7.2 Transcription and gene expression

Question 1

What does the promotor do?

Answer 1

Only some DNA sequences code for the production of polypeptides. These are called coding sequences. There are a number of non-coding sequences found in genomes. Some of them have functions such as those sequences that produce tRNA and rRNA.

Some non-coding regions play a role in the regulation of gene expression such as enhances and silencers.

THE PROMOTER is a sequence that is located near a gene. It is the binding site of RNA polymerase, the enzyme that catalyses the formation of the covalent bond between nucleotides during the synthesis of RNA. The promotor is not transcribed but plays a role in transcription.

Question 2

How is gene expression regulated by proteins in E.coli?

Answer 2

The genes responsible for the absorption and metabolism of lactose by E.coli are expressed in the presence of lactose so it can be broken down and not in the absence of lactose.
In the absence of lactose, a repressor protein blocks the genes that would need to be transcribed to make the protein so that mRNA cannot be transcribed and taken to ribosomes to make the enzyme that breaks down lactose.
In the presence of lactose this repressor is deactivated and then the enzymes can be made.

Question 3

What are enhancers?

Answer 3

These are regulatory sequences on the DNA that increase the rate of transcription.

Question 4

What are silencers?

Answer 4

These are regulatory sequences on the DNA that decrease the rate of transcription.

Question 5

What are promoter proximal elements?

Answer 5

These are sequences of DNA that do not code for anything but are near (above or below) the promoter and binding of proteins is also necessary to initiate transcription.

Question 6

How can environmental factors effect gene expression?

Answer 6

Sunlight causes the production of skin pigmentation in humans.
The lac operon in E.coli
In coat colour of cats the “C” gene codes for the production of the enzyme tyrosinase, the first step in the production of pigment. A mutant allele of the gene “c” allows normal pigment production only at temperatures below body temperature. This is the allele in Siamese cats near body temperature these pigments do not be produced so the cat is white in hot areas.

Question 7

How is transcription regulated?

Answer 7

Nucleosomes help regulate transcription in eukaryotic cells.
DNA is wrapped around histone proteins, how tightly is determined by the different types of modification that can occur to the tail. Normally the proteins have a positive charge and therefore bind to the DNA tightly reducing transcription. If you add an acetyl group to the protein you neutralise that charge and allow more transcription to occur.

Question 8

What are epigenetic tags?

Answer 8

These are chemical modification to the histone proteins that either increase or decrease transcription.
These epigenetic tags are passed on to the sperm and egg but when they meet they are erased. This is called “reprogramming” About 1% of the epigenome is not erased and so the environment is effecting inheritance a little. This is called “imprinting”

Question 9

What does methylation do to transcription?

Answer 9

Methylation prevents transcription by coiling up the histones.

Question 10

What is post transcriptional modification?

Answer 10

Eukaryotic cells modify mRNA after transcription

Question 11

What is the difference between the transcription and translation of DNA in prokaryotes and eukaryotes?

Answer 11

Because prokaryotes do not have a nuclear membrane around their DNA transcription and translation can be coupled. However the separation of the location of transcription and translation into separate compartments in eukaryotes allows for significant post-transcriptional modification to occur before the mature transcript exits the nucleus.

Question 12

What happens in eukaryotic cells after transcription?

Answer 12

The pre mRNA has to be modified. One of these modifications is called splicing. Interspersed throughout the mRNA are sequences that will not contribute to the formation of the polypeptide. They are referred to as intervening sequences or introns. They much be removed. These will be spliced together to form the mature mRNA.

• You also need to add:
  A 5’ cap
  A poly A tail

Question 13

What are introns?

Answer 13

Sequences in the mRNA that will not contribute to the formation of the polypeptide. They are removed by splicing

Question 14

What goes on the front of the mature mRNA?

Answer 14

5’ CAP

Question 15

What goes on the end of the mature mRNA?

Answer 15

A poly A tail

Question 16

How does DNA splicing work?

Answer 16

Splicing is a process where pre-mRNA is edited by a spliceosome, and sections are removed. In genes with multiple exons, some of these can be removed and if they are it forms different proteins to if it is, this means RNA splicing increases the number of different proteins an organism can produce.