7.2: Transcription & gene expression

Gene expression

Proteins regulate the expression of genes.

Prokaryotes express genes in response to their environment. E.g. figure 1, pg. 356, shows a repressor protein blocking the expression of the gene responsible for producing lactase.

This is controlled by negative feedback.

Eukaryotes also respond to environmental factors and regulate gene expression using proteins.

Regulation of transcription

Enhancers: Increase the rate of transcription

Silencers: Decrease the rate of transcription

Promoter-proximal elements: are near a promoter, there are required to initiate transcription.

Nature V’s Nurture…

What impact does the environment have on gene expression?

Qu. Pg. 356.

Morphogens regulate embryonic gene expression.

Siamese cats…

Coat colour in cats is determined by the ‘C’ gene. In Siamese cats the gene is ‘Cs’,

this is actually a mutation - the amino acid glycine has been replaced by arginine in the formation of the protein tyrosinase.

Tyrosinase is the protein which regulate coat colour. In warmer temperatures, the

coat appears grey/white. In cooler temperatures the colour changes to

black/brown.

Nucleosomes & transcription

Some groups can be added to the nucleosome e.g. acetyl group, methyl group or a phosphate group.

By adding, or removing these groups, the structure of the DNA strand will be altered. e.g.

Often there is a residual amount of the amino acid lysine on the tails of histones. Lysine has a slightly positive charge therefore it will attract the DNA strand. As a result the coil become tighter. Transcription slows.

Histone acetylation is the opposite of this process, by adding the acetyl group, the charge is neutralised and the DNA strand loosens. Transcription increases.

Some vocab.

Acetylation = adding an acetyl group

Deacetylation = removing a acetyl group

Methylation = adding a methyl group

Demethylation = removing a methyl group

Methylation of histones can either increase or decrease transcription but methylation can occur

directly onto the DNA strand (base C) in which case it decreases gene expression.

Epigenetics

The study of heritable cellular and physiological traits. Identifying patterns, trends and discrepancies.

Read green box pg. 358/9 What is an epigenetic tag? What is meant by the epigenome? Looking at fig. 7, how does the methylation of cytosine

regulate transcription?

Gene expression

Gene expression can occur at different stages.

Post-transcriptional modification

Only occurs in eukaryotes, the immediate prodtuct of transcription is called pre-mRNA. This contains introns (aka intervening sequences), these are non-coding regions, which must be removed before translation. See fig. 10, pg. 360.

Introns and exons (coding regions) increase the variety of proteins that can be produced from a single mRNA strand.

mRNA splicing

Splicing increases the number of proteins possible

Single genes can code for multiple proteins because exons may or may not be included.

In mammals, tropomysin (protein) can have up to 5 different forms depending on which tissue it is being produced in.

This splicing, occurs in other organisms too