The genetic code and protein synthesisEdexcel A-Level Biology A: Revision notes

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.

The genetic code is how the base sequence is turned into amino acids. It is:

• a triplet code: three bases, called a codon on mRNA, code for one amino acid
• non-overlapping: each base belongs to only one codon
• degenerate: most amino acids are coded for by more than one triplet

Four bases in groups of three give 4³ = 64 possible triplets, enough for 20 amino acids, which explains the degeneracy. Some triplets are start or stop signals.

Messenger RNA (mRNA) is a copy of a gene, made in the nucleus and taken to a ribosome. Transfer RNA (tRNA) is a folded molecule with an anticodon at one end and a specific amino acid at the other. Ribosomes are where translation happens.

The template (antisense) strand of DNA is the strand that is copied, so mRNA is complementary to it. The other strand, the coding (sense) strand, has the same base sequence as the mRNA (except T instead of U).

The start codon is AUG, which codes for methionine. The stop codons (UAA, UAG, UGA) do not code for amino acids.

Transcription makes mRNA from a gene in the nucleus.

1. RNA polymerase binds to the DNA at the start of the gene and breaks the hydrogen bonds, unwinding a short section.
2. Free RNA nucleotides pair with the exposed bases of the template strand by complementary base pairing (A with U, T with A, C with G).
3. RNA polymerase joins the nucleotides by phosphodiester bonds, forming mRNA.
4. The mRNA leaves through a nuclear pore.

Worked example: template TAC CGA GGT ATT gives mRNA AUG GCU CCA UAA.

Translation builds the polypeptide at a ribosome in the cytoplasm.

1. mRNA binds to a ribosome, and translation begins at the start codon, AUG.
2. A tRNA carrying a specific amino acid has an anticodon that pairs with the codon by complementary base pairing.
3. A second tRNA binds to the next codon, and a peptide bond forms between the two amino acids (a condensation reaction).
4. The ribosome moves along the mRNA, adding amino acids, until a stop codon is reached.
5. No tRNA matches a stop codon, so the polypeptide is released.

The order of codons on the mRNA decides the order of amino acids, which is the primary structure.

The flow of information is DNA → mRNA → polypeptide.

• DNA triplets on the template strand give mRNA codons, which give tRNA anticodons matching the template triplets.
• Each stage uses complementary base pairing.
• A polypeptide of n amino acids needs at least 3n nucleotides in mRNA, plus 3 for the stop codon.

Worked example: a polypeptide of 18 amino acids needs 18 × 3 + 3 = 57 nucleotides in its mRNA.