L9 - Prokaryotic Transcription

Question 1

Initiation:

Answer 1

RNA polymerase is directed to start site of transcription on DNA. Promotor region has two motifs which let polymerase know where to bind. -10 and -35. Pribnow box (-10 upstream TATAAT) (-35 upstream TTGACA). Sigma factor recognises initiation site and bind to anchor sequence with polymerase. Polymerase can start transcription and sigma factor then removed.

Question 2

Elongation:

Answer 2

Template strand of DNA is read to synthesise mRNA. Polymerase cause DNA to separate forming transcriptional bubble. Makes template strand accessible. Sigma factor detaches and polymerase move down gene. Add nucleotide at 3’ end by making phosphodiester bond.

Question 3

Termination:

Answer 3

Stop sequence (not codon) controls termination of transcription. These are basically G-C rich regions where mRNA strand forms a hairpin loop. Polymerase recognises this and removes mRNA. 
Termination may require protein, rho. This has helicase activity so separate mRNA from RNA polymerase. Coupled with ATP hydrolysis.

Question 4

Eukaryotic transcription:

Answer 4

Has more complexes:

1. More polymerase
2. More initiation factors
3. More elongation factors
4. Other helper proteins

Prokaryotes only have sigma factor whereas eukaryotes have more types of iniation factors.
Have additional mRNA processing. Genes are split as have exons and introns.
Exons are regions of DNA within a gene that is found in mature mRNA and in proteins so is expressed.
Introns are regions of DNA that are not found in mature mRNA. Don’t code for any amino acids and must be taken out before synthesis of proteins.

Question 5

mRNA processing of eukaryotic transcription:

Post-transcriptional:

Answer 5

1. Capping
   Form 7-methylguanosine cap at 5’ end at start of mRNA. Important for translation. Ribosome can find start of sequence.
2. Splicing
   Removes introns (non-coding regions). AAAAAAA at end so enzyme know where to cut. Uses spliceosomes. Exons separated and fitted together for protein synthesis.
3. Polyadenylation
   Addition of many adenines (AAAAAA) to 3’ end of mRNA. More stable with this. Nuclease remove RNA so would remove A tail and so protects original strand and only polyadenyl chain is disrupted.