Unit 1
Protein Synthesis and Gene Expression
Cell Division
Why would cells need to divide?
To let an organism grow or increase in size, to replace old or damaged cells, or to asexually reproduce.
Cell Cycle: A series of events that a cell goes through as it grows and divides.
M Phase
or
Cell Division
The Cell Cycle
Overview of Mitosis
The newly created cells are called daughter cells.
The genetic material (DNA) is exactly the same as the original cell.
Karyotype – A visual representation of an individual’s complete set of chromosomes arranged in numerical order.
Autosome – Any chromosome in an organism which is not a sex chromosome.
Sex chromosomes – The specific chromosomes that determine an organism’s sex.
Cells can be either somatic cells or gametes.
Somatic cells = regular body cells (46 chromosomes).
Somatic cells are diploid cells. Diploid cells have 2 full sets of chromosomes. One set has 23 chromosomes, so a somatic cell has 2 full sets of chromosomes (46 chromosomes).
Diploid cells = 2N
Gametes = Sex cells
Female gamete = egg (ovum)
Gametes are haploid cells, which contain only 1 full set of chromosomes, or 23 total.
Haploid = N
Male gamete = sperm
Meiosis (my-OH-sis)
Meiosis is a reduction division in which the number of chromosomes is cut in half.
This is the process used to create gametes.
Why would gametes (haploid cells) need only half the number of chromosomes as a somatic cell (diploid cell)?
Homologous chromosomes:
A pair of chromosomes in a diploid cell that have corresponding DNA sequences with one coming from each parent; also called homologues. These chromosomes have similar traits on them (hair color, eye color, etc.).
Meiosis has similar phases to mitosis with a few variations to what happens in these phases.
Meiosis involves 2 cell divisions referred to as Meiosis I and Meiosis II.
Meiosis produces 4 new cells with genetically different information and half of the chromosomes as the original cell.
Crossing Over
Crossing over is a process in which the chromatids of homologous chromosomes cross over and exchange genetic information. (Happens in Prophase I)
This allows for genetic variation!
Meiosis I
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
The DNA is copied in the S phase.
Homologous chromosomes form tetrads and crossing over occurs.
Meiosis I results in two haploid (N) daughter cells.
Telophase I and cytokinesis
Prophase II
Prophase II
Metaphase II
Metaphase II
Anaphase II
Anaphase II
Telophase II and Cytokinesis
Meiosis II
The process of meiosis in males is called spermatogenesis and results in 4 equal sized haploid cells.
The process of meiosis in females is called oogenesis and results in one viable egg and multiple polar bodies which are smaller in size.
Mitosis vs Meiosis
Nondisjunction: When a pair of homologous chromosomes fail to separate during anaphase.
This could result in gametes having an extra chromosome or having one less chromosome.
When Gametes Meet
When two gametes meet, or when the sperm meets the egg, a new organism is formed.
This fertilized egg is called a zygote!
The zygote will continue to divide and develop into a group of cells which will implant into the wall of the uterus.
Embryo: A group of cells which is developing into a new organism. This group of cells is called an embryo until it is 8 weeks old for a human.
Embryo
After 8 weeks of development the embryo is now know as a fetus, or unborn offspring, until the moment of birth.
Fetus
The zygote contains stem cells.
Stem Cells: Cells produced during the first few divisions of the zygote which have the potential to become any type of specialized cell in the body.
Determination vs Differentiation
Determination: Within a few weeks after fertilization, most stem cells become committed to develop into only one type of cell. These committed cells still retain all of the genetic material needed to build an entire organism. However, during determination, they lose the ability to express some of this information.
Cell Differentiation: The process by which committed cells acquire the structures and functions of highly specialized cells.
DNA vs. RNA
1. They contain different sugars.
2. RNA is single stranded, DNA is double stranded.
3. RNA contains Uracil instead of Thymine.
RNA has ribose
DNA has deoxyribose
There are 3 differences between DNA and RNA
3 Types of RNA
Messenger RNA (mRNA): A copy of the instructions needed to build proteins. It carries the message to the ribosomes.
Ribosomal RNA (rRNA): Ribosomes are made up of rRNA. Its role is to hold the mRNA in place.
Transfer RNA (tRNA): Carries needed amino acids to the ribosome to make proteins. The 3 base pairs on the bottom of the molecule is called an anti-codon.
Proteins
What are proteins and why are they important?
There are different types of proteins each with a function.
Protein Synthesis
Protein synthesis happens in two stages:
instructions are made and sent to the ribosome.
language and translates it into the “amino acid”
language.
Transcription
A transcript is a written copy of something. (Such as a transcript of your high school grades.)
Transcription takes place in the nucleus.
This involves making a copy of the information on the DNA strand by building a strand of mRNA.
Transcription
The enzyme RNA polymerase will unzip the DNA strand and copy the entire gene by building a strand of mRNA.
Remember to use the base Uracil instead of Thymine!
