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Translation
Translation is the process of converting an RNA sequence
into a protein.
The Genetic Code
- how to code for 20 amino acids with only four
different bases?
- a triplet code would allow 64 different amino
acids
- Tsugita and Frankel-Conrat used point mutations in
TMV to show that the code was nonoverlapping
- Crick used acridine mutagens (proflavin) to create
deletions and insertions
- no gaps in the code
- a specific start point
- as three insertions or deletions could produce
wild-type activity -the code must be triplet
- Nirenberg and Matthaei - in vitro translation of poly
U = polyphenylalanine
- Khorana - used poly(UC) to show that the code has an
odd number of bases (he got poly(serine-leucine))
- Nirenberg showed that a trinucleotide could bind an
aminoacyl-tRNA in vitro - he and Khorana then broke the
code
- the code is degenerate
- there is more than one codon for an amino acid
(up to six different codons for some amino acids)
except for methionine and tryptophan
- there are special start and stop codons
- AUG codes for methionine and is also the start
codon
- UAA, UAG and UGA don't code for amino acids and
are the stop codons (a special protein, releasing
factor, binds to them and stops translation)
- The complete sequence of the phage MS2 and of the
proteins coded for by the MS2 genes confimed the genetic
code
- The code is almost universal, with only a few minor
variations, particulary in mitochondria
tRNA
- tRNA is the translator
- aminoacyl synthetases puts amino acids on one end of
the tRNA
- if the amino acid is changed after it is linked to
the tRNA the new amino acid will be inserted as if it
were the original amino acid
- cysteine-tRNACys treated with nickel hydride
gives alanine-tRNACys
- proteins made with this tRNA have alanine where
cysteine should be
- each tRNA has an anticodon that can base pair with
the codon
- code is degenerate
- Crick's wobble hypothesis - the first base base of
the anticodon can form unusual base pairs (G to U for
instance), also unusual bases such as inosine can pair
with more than one nucleotide
- some amino acids have more than one tRNA
- only about 32 different tRNA's
- the aminoacyl synthetases recognize various
characteristics of the tRNA molecule
- three codons have no tRNA - the stop codons
- the stop codons are recognized by protein
releasing factors that terminate translation
- mutations that create premature stops can be
suppressed by a mutation in a tRNA that creates a new
tRNA that can recognize the stop codon
- UAG = amber, UGA = opal, UAA = ochre
- Try and answer this
question
about translation at the
Biology
Project
Translation
- Ribosomes - the site of protein synthesis
- two subparticles - 30S and 50S in prokaryotes, 40S
and 60S in eukaryotes
- 30S = 16SrRNA + 21 proteins
- 50S = 23SrRNA + 5S rRNA + 34 proteins
- 30S + 50S gives a 70S ribosome; in eukaryotes 40S
+ 60S gives an 80S ribosome
- self-assembly
- multiple ribosomes can traslate from one mRNA at the
same time (polysomes)
- initiation
- initiation factors + 30S + mRNA + GTP + fMet-tRNA
- requires ribosome binding sites on the mRNA
(Shine-Dalgarno sequence 5'-AGGAGGU-3'
- an AUG (sometimes GUG) downstream of the
Shine-Dalgarno sequence is the first codon used
- elongation
- mRNA is read 5' to 3'
- aminoacyl tRNA binds at the A site of the ribosome
- peptidyl transferase makes a peptide bond between
the amino acid at the P site and the one in the A
site, releasing the peptide from the P site
- the ribosome translocates 3 nucleotides moving the
peptide chain to the P site using one GTP for energy
- termination
- a protein release factor recognizes a stop codon
in the A site and releases the peptide chain from the
tRNA at the P site
Variations in
Transcription and Translation in Prokaryotes and
Eukaryotes
- Eukaryotic mRNA is heavily processed, unlike
prokaryotic mRNA
- the initial transcript is called hnRNA
(heterogenous RNA) because it was originally this
unknown RNA found with a large range of sizes only in
the nucleus
- the hnRNa is has sequences removed from each end
and then a 5' cap (methyl guanasine added 5' to
5') and a poly A tail (a string of about 300 As
added to the 3' end of the RNA) are added
- introns are then spliced out - various
stretches of RNA sequence are cut out of the
transcript and then the remaining exons are
spliced back together, the introns are presumabely
degraded
- the mature mRNA is then exprted to the cytoplasm
for translation
- Here is a more complete
description
of the discovery of introns, from the
"DNA from the
Beginning" site
- Try answering these quesions about eukaryotic
transcription from the
Biology
Project:
question1,
question2,
question3,
question4,
question5,
question6,
question7
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PROCARYOTIC
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EUCARYOTIC
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Promoter
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Pribnow's
Box
TATAATG
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TATA Box
TATAAATA
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RNA
Polymerase
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One Type
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Three
Types
Pol I -
rRNA
Pol II - mRNA
Pol III - tRNA
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Product of
Transcription
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mRNA, no cap or
tail
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hnRNA which is
modified with Methyl Guanosine Cap & Poly A
Tail
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Introns
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Very
rare
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Very
common
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Translation
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Starts while
the mRNA is still being synthesized (no
nucleus)
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Only in the
cytoplasm on processed mRNAs
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Ribosomes
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70S
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80S
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Start
Codon
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AUG
(f-met)
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AUG
(met)
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Initiation
Sequences
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Shine-Delgarno
sequences
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Weak consensus
sequence
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mRNA
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Polycistronic
possible
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Monocistronic
need cap for initiation
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Origin of Life
- Conserved code argues strongly for one origin
- DNA replication, transcription and translation are
too complicated
- Current hypothesis: an RNA origin
This document is maintained by: Jeff Bell
Last Update: Friday, March 1, 2002 |