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Birth of proteins by translation
Reading: Any of the biochemistry or Molecular biology
texts
Mol. Bio. of the Cell by Alberts et al (4 e) – Chapter 6
Molecular Cell Biol. by Lodish et al (5 e) – Chapter 4.3 – 4.5
Biochemistry by Voet and Voet (2 e) – Chapter 30
Key concepts in translation
Genetic information transcribed from DNA to mRNA as a nonoverlapping, degenerate triplet code
1 codon = 1 amino acid but 1 amino acid > 1 codon
2 key molecules responsible for decoding nucleotide sequence into amino acid sequence are tRNAs and aminoacyl-tRNA synthetases
3 base anticodon in tRNA allows base-pairing with corresponding sequence in mRNA
20 specific aminoacyl-tRNA synthetases present
Both pro and eukaryotic ribosomes have a large and small subunit
What is translation?
mRNA directed synthesis of polypeptides
Translates DNA sequence information into proteins
Genetic code dictates translation of specific RNA triplet codons to amino acids
Occurs in the cytosol
Genetic code
Triplet code Degenerate –
more than 1 triplet may encode same amino acid
Non overlappingE.g AUGCGTACT
Start codon mainly AUG (rarely GUG)
Stop codons are UAG, UGA, UAA
Exceptions!
UGA Stop Trp mycoplasma,
mitochondria (some spp)
CUG Leu Thr Yeast mitochondria
UAA, UAG Stop Gln Paramoecium,
Tetrahymena etc
CODON UNIV UNUSUALORGANISM
CODE CODE
Open Reading frames (ORF)Uninterrupted sequence of codons in mRNA
(from start to stop codon) that is translated into amino acids in a polypeptide chain
MAN CAN FLY- correct sequence
DAN CAN FLY – substitutionDAC ANF LY - frameshift
mutation
Mutations
Deletions or Insertions: 1bp to several MbpSingle base substitutions
Missense mutations: replace one amino acid codon with another
Nonsense mutations: replace amino acid codon with stop codonSplice site mutations: create or remove
exon-intron boundaries Frameshift mutations: alter the ORF due to base substitutionsDynamic mutations: changes in the length of tandem repeat elements
Main classes of mutations
Translation requires…..
1) mRNA
2) Aminoacyl- transfer RNA (aatRNA)3) Ribosomes
1) Messenger RNA (mRNA)
This class of RNAs are the genetic coding templates used by the translational machinery to determine the order of amino acids incorporated into an elongating polypeptide in the process of translation.
2) Transfer RNA (tRNA)class of small RNAs which form covalent bonds to
amino acids allows correct insertion of amino acids into the
elongating polypeptide chain.
Adaptor hypothesis
tRNA acts as a ‘shuttle’ linking amino acid to nucleic aid
Aligns correct amino acids to form a polypeptide
One tRNA per amino acid
Translation has 2 important recognition steps
Correct aminoacylation (‘charging’): Covalently attach the correct amino acid to tRNA (specified by anticodon)
Select the correct charged tRNA as specified by mRNA
1
2
How does the aaRSs select the right tRNA to be acylated especially since most tRNAs are structurally similar?
By recognising specific tRNA
identifiers present on the
acceptor step & anticodon
loop
e.g. AlaRSs recognise G3.U70
bp on the acceptor stem
2Select the correct charged tRNA as specified by mRNA
Less than 61 tRNAs found in cells
Ribosomes select aa-tRNA based only on their codon –anticodon interactions
This pairing is antiparallel and the base in the third position forms non standard base pairing (Wobble hypothesis)
tRNA anticodon 3’-A A G-5’ or 3’-A A G-5’mRNA codon 5’-U U C-3’ 5’-U U U-3’
Question?When does translation of the genetic code actually occur, during the charging of the tRNA synthetases or during the binding of tRNAs to the ribosome?
3) Ribosomes
Ribosomal RNA (rRNA) assembled, together with numerous ribosomal proteins, to form the ribosomes.
