7.2 DNA Transcription and Gene expression

Question 1

outline the process of transcription

Answer 1

5’-3’ direction (antisense strand in 3’-5’)

Initiation
1. RNA polymerase binds to the DNA at the promoter region and the double helix unwinds.

Elongation
2. RNA polymerase reaches coding region and begins the synthesises of a complementary RNA copy from the antisense DNA strand, by binding free nucleotides using complentary base pairings but replacing thymine by uracil .

Termination
3. process continues until the RNA polymerase reaches a terminator region of the DNA; transcription stops when the RNA polymerase detaches from the DNA. DNA refroms double helix.

results in pre- mRNA

Question 2

Describe the three post-transcriptional modifications of pre-mRNA in eukaryotes

Answer 2

Capping
involves the addition of a methyl group to the 5’-end of the transcribed RNA
provides protection against degradation
allows the transcript to be recognised by the cell organelles such as Ribosomes

Polyadenylation

addition of a long chain of a poly-A tail to the 3’-end of the transcript
improves the stability of the RNA transcript and facilitates its export from the nucleus

Splicing
The process by which introns are removed

Question 3

Why is splicing necessary for gene transcription

What is alternative splicing?

Answer 3

Introns are intruding sequences whereas exons are expressing sequences

exons can be spliced together after the removal of introns in any order which increases the range of coded proteins (not all exons need to be spilced)

Question 4

List two major differences in gene expression between prokaryotic cells and eukaryotic cells

Answer 4

Prokaryotes: no nucleus or nuclear membrane so once mRNA has been synthesized, translation begins immediately.

Eukaryotes: pre mRNA produced needs to be prepared for translation, the process is called post-transcriptional modification of mRNA. The reasons is due to existence of introns and exons in eukaryotic genes.

Question 5

How is transcription regulated in eukaryotic DNA?

Answer 5

through transcription factors that bind to DNA during transcription before RNA polymerase

they can be enhancers (speed up transcription) or silencers (slow down trans.)

Question 6

How is transcription regulated in prokaryotic DNA?

give example

Answer 6

proteins, known as protein repressors, bind to DNA at the repressor binding region and decrease transcription.

(response to environmental factors)

example:

if lactose is present the gene with the lactase enzyme production will be expressed.

however if no lactose in present then a repressor protein will block RNA polymerase from transcibing the genes which express lactase enzyme production.

Question 7

what is a Lac operon

Answer 7

collective term for promoter, repressor and coding region in prokaryotic DNA during transcription.

Question 8

Outline histone methylation

Outline histone acetylation

Answer 8

adition of methyl group to nucleosome will generally decrease transcription as nucleosomes bind closer together

addition of an acetyl group which prevents binding between nucleosomes.
This partially unwinds the DNA, allowing the enzymes involved in transcription to access it.
increase transcription

Question 9

Define epigenetics

Answer 9

Epigenetics is the study of heritable changes in organisms that are brought about by changes in gene expression rather than by modification of the genetic code.

Question 10

List types of epigenetic tags

Answer 10

• GpG
• environmental factors: pollution, diet, temperature or stress can affect methylation

Question 11

how does methylation / acetlylation occur?

Answer 11

have a genetic basis but are influenced by environment

various factors including age, diet, environmental conditions during important developmental stages and exposure to infectious agents.