B: Transcription

Question 1

Explain how RNA polymerase synthesises RNA

Answer 1

1. RNA polymerase starts at promotor site and transcription starts at +1 site.
2. RNA polymerase either binds directly or indirectly to the promotor.
3. RNA polymerase recruits RNA precursors - NTPs.
4. RNA polymerase can synthesise RNA by using DNA as template
5. RNA polymerase synthesises RNA from 5’ to 3’ direction.
6. RNA polymerase catalyses the formation of 5’-3’ phosphodiester bonds between phosphate on 5’ carbon on NTP and 3’ OH on another nucleotide and released pyrophosphate.

Question 2

Explain how gene is transcribed into mRNA

Answer 2

• RNA polymerase initiates transcription by producing RNA
• Introns are removed by RNA splicing, and exons are rejoined in various ways to make different versions of mRNA to code for different versions of the protein.
• Strand is cleaved at polyA adenylation site on 3’ end (AAUAAA)
• Enzyme PolyA polymerase adds hundreds of adenines (polyA tail)
• 7 methyl guanine is added to 5’ end via 5’ to 5’ triphosphate bond –> mRNA is capped

Question 3

What is the +1 site

Answer 3

Position on DNA where RNA polymerase begins to synthesise RNA

Question 4

Function of CAP

Answer 4

Translation initiation, protection, transport, splicing

Question 5

Function of polyA tail

Answer 5

Transport, protection + termination.

Question 6

Difference between mRNA in euk and prok?

Answer 6

Euk - splicing, cap, polyA, not coupled.
Prok - Ribosome binding site, coupled.

Question 7

mRNA cap

Answer 7

7-methylguanine added to 5’ end of mRNA via 5’-5’ triphosphate linkage in eukaryotic cells.
Protects mRNA against degradation by 5’ exonucleases, and is a ribosome binding site, so initiates translation.

Question 8

PolyA tail

Answer 8

10-30 bases down from polyA adenylation cleavage site (AAUAAA) polyA polymerase adds polyA tail (hundreds of adenines).