Transcription + RNA splicing

Question 1

why doesn’t transcription have to go to completion for translation to occur in prokaryotes?

Answer 1

because there is no compartmentalisation in prokaryotes. must go to completion in eukaryotes for this reason. compartmentalisation = arrangement of genes into chromatin

Question 2

what is the general function of transcription

Answer 2

make an mRNA copy of the information in a gene that codes for proteins

Question 3

what three sections can be found in a gene

Answer 3

promoter, terminator and regulatory sequences

Question 4

describe the structure of promoters

Answer 4

these are not all identical, but they all tend to have a position -35 (TTGACG)and Pribnow box position -10 (TATAAT) right before the transcription start site. The promoter ability is stronger when it resembles these sequences closely

Question 5

what enzyme catalyses Transcription

Answer 5

RNA polymerase

Question 6

describe the structure of RNA polymerase

Answer 6

multi subunit enzyme B’, B, sigma , 2alpha, omega (5 subunits)

• alpha= the two subunits assemble enzyme and bind the regulatory factors
• B = has polymerase activity including chain initiation and elongation
• B’ binds to DNA nonspecifically
• w (omega)= restores denatured RNA polymerase to a functional form in vitro
• sigma = reduces affinity of enzyme for non-specific DNA, increases affinity for promoter sites by directing the enzyme where to go. dissociates from core enzyme when a promoter has been bound

Question 7

what are the three RNA polymerases in eukaryotes

Answer 7

I, II and III
most important is RNA polymerase II is most important + studied

Question 8

how does transcription initiation occur

Answer 8

RNA polymerase does not recognise the core promoter sequences, so transcription factors mediate the binding of RNA polymerase to form a transcription initiation

Question 9

what is the polyemerase doing to the DNA double helix during elongation in transcription

Answer 9

it unwinds the template DNA ahead of it, and rewinds the DNA behind it. there is around 17 base pairs in the unwound region

Question 10

how does transcription termination occur in eukaryotes

Answer 10

not very well understood but it occurs with polyadenylation of the transcripts

Question 11

how does termination of transcription occur in bacteria

Answer 11

two types
- rho-dependent
- rho-independent

both lead to release of mRNA transcript

Question 12

describe rho dependent termination

Answer 12

where a protein, rho, terminates transcription. it binds to the RNA strand and uses helicase activity to release the mRNA from enzyme complex

Question 13

describe rho independent termination

Answer 13

transcription is terminated due to a specific sequence in terminator DNA. the terminator DNA has a GC rich area which causes a hairpin structure to form. after this, there is an AU sequence which is weakly bonded together, and with the hairpin causes transcription to stall and for the instability to be enough to break away the mRNA

Question 14

what are the coding and template strands

Answer 14

TEMPLATE STRAND
- runs in 3’ to 5’ direction,
- is the template for transcription, so the RNA sequence that is synthesised is synthesised against this strand

CODING STRAND
- this strand runs in 5’ to 3’
- has the same nucleotide sequence as the mRNA strand

Question 15

what is RNA splicing

Answer 15

eukaryotic trc makes a longer mRNA called preRNA which has to be made into mature mRNA.

introns (non coding regions) and exons are both transcribed but introns are not translated

splicing is the removal of introns and the joining of the exons

Question 16

what is a splicesome

Answer 16

a complex of snRNPS (small nuclear RNA and protein) which precisely removes introns from the pre-RNA

Question 17

what are the intron-exon boundaries

Answer 17

5’ splice site (GU) a branch site, and 3’ splice site (AG)

Question 18

what is RNA capping

Answer 18

covalent attachement of a modified guanosine to the 5’ end of the mRNA

Question 19

when doe RNA capping occur

Answer 19

while transcription occurs after the first 20-25nt have been transcribed

Question 20

what are the functions of RNA cappings

Answer 20

prevents degradation from 5’ end by nucleases
allows proper exiting of mRNA from nucleus to cytoplasm
CAP structure enables ribosome binding to mRNA

Question 21

what is the PolyA tail function

Answer 21

gives stability and is good for RNA deciphering

can be found at the 3’ end of most mature mRNA - consists of 100-300 adenine residues