Genes

The genetic code is non-overlapping and directional: reads 5’ → 3’

The code is degenerate - several codons can encode for the same amino acid, reducing the impact of mutations

The Relationship between DNA Mutations and Protein Mutations:

  • If there is a mutation in the DNA (e.g. insertion), the mRNA will contain the wrong base, the codon will change so the amino acid may change and the protein’s structure and function may change.

  • Deamination of cytosine → uracil, resulting in a mutation

  • Nonsense mutation is when there is a premature stop codon

  • Missense - mutation causes another amino acid to form

Basic structural anatomy of a gene:

  1. A transcription unit - promotor, coding region (exons and introns), terminator

RNA polymerase binds to the promotor.

Structure of the mRNA:

1. Untranslated regions (UTRs)

  • 5′ UTR (before the start codon)

  • 3′ UTR (after the stop codon)

These are not translated, but are essential for:

  • stability

  • translation efficiency

  • regulation

2. Open Reading Frame (ORF)

This is the coding region that is translated into protein.

3. Cap and Poly(A) tail (eukaryotes)

  • 5′ cap (7‑methylguanosine, 5′–5′ linkage)

  • Poly(A) tail added at the 3′ end

These protect the mRNA and regulate translation.

A gene consists of a transcription unit controlled by promoter and terminator sequences; the resulting mRNA contains a coding region (ORF) embedded within untranslated regions and undergoes capping, polyadenylation, and splicing to become a mature transcript.

Lagging Strand Synthesis

Max 6 marks

  • Helicase unwinds DNA (1)

  • Primase synthesises short RNA primers (1)

  • DNA polymerase III extends primers → Okazaki fragments (1)

  • DNA polymerase I removes RNA primers (5’→3’ exonuclease) (1)

  • DNA polymerase I replaces RNA with DNA (1)

  • DNA ligase seals nicks between fragments (1)

PCR

Max 5 marks

  • Denaturation (≈94°C): strands separate (1)

  • Annealing (≈55°C): primers bind (1)

  • Extension (≈72°C): Taq polymerase synthesises DNA (1)

  • Taq used because it is heat‑stable (1)

  • Exponential amplification (1)

Transcription → RNA processing → Translation