Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.2.3Additional lessons

The Genetic Code — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 32 of 101

📚 IB Biology HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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.
0%read

Part 1 of 5Properties of the Code

The genetic code links base sequences to amino acid sequences.

Triplet code
  • Triplet code Each codon is three bases long, because two bases would give only 16 combinations, which is too few for 20 amino acids.
Degenerate
  • Degenerate Most amino acids are coded by more than one codon, so some base changes do not alter the protein made.
Universal
  • Universal Nearly all organisms use the same code, which supports a common ancestry and allows genes to be moved between species.
Non-overlapping
  • Non-overlapping Bases are read in separate groups of three, so each base belongs to one codon only.
CodonsNumberMeaning
Total possible64 (4³)All triplets of four bases
Amino acid codons61Specify 20 amino acids
Stop codons3UAA, UAG, UGA
Start codon1AUG, also codes for methionine

Part 2 of 5Using a Codon Table

1
Write down the mRNA sequence in the 5' to 3' direction and mark the first AUG as the start of translation.
2
Split the sequence into groups of three bases from the start codon without leaving gaps or overlaps.
3
Look up each codon using the first base, then the second, then the third, to find the amino acid.
4
Continue until a stop codon appears, then list the amino acids in order as the primary structure.

Part 3 of 5Consequences of the Code

Consequences of the Code
Genetic engineering
  • Genetic engineering Because the code is universal, a human gene can be expressed in bacteria which then make a human protein.
Mutation effects
  • Mutation effects Degeneracy means a change in the third base of a codon often has no effect on the amino acid, called a silent change.
Efficiency
  • Efficiency Having 61 sense codons for 20 amino acids gives a buffer against the damage caused by point mutations.
Exceptions
  • Exceptions A few organelles and organisms use slightly different codon meanings, so universality is almost but not entirely complete.

Part 4 of 5Worked Example

Question

  • A polypeptide of 150 amino acids is made from a gene.
  • Calculate the minimum number of bases in the mRNA coding region including the stop codon.

  1. Each amino acid needs one codon of three bases: 150 × 3 = 450 bases.
  2. Add one stop codon of three bases: 450 + 3 = 453.

Answer 453 bases (a longer mRNA is expected because of untranslated regions).

Part 5 of 5Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Codon Three consecutive bases of mRNA that code for an amino acid or stop.
  • Degenerate Describes a code in which several codons can specify the same amino acid.
  • Universal Used with the same meanings by almost all living things.
  • Stop codon A codon that ends translation and does not code for an amino acid.
  • Reading frame The way the base sequence is divided into triplets.

Common Mistakes

  • Stating that the code is universal without the word nearly or almost, because exceptions exist.
  • Mixing up degenerate with ambiguous; each codon gives one amino acid only.
  • Counting 64 amino acids; there are 20 amino acids but 64 codons.
  • Reading the DNA template instead of the mRNA when using a codon table.

Last updated:

\n