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Translation

Biochem and medical genetics

QuestionAnswer
How is DNA translated to protein A two stage process The nucleotide sequence in the DNA molecule is transcribed into an RNA intermediate The RNA intermediate is the translated into protein
How does mRNA act as a template Each three nucleic acids is a codon Each codon codes for one amino acid The instruction in the RNA needs to be decoded - role of tRNA
Genetic code Code is redundant (multiple codons code for the same amino acid) but unambiguous (a codon always codes for the same amino acid)
Start codon AUG specifies methionine and is also the start codon from which translation begins All polypeptides begin with methionine
Stop codons UAA, UAG and UGA specify stop codons that are not recognised by tRNA molecules but by proteins instead These proteins force the termination of translation
Open reading frame The genetic code is read in non-overlapping triplets from a fixed starting point - start codon AUG An open reading frame refers to a continued region from the start codon that contains exclusively triplets that encode for amino acids
Nonsense mutations A stop codon is created. This causes the ribosome to stop translation at the wrong place and a shortened protein with aberrant function/regulation is formed E.g. Beta-thalassemia change of UGG to UGA (stop codon)
Missense mutations An amino acid is replaced This may have consequences for the structure and function of the protein Can cause no harm at all so are silent E.g. Sickle cell disease change of GAG (Glu) to GUG (Val)
Frame shift mutations If a number of nucleotides in an open reading from are deleted/inserted and this number is not a multiple of three, the reading frame from that point onwards will change and can result in incorporation of incorrect amino acids
What is required for translation tRNA - physical representation of the 1:1 correspondence between nucleic acid and protein structure Ribosomes Translation factors (initiation, elongation and termination) Phosphorylation of some factors Energy from GTP
tRNAs Strands of RNA which base pair to form 3 stems each with a loop Anticodon binds to the mRNA codon 3' end is CCA - binds to specific amino acid Assembled by specific aminoacyl tRNA synthetases
tRNA synthetases Associate with their specific amino acid and its tRNA This is highly specific and proof-reading ensures accuracy of this process Contain two highly specific pockets - correct tRNA can be charged with its amino acid Specific forms for each amino acid
How does decoding work The tRNA recognises its corresponding codon in the mRNA via base pairing between nucleotides in the anticodon and the codon This occurs in ribosomes
Wobble bases There should be 61 tRNAs, but most organisms only have up to 45 Some tRNA must be able to recognise more than one codon These usually have a different 3rd base. Base pairing at this base is less strict do it binds to multiple bases
Structure of the ribosome Consist of protein and RNA - the catalytic component Ribosomal subunits assembled in nucleolus and exported Total 80s made of large 60s subunit and small 40s subunit
60s ribosomal subunit Contains three sites to accommodate tRNA molecules E site - exit P site - peptidyl A site - aminoacyl
Process of translation The P site is occupied by methionine initiator tRNA A site is lined up with next codon to accommodate tRNA When correct tRNA enters A site peptidyl transferase catalyses peptide bond formation Ribosome shifts by one codon releasing initiation tRNA
Creating a peptide bond Formation between amino acids in the P and A site is catalysed in the peptidyl transferase activity residing in the large subunit of the ribosome This triggers a nucleophilic attack to form a peptide bond and release the tRNA
Translation cycle Ribosome binds to mRNA Moves 3 nucleotides at a time - gives directionality to the protein The stop codon is not recognised by tRNA - recognised by a release factor that triggers cleavage of the polypeptide to release it and disassembles the ribosome
Role of cap and poly(A) tail Cap binding proteins and proteins on the poly(A) tail interact with each other and create a circular mRNA molecule This recruits the ribosome to mRNA
Role specifically of the cap The cap structure is critical for the initiation of translation The small ribosomal subunit is recruited to the cap and scans the mRNA for the AUG start codon Once the start codon is identified the large ribosomal subunit joins and translation starts
Role of 4E proteins - regulation of translation These can bind to a cap protein and block recruitment of the small ribosomal subunit Phosphorylation of these proteins in response to insulin prevents its interaction with the cap - ribosome can be recruited This gives a rapid change in protein making
Role of UTRs - iron metabolism In low FE iron responsive proteins bind to 5' UTR of ferratin mRNA preventing its translation In high FE these bind to FE so do not bind to ferratin, allowing it to be translated These are therefore key regulatory sequences
How can protein expression be regulated Transcriptional control Transcription factors Co-transcriptional pre-mRNA processing - alternative splicing mRNA export and localisation Translation initiation Degrading of mRNA and half life modification
Importance of difference between eukaryotes and prokaryotes Antibiotics exploit subtle differences between translation in prokaryotes and eukaryotes This can inhibit bacterial transcription (rifamycin) or translation (tetracycline, erythromycin, streptomycin etc) without affecting human processes
Mode of action of some translation inhibitors Aminoglycosides - change 30s subunit shape for mRNA is misread Tetracycline - blocks ribosome docking site of tRNA Chloramphenicol - inhibits peptide bond formation Macrolide - binds 50s subunit and prevents mRNA moving through ribosome
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