2.3.3Unit 2: Cells, Development, Biodiversity and Conservation
How Bases Code for a Polypeptide Chain — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 19 of 26 Unit quiz
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Section 1 of 5From Gene to Polypeptide
The base sequence of a gene determines the amino acid sequence of its polypeptide by way of mRNA.
Template strand
- The DNA strand that is read by RNA polymerase, and it is also called the antisense strand.
Coding strand
- The other DNA strand, which has the same base sequence as the mRNA except that it has thymine instead of uracil.
mRNA codon
- Each set of three mRNA bases is complementary to a triplet on the template strand and specifies one amino acid.
tRNA anticodon
- The three bases on a transfer RNA that are complementary to a codon and pair with it during translation.
Amino acid order
- The order of codons along the mRNA decides the order of amino acids, which is the primary structure of the protein.
Protein function
- The primary structure determines folding, and so one change in the base sequence can change the shape and function.
Section 2 of 5Working Out the Sequence
1
Write out the DNA template strand in groups of three bases, which are the triplets read in order.
2
Build the mRNA by pairing each template base with its complement, using A–U, T–A, C–G and G–C.
3
Divide the mRNA into codons starting from the start codon AUG, which also codes for methionine.
4
Use the codon table to find the amino acid for each codon until you reach a stop codon.
5
Write the sequence with the amino acids joined by dashes, and do not include any amino acid for the stop codon.
Section 3 of 5Sample Codons and Their Amino Acids
Sample Codons and Their Amino Acids
AUG
- Codes for methionine and acts as the start codon of an mRNA.
CCA
- Codes for the amino acid proline, a codon that appears in many proteins.
GGU
- Codes for the amino acid glycine, which has the smallest R group of all amino acids.
UAA
- A stop codon that causes the ribosome to release the finished polypeptide.
| Codon | Amino acid |
|---|---|
| AUG | Methionine (Met) |
| CCA | Proline (Pro) |
| GGU | Glycine (Gly) |
| UAA | Stop |
Section 4 of 5Worked Example
Question
- The template strand of a gene reads TAC GGT CCA ATT.
- Write the mRNA sequence and the amino acid sequence, using the codons AUG = Met, CCA = Pro, GGU = Gly and UAA = stop.
- Pair each template base with its complement, replacing thymine by uracil in RNA: TAC gives AUG, GGT gives CCA, CCA gives GGU and ATT gives UAA.
- The mRNA is AUG CCA GGU UAA.
- TranslateAUG = Met, CCA = Pro, GGU = Gly, UAA = stop.
Answer
- mRNA: AUG CCA GGU UAA;
- polypeptide: Met–Pro–Gly, then stop.
Section 5 of 5Exam Checklist & Tips
2.3.3 – Key Points for the Exam
- Template strand The DNA strand used by RNA polymerase to make mRNA.
- Codon Three mRNA bases that code for one amino acid.
- Anticodon Three bases on tRNA that pair with a codon on mRNA.
- Primary structure The sequence of amino acids in a polypeptide.
- Start codon AUG, where translation begins.
Common Mistakes
- Reading the coding strand as if it were the template, which gives the wrong mRNA.
- Writing T in the mRNA sequence instead of U.
- Forgetting that stop codons add no amino acid to the polypeptide.
- Starting codon groups from the wrong base, which changes every later codon.
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