Ribonucleic acid RNA:

1
Transcription makes RNA
Ribonucleic acid (RNA)

• Linear polymer of ribonucleotides
• Similar structure to DNA - BUT
• Sugar group is ribose, not
• deoxyribose (-OH not -H on 2 carbon)
• Thymine is replaced by uracil
• DNA ATCG
• RNA UAGC
• RNA is largely single stranded

2
THREE FORMS OF RNA

• mRNA (messenger RNA)
• encodes for proteins
• rRNA (ribosomal RNA)
• forms part of the ribosome
• structure
• tRNA (transfer RNA)
• adapter for protein synthesis

3
Mechanism of RNA synthesis
Non-template strand
3
5
DNA
C C T A G
Template strand
3
5
RNA
4
REPLICATION TRANSCRIPTION Entire
Selected chromosome segments Requires No
requirement primers for primers Both
strands Only one strand copied is copied
(within a gene) Polymerase has Polymerase
has proof-reading no proof-reading
activity activity
5
PROMOTERS DIRECT START OF TRANSCRIPTION

• Much of the DNA is not
• transcribed into RNA
• Initiation of RNA synthesis
• does not occur at random
• sequences of DNA
• RNA polymerase binds
• specific sequences of DNA
• (PROMOTERS) and directs
• transcription of adjacent genes

6
Enzyme responsible for transcription
is DNA-dependent RNA polymerase
Prokaryotes 5 subunits (core enzyme) plus a
sigma factor. Transcribes all forms of
RNA Eukaryotes 3 different polymerases RNA
pol I synthesises rRNA RNA pol II synthesises
mRNA RNA pol III synthesises tRNA
7
4 STAGES OF TRANSCRIPTION

• Template recognition
• Initiation
• Elongation
• Termination

8
1. Template recognition
RNA polymerase

• RNA polymerase binds to duplex DNA
• Formation of closed promoter complex
• RNA polymerase contacts -35 region and -10 region
• Direct contact between s-factor and -35 and -10
  regions
• s-factor has domains that recognise the promoter
• Change from closed to open complex results in
  unwinding approx. 12bp from the -10 region to the
  start site

9
2. Initiation Formation of open promoter complex
RNA polymerase

• DNA is unwound at the
• promoter forming
• TRANSCRIPTION BUBBLE

10
3. Elongation
5

• RNA polymerase moves
• RNA is synthesised by base
• pairing with one strand
• of DNA
• Unwound region moves with
• polymerase

11
4. Termination
DNA
RNA transcript
RNA polymerase

• RNA and RNA polymerase
• dissociate at the terminator
• DNA duplex reforms