28. Transcription

Question 1

What are the 5 components of a gene that punctuate it?

Answer 1

1. Regulatory sequence
2. Promotor
3. Coding sequence
4. Terminator
5. Untranslated sequence

Question 2

What are the three broad processes of transcription?

Answer 2

1. Initiation - start where RNA polymerisation binds
2. Elongation - generation of mRNA, RNA polymerase moves along
3. Termination - stop of transcription

Question 3

What are the differences in the number of polymerase prokaryotes and eukaryotes?

Answer 3

• Prokaryotes have one RNA polymerase

- Eukaryotes have 3 different RNA polymerises which transcribe different types of RNA

Question 4

Which is the main RNA polymerase for mRNA?

Answer 4

RNA polymerase II

Question 5

How does RNA polymerase know where to bind to start transcription?

Answer 5

RNA polymerase looks for consensus sequences in the promotor region. Consensus sequences are highly conserved in different genes

Question 6

What does the prokaryote RNA polymerase consist of in its subunits?

Answer 6

• 5 subunits and a detachable sigma factor

- Core enzyme + sigma factor = holoenzyme

Question 7

What is the role of the sigma factor in prokaryote RNA polymerase?

Answer 7

• Sigma factor binds to the promotor = closed complex
• RNA polymerase switches to open complex and melts the double strands of DNA to form the transcription bubble
• Sigma factor then dissociates which is required for RNA polymerase to escape the promotor

Question 8

What is the role of the alpha subunit in prokaryotic RNA polymerase?

Answer 8

It determines the DNA to be transcribed

Question 9

What does the beta and beta’ unit in prokaryotic RNA polymerase do?

Answer 9

Beta catalyses polymerisation and Beta’ binds and opens the DNA template

Question 10

What are the four roles of general transcription factors in the eukaryotic promotor binding process?

Answer 10

1. Recognise the consensus sequence
2. Bind to the DNA in a particular sequence
3. Recruit RNA polymerase
4. Unwind the DNA (helicase activity)

Question 11

What are the specific transcription factors needed for in eukaryotic promotor binding?

Answer 11

Specific transcription factors are required for binding and important for differential gene expression. They can be proximal or distal

Question 12

When transcription is occurring, how are the two DNA strands named?

Answer 12

• DNA coding strand is 5’ to 3’

- DNA template strand is the one used to generate the complementary mRNA sequence and is 3’ to 5’

Question 13

Which DNA strand acts as the template during transcription?

Answer 13

Either DNA strand can act as the template

- This differs between genes but is always the same for a given gene

Question 14

Which direction does RNA transcription occur in?

Answer 14

5’ to 3’ direction

Question 15

How does RNA polymerase access the DNA to keep unwinding it during transcription?

Answer 15

• The transcription bubble describes the unwound DNA base pairs
• DNA that has already been transcribed rewinds behind the bubble

Question 16

In prokaryotes how is transcription terminated?

Answer 16

1. RNA polymerase encounters the chain termination sequence
2. The self complementary RNA forms a hairpin (stem and loop structure)
3. RNA and RNA polymerase dissociate from the DNA

Question 17

In eukaryotes how is transcription terminated?

Answer 17

For RNA polymerase II

1. Polymerase passes the termination sequence
2. RNA is cleaved by the cleavage complex

Question 18

What happen in the elongation part of transcription, within the transcription bubble?

Answer 18

• Using unwound DNA template strand to add mRNA building blocks on at a time
• Inside the bubble individual RNA nucleoside triphosphate (NTP) are added