Lecture 15 Flashcards
Discovery of mRNA
DNA is in the nucleus
protein synthesis occurs in ribosomes, in the cytoplasm
ribosomes have RNA and protein
Difficulties of perspective
some proposals: DNA were 3D structural moulds for protein synthesis
ribosomes were gene-specific, specificity conferred by RNA
genetic code was not yet determined
Brenner, Jacob and Meselson
used again the T4/E coli infection system,
T4 infection means the injection of phage DNA and synthesis of phage proteins to produce new viral particles
it was known that a burst in RNA production occurred shortly after infection but what was that RNA. A structural component for new gene-specific ribosomes or messenger
They labelled all E coli ribosomes with 15N and 13C
the infected with T4 and removed 15N and 13C from the medium
so ribosomes assembled before infection were heavy, new ribosomes were light
then they added 32p to label newly synthesised RNA and saw to which ribosomes it went
new RNA goes to pre-existing ribosomes to make protein
RNA polymerase
pries DNA strands apart
reads sequence by base-pairing
uses the base uracil instead of thymine
polymerises ribonucleotides 5’–>3’
does not need a primer
is a multiprotein gene
Promoter
DNA sequence where RNA polymerised binds
it directs the initiation of transcription
can overlap a few tens of nucleotides with the transcription unit
Transcription unit
DNA sequence that is copied into RNA
Terminator
DNA sequence that signals for the RNA polymerse to detach from DNA
directs the termination of transcription
Initiation of transcription
RNA polymerase starts at the promoter
As RNA polymerase moves along the DNA it untwists the double helix 10 20 bases at a time
Elongation of transcription
5’–>3’
transcription progresses at a rate of around 60nt/s in E coli
a gene can be transcribed simultaneously by several RNA polymerase
Termination of transcription
the mechanisms of termination are different in bacteria and eukaryotes
in bacteria the polymer stops transcription at the end of the terminator region and the mRNA can be translated without further modification
splicing in eukaryotes
Regulation of gene expression
proteins fulfil many functions- some are needed all the time but some should only be used when necessary
therefore genes need to be turned on and off again
- positive and negative regulation
Brenner, Jacob and Meselson
used the T4/E coli infection system
it was known that a burst in RNA production occurred shortly after infection
they labelled all E coli ribosomes with 15N and 13C
then infected with T4 and removed 15N and 13C from the medium
so ribosomes assembled before infection were heavy but after were light
they added 32P to level newly synthesised RNA, and saw to which ribosomes went
new RNA goes to pre-existing ribosomes