BIOB 160 Principles of Living Systems · Unit 4: Molecular genetics · on Final exam
4.4 Translation and mutations
Study notes for Translation and mutations in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- The genetic code reads mRNA in codons of three bases: 64 codons, 61 for amino acids and 3 stop codons (UAA, UAG, UGA). AUG codes for methionine and is the start codon. The code is redundant and nearly universal.
- tRNA carries an amino acid and has an anticodon that pairs with a codon. Aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA.
- The ribosome (rRNA + protein) has three sites: A (the incoming aminoacyl-tRNA), P (holds the growing polypeptide) and E (exit). The rRNA catalyzes peptide bonds.
- Stages: initiation (small subunit, mRNA and initiator tRNA at AUG, then the large subunit), elongation (codon recognition, peptide bond, translocation) and termination (a release factor at a stop codon). Many ribosomes can translate one mRNA (polyribosomes).
- Point mutations: silent (same amino acid), missense (a different amino acid, as in sickle-cell disease) and nonsense (an early stop codon). Insertions or deletions not in multiples of three cause frameshifts.
Worked example
Translate the mRNA 5′-AUG GCU UGG UAA-3′. Then say what happens if the G in UGG mutates to A.
Show the solution
AUG = Met, GCU = Ala, UGG = Trp, UAA = stop: Met-Ala-Trp. Changing UGG to UAG creates a stop codon, a nonsense mutation: the protein ends after Met-Ala.
Common mistakes
- Reading codons from the 3′ end. Translation reads mRNA 5′ → 3′.
- Pairing the tRNA anticodon in the same direction as the codon. They are antiparallel.
- Thinking every base substitution changes the protein. Many are silent because the code is redundant.
Exam tipReading: Campbell ch. 17 (pp. 335–358). Practise translating short mRNAs with the codon table until you can find codons quickly.
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