The strand of mRNA is then edited when the introns are removed and the exons are spliced together.
Introns: regions of DNA not involved in coding for proteins.
Exons: regions of DNA used to code for proteins.
Transcription
Once the exons are spliced together, the now complete mRNA strand will leave the nucleus through a nuclear pore and travel to the ribosome for translation.
Translation
Translation takes place in the cytoplasm on a ribosome.
The ribosome will “read” the base sequence on the mRNA strand and “translate” it into the amino acid language of proteins.
Translation
The ribosome “reads” 3 mRNA nucleotides at a time.
This 3 base sequence of mRNA is called a codon.
Translation
Each codon specifies or calls for a specific amino acid.
How many different amino acids are there?
20
Some amino acids are called for by more than one codon.
Translation
A mRNA strand reads18 bases long:
AUGCCAAUCGUAUCCUGA
How many codons are in the strand above?
6
What amino acids are being called for by the mRNA strand above?
Methionine
Proline
Isoleucine
Valine
Serine
Stop
Translation
When a codon calls for a specific amino acid, a tRNA molecule will bring the needed amino acid over.
Each tRNA molecule carries only one kind of amino acid. How does the correct molecule of tRNA and the correct amino acid get to where it needs to go?
Translation
Each tRNA molecule has 3 unpaired bases on one end of the molecule called an anti-codon.
The anti-codon must be the complementary match to the codon on the mRNA strand.
Codon
Anti-codon
AUG
UAC (Methionine)
Matches with
As each codon is read, the tRNA with the matching anti-codon will bring in the correct amino acid.
Translation
Translation
A peptide bond is formed between each new amino acid brought by the tRNA.
Translation
The process goes on until it reaches a stop codon.
A chain of amino acids is a protein.
A summary of protein synthesis.
Gene Expression
Remember that DNA contains and stores the instructions for how to build proteins.
Regions of DNA not involved in coding for proteins.
Regions of DNA used to code for proteins.
All somatic cells contain a complete set of DNA for the organism but only some parts of it are “activated”.
What are introns?
What are exons?
Gene Expression
The expression of traits is more complicated than just having a dominant or recessive trait.
There are things such as incomplete dominance, codominant alleles, polygenic traits, sex linked traits, internal factors and external factors.
Gene Expression – Incomplete Dominance
The phenotype of the offspring is a blend of the two traits and looks like neither parent.
Red flower X White flower
Pink flower
Gene Expression - Codominance
Codominance: Both alleles contribute to the phenotype.
Ex. Roan cow or blood types
Gene Expression – Polygenic Traits
Polygenic trait: A trait controlled by two or more genes.
Ex. Skin color, eye color, or height.
Ex. AABBCCDDEEFF
Ex. aabbccddeeff
Ex. AabbCcDdEeff
Gene expression – Sex linked traits
These are traits found on the sex chromosomes. Examples include hemophilia, red-green color blindness, congenital night blindness, Duchenne muscular dystrophy, and Fragile X syndrome.
Example
Gene Expression – Internal Factors
Molecule distribution (Proteins, mRNA, and organelles) in the cytoplasm of the egg can be uneven and regulate gene expression.
Cells developing in an embryo are influenced by surrounding cells and communicate using signal molecules and these can cause genes to be turned on or off.
Ex. Proteins that control when a cell will enter mitosis.
Ex. Eating breakfast (dietary choices) will influence blood sugar levels.
Gene Expression – External Factors
Gene C controls fur pigmentation in Himalayan rabbits. The gene is active when environmental temperatures are between 15-25° C. The rabbit on the left was reared at 20° C while the rabbit on the right was reared at temperatures above 30°. Gene C in inactive at higher temperatures.
Examples of external factors that can affect gene expression are:
Temperature
Light
Drugs or chemicals
Gene Expression – External Factors
Temperature – Sea turtles
Eggs in warmer temperatures develop into females.
Eggs in cooler temperatures develop into males.
Identical twins – Size or height differences
Position in the uterus, different nutrition, and health
care options can result in different heights and other physical traits.
Mutations: Permanent, inheritable changes in the genetic material.
Mutations
Mutations are a source of variation and contribute to evolution.
Mutations
A chromosomal mutation is a change in the structure or number of chromosomes in an organism.
Types of chromosomal mutations include:
Deletions
Insertions (Duplications)
Inversions
Translocations
Types of Chromosomal Mutations
Types of Chromosomal Mutations
Types of Chromosomal Mutations
Point Mutations
A point mutation is a change in a single base pair of the DNA.
This change is a substitution only, not a deletion or insertion of a base.
Frameshift Mutation
A frameshift mutation happens when a single base is inserted or deleted from the DNA.
A frameshift mutation causes a shift in how the ribosome “reads” the grouping of codons.
Frameshift mutations cause more dramatic changes and alters a protein so much that it is unable to perform its normal functions.
Frameshift Mutation
Causes of Mutations
Mutagens: Any agent which can cause a change in DNA.