Biology 1002B Lecture Notes - Lecture 12: Release Factor, Start Codon, Ribosomal Rna

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Lecture 12: Prokaryotic Gene Function
1. Identify the sequence of standard "start" and "stop" codons
Start: nontemplate 5’ AUG 3’; template 3’ TAC 5’ (RNA understood)
Stop: non template 5’ UAA, UAG, UGA 3’; template 3’ ATT, ATC, ACT 5’
Eukaryotic Gene Expression
Prokaryotic Gene Expression
More genes
Less genes
Genes more spread out on many chromosomes
Genes located on one chromosome
Groups of genes producing proteins with related
functions are NOT organized into operons
Groups of genes producing proteins with related
functions are organized into operons
mRNA that must have its introns excised
Lack introns
mRNA kept away from ribosomes
mRNA transported out of nucleus to ribosomes
5’ cap, poly-A tail added before going to
translation (post translational)
Transcription and translation can occur
simultaneously since ribosomes are readily there
mRNA doesn’t have a nucleus or nuclear
membrane protecting genes (but not floating)
mRNA tends to be stable, used multiple times
Efficient, but prevents eukaryotes to make fast
response changes to environmental disruptions
mRNA of prokaryotes is unstable, allow creation
of new mRNA, which has more opportunities to
adjust for changing environmental conditions
Synthesis of protein is normally off until activated
Allow for continual synthesis of protein to occur
Gene-regulating before & after transcription in
nucleus, in cytoplasm before & after translation
2. DNA sequence “signals” as promoter, 5’ & 3’ UTR, SD box, start & stop codon, transcription terminator etc.
Promoters attract the attention of RNA polymerase
o RNA polymerase and DNA interaction create bubble
o -10 sequence upstream from start point (+1), TATA promoter (euk promoter is -35)
o Start point where transcription begins reading template strand and is downstream of bubble
o Change coding strand to be called non template
o 3’ end = downstream; 5’ end = upstream
-35 bases upstream (TTGACA)
Black arrows = where polymerase touches DNA
Green/purple = sites of mutations affect promoter efficiency
Start codon
o Signal for translation not transcription
o Universal genetic code (same in pro and euks)
o Not necessarily the first 3 bases because lots of UTR
terminator is after/downstream stop codon
o gets transcribed from DNA into RNA (signal in DNA but
understood in RNA)
o makes hair pin structure by pairing with itself to signal polymerase to stop
translation initiation is stabilized by mRNA/rRNA base pairing
o 5’ UTR (untranslated region since upstream of mRNA start codon) but transcribed
UTR in ribosome is facing up and pair with rRNA since no tRNA will pair with them
rRNA SD box (DNA transcribed, understood in mRNA) pair with UTR (mRNA)
SD box helps initiating translation in bacteria
SD box is an example of complementary base pairing of rRNA and mRNA
more than one polymerase transcribing and translating at the same time on the gene
o translated before it is finished transcription
o as soon as SD box is transcribed, ribosomes start transcription once there is mRNA
o the ribosome that has translated the longest is the one that has its mRNA translated first
o rRNA polymerase travels from 5’ to 3’ on template
o ribosomes travels 5’ to 3’ starting on mRNA
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Document Summary

Identify the sequence of standard start and stop codons. Start: nontemplate 5" aug 3"; template 3" tac 5" (rna understood) Stop: non template 5" uaa, uag, uga 3"; template 3" att, atc, act 5". Genes more spread out on many chromosomes. Groups of genes producing proteins with related. Groups of genes producing proteins with related functions are not organized into operons. Mrna that must have its introns excised. Mrna transported out of nucleus to ribosomes. 5" cap, poly-a tail added before going to translation (post translational) functions are organized into operons. Transcription and translation can occur simultaneously since ribosomes are readily there. Mrna doesn"t have a nucleus or nuclear membrane protecting genes (but not floating) Mrna tends to be stable, used multiple times. Efficient, but prevents eukaryotes to make fast response changes to environmental disruptions. Mrna of prokaryotes is unstable, allow creation of new mrna, which has more opportunities to adjust for changing environmental conditions.

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