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2.3.2Unit 2: Cells, Development, Biodiversity and Conservation

The Nature of the Genetic Code — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 26 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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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.
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Part 1 of 5A Triplet Code

The sequence of bases in a gene is a code that the cell reads to build proteins.

Gene
  • Gene A length of DNA with a base sequence that codes for the amino acid sequence of one polypeptide.
Triplet
  • Triplet A group of three bases in DNA that codes for one amino acid, and the transcribed mRNA version is called a codon.
Number of codons
  • Number of codons Four bases taken three at a time give 4³ = 64 possible codons, which is more than enough for 20 amino acids.
Start signal
  • Start signal The codon AUG codes for methionine and also signals where translation begins on an mRNA.
Stop codons
  • Stop codons UAA, UAG and UGA do not code for an amino acid but signal the ribosome to end translation.
Reading direction
  • Reading direction Codons are read in order along the mRNA from the start codon, three bases at a time.

Part 2 of 5Properties of the Code

Properties of the Code
Degenerate
  • Degenerate Most amino acids are coded for by more than one codon, so some mutations have no effect on the protein.
Non-overlapping
  • Non-overlapping Each base is part of only one codon, so the sequence is read as separate consecutive groups of three.
Universal
  • Universal Nearly every organism uses the same codon for the same amino acid, which makes genetic engineering between species possible.
Triplet
  • Triplet Three bases are needed because two bases would give only 4² = 16 combinations, which cannot cover 20 amino acids.

Part 3 of 5Consequences of the Code

1
A gene is transcribed into mRNA, which carries the code to ribosomes where amino acids are assembled in the correct order.
2
Because the code is non-overlapping, inserting or deleting a single base shifts every codon after that point.
3
Because the code is degenerate, a change in the third base of a codon often still codes for the same amino acid.
4
Because the code is universal, a human gene placed in a bacterium can be translated into the human protein.
5
The stop codon ends the polypeptide, so the number of amino acids is one fewer than the number of codons.

Part 4 of 5Worked Example

Question

  • The coding region of an mRNA contains 303 nucleotides, from the start codon to the end of the stop codon inclusive.
  • Calculate the number of amino acids in the polypeptide.

  1. Number of codons = 303 ÷ 3 = 101.
  2. The last codon is a stop codon, which does not code for an amino acid.
  3. Amino acids = 101 − 1 = 100.

Answer The polypeptide contains 100 amino acids.

Part 5 of 5Exam Checklist & Tips

2.3.2 – Key Points for the Exam

  • Codon A sequence of three bases on mRNA that codes for one amino acid or a stop signal.
  • Degenerate code A code in which most amino acids are coded for by more than one triplet.
  • Non-overlapping Each base is read only once and belongs to a single codon.
  • Universal code The same triplet codes for the same amino acid in nearly all organisms.
  • Stop codon A codon that signals the end of translation and codes for no amino acid.

Common Mistakes

  • Writing that the code is overlapping; each base is read as part of only one triplet.
  • Saying that degenerate means a codon can code for several amino acids, when it is the amino acid that has several codons.
  • Stating there are 64 amino acids, when 64 is the number of possible codons and 20 the number of amino acids.
  • Forgetting that stop codons code for no amino acid.

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