dna transcription process

transcription process

what is the relationship between mRNA strand and the coding and template strand

mRNA is identical to the coding strand

mRNA is complementary to the template strand

what direction is RNA synthesised in?

5’ —> 3’

what direction does RNA polymerase move?

3’ —> 5’

what are the three main stages of DNA transcription

1. INITIATION (steps 1-3)

2. ELONGATION (step 4)

3. TERMINATION (step 5)

step 1 (initiation)

• RNA polymerase and initiation factors recognise and bind to promoter

• “closed complex”

step 2 (initiation)

• RNA polymerase and initiation factors separate DNA strands

• bases in template strand are made available for base pairing with incoming rNTPs

• “open complex”

step 3 (initiation)

• 12-14 base pairs around start site are separated forming transcription bubble

• template strand enters active site of RNA polymerase

• active site catalyses phosphodiester bond formation between rNTPs

• initiation is considered complete once two rNTPs have been joined together

step 4 (elongation)

• RNA polymerase dissociates from promoter region and initiation factors

• RNA polymerase moves along template strand opening DNA strands in front of its direction of movement

• polymerase maintains a melted region of 14-20 nucleotides

• once rNTPs have been joined, polymerase guides DNA strands back together and they reassociate

• “elongation complex”

step 5 (termination)

• completed RNA molecule is released from RNA polymerase

• polymerase dissociates from DNA

how are rNTPs added?

• nucleophilic attack from 3’ oxygen (RNA chain) on alpha phosphate of incoming rNTP

• phosphodiester bond and pyrophosphate molecule produced