4.2.7Additional lessons
Translation and the Proteome — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 36 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 5Ribosome Binding Sites
A ribosome has several sites that let it work like a production line.
A site
- A site The aminoacyl site where an incoming tRNA carrying the next amino acid first binds to its codon.
P site
- P site The peptidyl site that holds the tRNA attached to the growing polypeptide chain.
E site
- E site The exit site from which an empty tRNA is released after it has delivered its amino acid.
mRNA binding site
- mRNA binding site A groove on the small subunit that holds the mRNA so that codons are presented in order.
| Site | Occupied by | Event |
|---|---|---|
| A | Incoming aminoacyl tRNA | Codon-anticodon pairing |
| P | tRNA with polypeptide | Peptide bond formation |
| E | Empty tRNA | Release from ribosome |
Section 2 of 5Three Phases of Translation
1
Initiation: the small subunit binds mRNA near the start codon, an initiator tRNA with methionine pairs at AUG, and the large subunit assembles.
2
Elongation: an aminoacyl tRNA enters the A site, a peptide bond joins its amino acid to the chain at the P site, and the chain is transferred to the A site tRNA.
3
Translocation: the ribosome shifts three bases along the mRNA, moving tRNAs from A to P and from P to E.
4
Termination: a stop codon in the A site is recognised by a release factor, the polypeptide is freed and the ribosome dissociates.
Section 3 of 5Proteome and Ribosome Types
Proteome and Ribosome Types
Proteome
- Proteome This is the entire set of proteins that a cell, tissue or organism makes at a given time.
Variation
- Variation Different cell types express different genes, so a muscle cell and a neuron have different proteomes despite identical DNA.
Polysomes
- Polysomes Several ribosomes can read one mRNA at once, producing many copies of a polypeptide very quickly.
Bound ribosomes
- Bound ribosomes Ribosomes attached to rough ER make proteins for secretion, lysosomes or membranes, guided by a signal peptide.
Section 4 of 5Worked Example
Question
- A ribosome translates an mRNA at 15 amino acids per second.
- How long does it take to make a polypeptide of 450 amino acids, ignoring initiation and termination?
- Time = number of amino acids ÷ rate.
- 450 ÷ 15 = 30 seconds.
- Polysomes would give many chains in this same period.
Answer 30 seconds
Section 5 of 5Exam Checklist & Tips
4.2.7 – Key Points for the Exam
- Proteome All the proteins expressed by a cell or organism at a given time.
- Polysome A group of ribosomes translating the same mRNA at once.
- A site Ribosome site where an incoming tRNA binds.
- Translocation Movement of the ribosome by one codon along the mRNA.
- Release factor Protein that recognises a stop codon and ends translation.
Common Mistakes
- Placing a new tRNA straight into the P site; it always enters at the A site.
- Stating that the genome and proteome are identical; the proteome changes with conditions and cell type.
- Forgetting that translocation moves the ribosome by exactly three bases.
- Saying a stop codon is read by a tRNA; a release factor recognises it.
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