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4.2.2Additional lessons

Translation in Protein Synthesis — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 103 of 142

📚 IB Biology SL Hub / Topical Revision
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Section 1 of 5Components of Translation

Translation takes place in the cytoplasm on ribosomes, and the amino acid sequence is determined by the mRNA codons.

Ribosome
  • Ribosome Made of rRNA and protein with a large and a small subunit, it holds mRNA and two tRNAs and catalyses peptide bonds.
mRNA
  • mRNA Carries the sequence of codons that determines the order of amino acids in the polypeptide.
tRNA
  • tRNA A folded RNA molecule with an anticodon at one end and a specific amino acid attached at the other.
Codon
  • Codon A sequence of three mRNA bases that specifies one amino acid or a stop signal.
Anticodon
  • Anticodon A sequence of three tRNA bases that is complementary to a codon.

Section 2 of 5Steps of Translation

1
The mRNA binds to the small ribosomal subunit and a tRNA carrying methionine pairs with the start codon AUG.
2
A second tRNA, with the anticodon complementary to the next codon, pairs with the mRNA on the ribosome.
3
A peptide bond forms between the two amino acids by condensation, which uses energy from ATP.
4
The ribosome moves along the mRNA by one codon and the first tRNA is released to pick up another amino acid.
5
These steps repeat until a stop codon is reached, when the polypeptide is released and the ribosome parts leave the mRNA.

Section 3 of 5Matching Codons and Anticodons

Matching Codons and Anticodons
Complementary pairing
  • Complementary pairing Adenine pairs with uracil and cytosine pairs with guanine between codon and anticodon.
Specificity
  • Specificity Each tRNA carries only one type of amino acid, matched to its anticodon by the cell's enzymes.
Two at a time
  • Two at a time The ribosome holds two tRNAs at once, which brings their amino acids together so a peptide bond can form.
Stop codons
  • Stop codons Stop codons do not code for an amino acid, so no tRNA binds and translation ends.
mRNA codontRNA anticodon
AUGUAC
GAACUU
CCUGGA
CAGGUC

Section 4 of 5Worked Example

Question

  • An mRNA reads AUG GAA CCU.
  • State the anticodons of the three tRNAs in order.

  1. Pair each base of the codon with its complement: A with U, U with A, G with C and C with G.
  2. AUG pairs with UAC.
  3. GAA pairs with CUU and CCU pairs with GGA.

Answer The anticodons are UAC, CUU and GGA.

Section 5 of 5Exam Checklist & Tips

4.2.2 – Key Points for the Exam

  • Translation The synthesis of a polypeptide at a ribosome using the sequence of mRNA codons.
  • Codon A triplet of mRNA bases specifying an amino acid.
  • Anticodon The triplet of tRNA bases complementary to a codon.
  • Peptide bond The covalent bond linking adjacent amino acids in a polypeptide.
  • Ribosome The organelle that assembles polypeptides during translation.

Common Mistakes

  • Putting translation in the nucleus; it is in the cytoplasm or on the rough endoplasmic reticulum.
  • Writing T in the anticodon; tRNA contains uracil.
  • Saying tRNA brings DNA to the ribosome; it brings amino acids.
  • Forgetting that making peptide bonds requires ATP.

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