3 MAJOR Types OF RNA
1) tRNA (transfer RNA, used in translation)
2) rRNA (ribosomal RNA, part of ribosomes)
3) mRNA (messenger RNA, codes for proteins)
Codons
A series of non overlapping three letter codes made of the 4 nucleotide bases
As in DNA replication, RNA is synthesized in a
5’ —> 3’, antiparallel direction
Codons are read in a
5’-3’ direction
Redundancy in codons
Some amino acid are specified by more then one codon
AUG has a dual function:
1) codes for methionine
2) START translation
UAA, UGA, UAG
Don’t specify an amino acid, but STOPs translation
Transcription is done by an enzyme called
RNA polymerase (RNA pol or RNAP)
RNA pol
- adds nucleotides (5’—>3’)
- doesn’t need primer
- catalyzes phosphodiester bonds
Bacteria and archaea have one RNA pol for all
Transcription
Eukaryotes have 3 classes of RNA pol :
1) RNA pol l —> synthesized rRNA
2) RNA pol ll —> synthesizes mRNA
3) RNA pol lll —> synthesizes tRNA
RNA pol initially binds to specific DNA sequence called a
Promoter
Promoter
- upstream of transcription
- provides a target for other proteins to bind to, which then recruit RNA pol in the correct orientation
- directs the timing and frequency of initiation
Initiation
The recruitment of RNA pol, unwinding DNA helix, and starting transcription
Do RNA require primer to start synthesizing RNA
NO
What’s the rate-limiting step in gene expression ?
Initiation
Bacteria promoter sequence
- pribnow box (-10 Element) TATAAT
- -35 element
TTGACA
Eukaryote and archaea promoter sequence
Contains a TATA box (TATAAA), upstream of transcription start site
RNA pol needs help to bind to the promoter sequence in bacteria
- protein called SIGMA binds directly to -10 and -35 elements
- one transcription begins, SIGMA dissociates
RNA pol needs help to bind to the promoter sequence in eukaryotes
- non of the 3 classes of RNA pols can bind promoters on their own
- several proteins called general transcription factors (GTFs) bind to promoter site, helping recruiting RNA pol to promoter
- as transcription beings, some GTFs dissociate and are replaced by proteins required for elongation