Ribosomes engage the mRNAs and form a catalytic domain into which the tRNAs enter with their attached amino acids. The proteins of the ribosomes catalyze all of the functions of polypeptide synthesis
Ribosomes
Ribosomes
Made of rRNA & ribosomal proteins2 subunits – large - small
Subunits are self assemblingcombine only in the presence of mRNA and a
charged (aminoacylated) tRNA
What is the structure of the What is the structure of the ribosome?ribosome?
A, P & E sitesA, P & E sites
Polypeptide synthesis (overview)
3 distinct steps
1. Chain initiation2. Chain elongation3. Chain termination
Initiation in eukaryotesStep 1 : Formation of pre-initiation complex 40S-eIF3 bound by eIF1A to a ternary complex of
tRNAimet, eIF2 and GTP
Fig 4-26 MCB by Lodish et al)
InitiationStep 2: Formation of initiation complex
(cap binding of mRNA to 40S)
InitiationStep 3: positioning at start codon – initiation complex unwinds mRNA using eIF4
helicaseInitiation complex stops at the start site AUGThis recognition allows an irreversible GTP
hydrolysis of eIF2 preventing any further unwinding
Kozak sequenceACCAUGG
InitiationStep 4: Association of large subunit (60S) Irreversible GTP hydrolysis mediates the
association of 60S-eIF6 (large subunit ) to the small subunit by the action of eIF5
This becomes the P site
Initiation in eukaryotes
Fig 4-25 MCB by Lodish et al)
Chain elongation
4 stage reaction cycle1) aatRNA binding
aatRNA binds to A site on ribosome by base pairing with codon
2) Conformation change in ribosome: induced by GTP hydrolysis of EF1
3) TranspeptidationC terminal of polypeptide uncoupled from P site tRNA and peptide bond transferred to amino acid on A site tRNAcatalysed by peptidyltransferase
4) TranslocationGTP hydrolysis of EF2 causes 2nd conformational change P site tRNA is transferred to E siteSimultaneous transfer of A site tRNA moved to P site
How does elongation proceed?How does elongation proceed?
Which end of the protein is produced first?Which end of the protein is produced first?
Chain elongation
4 Steps
Step 1 : aatRNA binding
Step 2: conformational change
Step 3 : Transpeptidation
Step 4 : Translocation
termination
Release factors (eRFs) recognise and bind to stop codons
This induces peptidyl transferase to transfer peptidyl group to water instead of aatRNA
Uncharged tRNA released from ribosome
Inactive ribosome then release mRNA
Typically the entire process takes 30-60sec!!
Antibiotics that inhibit translation
Only prokaryotes Streptomycin prevents initiation-
elongation Chloramphenicol blocks peptidyltransferase
Only eukaryotes Cycloheximide blocks translocation
Both Puromycin causes premature release of
polypeptide
During the process of translation:
A. the peptide is ‘passed’ from the tRNA in the P- site to the tRNA in the A-site.B. incoming tRNAs must first bind to the E-site.C. initiation begins with the binding of the ribosomal SSU to the poly-A tail of the mRNA.D. the mRNA is translated by one ribosome at a time.
Translation begins:
A. at the “start codon” of the mRNA.B. at the 3’ end of the mRNA.C. at the promoter site on the mRNA.D. in the nucleolus of the nucleus.
During the ‘elongation’ stage of translation, after the arrival of each new tRNA
A. the amino acid is ‘passed’ from the tRNA in the A-site to the tRNA in the P-site.
B. newly arriving tRNAs must first bind to the E-site.
C. the peptide is ‘passed’ from the tRNA in the P-site to the tRNA in the A-site.
D. the new tRNA must first bind to the P-site of the ribosome.
Initiation of translation begins when
A. the large and small subunits link together, then bind to the mRNA.
B. the ribosomal small subunit binds to the 5’ end of the mRNA.
C. after ribosome binds to the start codon and an initiator tRNA enters the ribosome.
D. the initiator tRNA binds to the start codon, followed by binding of the ribosomal small subunit.