A-Level Biology October/November 2024 Q8(a): Sometimes the gene for genetic engineering cannot be extracted from the donor organism.…

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  There are two main methods for synthesising a gene.

  Method 1: Using Reverse Transcriptase

  This method starts with messenger RNA (mRNA) that has been extracted from a cell known to be expressing the target gene. The enzyme reverse transcriptase is then used to synthesise a single strand of complementary DNA (cDNA) using the mRNA molecule as a template. This cDNA strand is a copy of the gene, but without introns.

  Method 2: Chemical Synthesis

  This method uses a 'gene machine' or oligonucleotide synthesiser. The process requires the amino acid sequence of the desired protein to be known, which can be obtained from an online database. From this sequence, the corresponding DNA nucleotide sequence can be determined. The machine then chemically joins individual nucleotides together in the correct, pre-determined order to build the gene from scratch.

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  How the marks are awarded

  • B1 — Correctly identifying that complementary DNA (cDNA) is made from a messenger RNA (mRNA) template, as stated in the first method: '...synthesise a single strand of complementary DNA (cDNA) using the mRNA molecule as a template.'
  • B1 — Naming the key enzyme, reverse transcriptase, for the first method. The answer states: 'The enzyme reverse transcriptase is then used...'
  • B1 — Describing the second method as the chemical synthesis or joining of nucleotides, as written: 'The machine then chemically joins individual nucleotides together...'

  Common mistakes

  • Confusing reverse transcriptase with other enzymes, such as DNA polymerase or ligase.
  • Incorrectly stating that reverse transcriptase uses a DNA template instead of an mRNA template.
  • Describing the chemical synthesis method too vaguely, for example, 'a machine makes the gene', without mentioning that it joins nucleotides based on a known sequence.
  • Describing PCR (Polymerase Chain Reaction), which is a method for amplifying existing DNA, not synthesising a gene from scratch or from mRNA.

  Examiner tip: For topics with multiple named processes, such as gene synthesis, create a summary table listing the process name, starting material, key enzyme(s) or machinery, and the final product to aid recall and prevent confusion.

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