Central dogma
DNA can be copied multiple times into messenger RNA molecules or mRNA. These mRNA molecules can then be translated, from the nucleotide language of the mRNA into the amino acid language of the protein
Transcription
produces an mRNA with a nucleotide sequence complementary to one of the DNA strands
Gene expression in prokaryotes
Initiation, elongation, termination. Transcription and translation occur simultaneously.
Gene expression in eukaryotes
initiation, elongation, termination, mRNA processing, RNA export. Transcription and translation occur seperately
RNA polymerase
reads the DNA template and synthesizes a complementary RNA strand
- makes RNA in the 5’ to 3’ direction
- globular protein, active site, phosphodiester bonds
non template strand
coding strand, has thymine instead of uracil
Sigma
E coli protein, binds to RNA polymerase before transcription, makes holoenzyme, binds to DNA at promoters
Consensus sequences
the most common nucleotides found at a specific DNA or RNA location
2 parts of the promoter region
- 10 region: TATAAT
- 35 region: TTGCCA
What happens when Sigma binds to the promoter?
- DNA double helix opens
- Template strand threaded through channel in RNA p-ase to active sight
- Ribonucleotide triphosphates enter another channel to more to active sight and incorporated into mRNA
RNA polymerase moves
3’ to 5’ direction
as RNA polymerase moves
- it unwinds the double helix
- reads the DNA template and brings in the complementary nucleotide
- catalyzes the addition of the nucleotide at 3’ end
- inserts U where there is A
Rho dependent termination
Occurs by binding of Rho to ribosome-free mRNA
Rho independent termination
formation of the stem-loop structure, which is facilitated by RNA polymerase pausing during transcription of the T-rich tract
Transcription in eukaryotes
- no sigma
- recognize consensus sequences
- introns (not translated) and exons (translated)
- 5’ cap, poly(A) tail
RNA pol I
transcribes genes that code for ribosomal RNAs (most rRNAs)
RNA pol II
transcribes genes that code for proteins (mRNA synthesis)
RNA pol III
transcribe genes that code for tRNAs and other small RNAs
Why do introns exist/intron splicing
allowing new combinations of exons to be created
5’ cap
- 7-methylguanosine attached to mRNA 5’ to 5’ triphosphate linkage
- recognition signal for translational machinery and protects transcript
3’ poly a tail
- 150-200 adenines added to 3’ end of mRNA
- protects the transcript from degradation
