This free quiz has 12 real AQA A-level Biology exam questions on genome projects and gene technologies (25 marks, about 35 minutes). It marks itself: the table question is marked for you, and for written answers you check your work against the real AQA mark scheme.
What the quiz covers (AQA 3.8.3–3.8.4)
the role of reverse transcriptase in RT-PCR
naming the regions either side of a transferred gene, and the purpose of a marker gene
reading a gel from DNA probe screening for viral DNA fragments
how many fragments restriction enzymes produce from a plasmid
what a DNA probe is, and inserting a gene into a plasmid with restriction endonuclease and ligase
primers in PCR, and the features of STRs that let electrophoresis separate them
why the proteome of a prokaryote is easier to determine from its genome
using PCR to amplify DNA (4 marks), and evaluating a gene therapy for haemophilia (5 marks)
How to use it
Write each answer in full before you open the mark scheme, then tick only the points you really made. Name every enzyme precisely and say what it acts on: restriction endonuclease cuts the plasmid, DNA polymerase joins nucleotides. For the gel question, use the bands of known length to identify each fragment before you tick.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Genomes and proteomes. Most students who scored nothing simply said that prokaryotic DNA is shorter or has fewer bases or genes. Less than 5% gained a second mark for regulatory genes.
✔ Gains credit: prokaryotes have no introns (non-coding DNA), whereas eukaryotes contain introns and more regulatory genes, so the proteome is harder to determine from a eukaryotic genome.
Restriction endonuclease and ligase. Many students named the wrong enzymes, such as DNA helicase, reverse transcriptase or DNA polymerase. Others said the restriction enzyme “cuts the gene”, said ligase makes sticky ends, or wrote “joins DNA together” with no mention of the plasmid.
✔ Gains credit: restriction endonuclease cuts the plasmid, producing sticky ends, and DNA ligase joins the gene and the plasmid by forming phosphodiester bonds.
Cutting at recognition sites. A significant number only said the enzyme cuts at a “specific site”, and some suggested heating the DNA to break it down. Weaker answers said the enzyme is complementary to a base sequence.
✔ Gains credit: restriction endonucleases cut DNA at a specific base sequence (a recognition or restriction site), breaking phosphodiester bonds.
The steps of PCR. A common error was leaving out nucleotides, or one of the temperature changes. Weaker answers named the wrong enzyme, most often RNA polymerase, or said DNA polymerase joins bases rather than nucleotides.
✔ Gains credit: heat to 90–95 °C to break hydrogen bonds and separate the strands, cool to 40–65 °C so primers bind, then raise to 70–75 °C so DNA polymerase joins DNA nucleotides, and repeat.
What a DNA probe is. Many described what a probe is used for or how it is labelled, rather than what it is. Others wrote “short sections” or “fragments” of DNA instead of a single strand, or confused probes with marker genes.
✔ Gains credit: a DNA probe is a short, single strand of DNA with bases complementary to the allele or gene being looked for.
Separating DNA by electrophoresis. Answers that said “size” or density were ignored, and a positive charge was rejected. Weaker answers wrote about amino acids or the charge on R groups instead of bases and nucleotides.
✔ Gains credit: DNA fragments differ in length (the number of bases, base pairs or repeats) and carry a negative charge from their phosphate groups.
Promoters, terminators and markers. Students who failed often called the regions either side of a gene start and stop codons, introns and exons, or VNTRs. Vague answers such as “to see the gene” got no credit for the marker gene.
✔ Gains credit: the regions are named as a promoter and a terminator, and a marker gene shows that the gene has been taken up, allowing detection of genetically modified cells.
Keep practising
The other AQA 3.8 self-marking quiz: Gene expression
Topic 3.7 quizzes start with inheritance
The AQA topic index
weekly AQA group classes: Year 12 on Tuesdays at 5.15pm, Year 13 on Wednesdays or Thursdays at 6.30pm
Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.