1. Define
1. Translation
2. Polymorphism

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2. Why is the Human Genome project called a mega project?

SEE YOU IN NEXT CLASS!