4.2.3Additional lessons
The Genetic Code — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 104 of 142
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 5Features of the Code
The genetic code links the base sequence of a gene to the amino acid sequence of a polypeptide.
Triplet
- Triplet Each sequence of three bases codes for one amino acid or a start or stop signal.
Degenerate
- Degenerate There are 64 possible codons (4³) but only 20 common amino acids, so most amino acids have more than one codon.
Universal
- Universal Almost all organisms use the same code, so genes can be transferred between species in genetic engineering.
Non-overlapping
- Non-overlapping Each base belongs to only one codon, so the sequence is read in successive groups of three.
Start and stop
- Start and stop AUG codes for methionine and starts translation, while UAA, UAG and UGA are stop codons.
Section 2 of 5Deducing a Sequence
Following the steps in order avoids errors when deducing sequences.
1
Start with the coding strand or the template strand of the DNA, making sure you know which is given.
2
If given the coding strand, the mRNA matches it with uracil instead of thymine.
3
If given the template strand, write the complementary mRNA base for base.
4
Divide the mRNA into codons, beginning at the start codon if required.
5
Use the codon table to name each amino acid in turn until a stop codon is reached.
Section 3 of 5Why the Code Matters
Features of the code explain why genes can be shared between species and why some mutations are harmless.
Why the Code Matters
Genetic engineering
- Genetic engineering Because the code is universal, human insulin can be made by bacteria that read the human gene correctly.
Limiting mutations
- Limiting mutations Degeneracy means some base substitutions do not change the amino acid, reducing harm from mutations.
Evolution
- Evolution A shared code is evidence that all living things share a common ancestor.
Reading frame
- Reading frame Starting at the wrong place gives a different set of codons and a different polypeptide.
| Codon | Amino acid |
|---|---|
| AUG | Methionine (start) |
| UUC | Phenylalanine |
| GAG | Glutamic acid |
| CAU | Histidine |
| UAC | Tyrosine |
| GCC | Alanine |
Section 4 of 5Worked Example
Question
- A DNA coding strand reads TTC GAG CAT TAC GCC.
- Deduce the amino acid sequence.
- The mRNA matches the coding strand with U for T: UUC GAG CAU UAC GCC.
- UUC is Phe and GAG is Glu.
- CAU is His, UAC is Tyr and GCC is Ala.
Answer Phe-Glu-His-Tyr-Ala.
Section 5 of 5Exam Checklist & Tips
4.2.3 – Key Points for the Exam
- Genetic code The set of rules by which base triplets specify amino acids.
- Degenerate Describes a code in which several codons can code for the same amino acid.
- Universal Describes the fact that the same code is used by nearly all organisms.
- Stop codon A codon that signals the end of translation.
Common Mistakes
- Counting 64 codons as 64 amino acids; there are only 20 amino acids.
- Using the template strand as if it were the mRNA sequence.
- Saying the code is overlapping.
- Calling the code universal without noting that there are a few minor exceptions.
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