4.2.2Additional lessons
Translation in Protein Synthesis — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 31 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Components of Translation
Translation needs several molecules acting together.
mRNA
- mRNA This carries the genetic message as a sequence of three-base codons, each of which specifies one amino acid or a stop signal.
Ribosome
- Ribosome A structure made of rRNA and protein that holds mRNA and tRNA together and catalyses the joining of amino acids.
tRNA
- tRNA Each tRNA carries one specific amino acid and has an anticodon that pairs with a matching codon on the mRNA.
Amino acids
- Amino acids Twenty different amino acids are the building blocks, joined by peptide bonds in the order dictated by the codons.
| Molecule | Role | Location of key feature |
|---|---|---|
| mRNA | Carries codons | Along the strand |
| tRNA | Brings amino acid | Anticodon at one end, amino acid at the other |
| Ribosome | Site of synthesis | Small and large subunits |
Section 2 of 5Steps of Translation
1
The small ribosomal subunit binds to mRNA, and a tRNA carrying methionine pairs with the start codon AUG.
2
The large subunit joins, and a second tRNA with a matching anticodon enters the neighbouring site on the ribosome.
3
A peptide bond forms between the two amino acids, and the ribosome moves along the mRNA by exactly one codon.
4
The first tRNA leaves, a new tRNA enters, and the cycle repeats so the polypeptide chain lengthens one amino acid at a time.
5
When a stop codon is reached no tRNA fits it, so the polypeptide is released and the ribosome parts dissociate.
Section 3 of 5Linking Structure to Function
Linking Structure to Function
Anticodon pairing
- Anticodon pairing Complementary pairing between codon and anticodon ensures that the correct amino acid is added at every position.
Activating enzymes
- Activating enzymes A specific enzyme attaches each amino acid to its own tRNA, so accuracy depends on this first step.
Energy
- Energy Making peptide bonds and moving the ribosome need energy, supplied by ATP and GTP.
Free ribosomes
- Free ribosomes These make proteins for use inside the cell, whereas ribosomes on rough ER make proteins for export or membranes.
Section 4 of 5Worked Example
Question
- An mRNA reads 5' AUG GAA CCU UAA 3'.
- State the anticodons that pair with the first three codons and the number of amino acids made.
- Anticodons are complementaryAUG pairs with UAC, GAA pairs with CUU, CCU pairs with GGA.
- The fourth codon UAA is a stop codon, so no tRNA binds.
- Three codons were translated, so three amino acids are joined.
Answer
- Anticodons UAC, CUU, GGA;
- a tripeptide of three amino acids.
Section 5 of 5Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Codon A triplet of mRNA bases that codes for one amino acid or a stop signal.
- Anticodon Three bases on tRNA complementary to an mRNA codon.
- Peptide bond Covalent bond between the carboxyl group of one amino acid and the amino group of the next.
- Start codon AUG, which codes for methionine and begins translation.
- Polypeptide A chain of amino acids joined by peptide bonds.
Common Mistakes
- Writing that the anticodon is on the mRNA; the anticodon is part of tRNA.
- Thinking that translation happens in the nucleus of eukaryotes;
- it occurs on ribosomes in the cytoplasm or on rough ER.
- Believing that tRNA reads DNA; it pairs only with mRNA codons.
- Forgetting that stop codons do not code for an amino acid.
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