This free quiz has 12 real AQA A-level Biology exam questions on DNA, genes and chromosomes (24 marks, about 30 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 3.4.1)
the protein associated with DNA in a chromosome
what genes, loci and exons are
molecules other than polypeptides that a gene can code for
the minimum number of bases in a gene for a polypeptide
features of DNA in prokaryotes, nuclei and chloroplasts
the universal, non-overlapping and degenerate genetic code
why a deletion can change a polypeptide when a substitution doesn't
comparing DNA in eukaryotic and prokaryotic cells (5 marks)
How to use it
Write each answer in full before you open the mark scheme. In definitions, say exactly what codes for what: a base sequence codes for a sequence of amino acids, and it is codons or triplets, not single bases, that make the code universal and degenerate. For the comparison question, write matched pairs about the DNA itself, not the whole cell.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
What a gene codes for. Only 2% of students gained the mark for defining a gene, because most wrote vaguely about sections of DNA coding for proteins. Only a small minority could name molecules other than proteins that a gene can code for.
✔ Gains credit: a gene is a sequence of DNA bases that codes for a polypeptide (a sequence of amino acids) or for a functional RNA, such as rRNA, tRNA or mRNA.
Defining an exon. Many students oversimplified this, writing that an exon codes for a protein or a polypeptide. They left out that it is the base sequence that codes for the amino acid sequence.
✔ Gains credit: an exon is a base sequence that codes for a sequence of amino acids.
Non-coding DNA isn't only introns. Most students thought the only non-coding DNA is in introns. Saying non-coding sequences “do not code for anything”, or don't code for “an amino acid”, wasn't enough.
✔ Gains credit: non-coding DNA does not code for polypeptides (sequences of amino acids), and non-coding multiple repeats are positioned between genes, or at the ends of chromosomes in telomeres.
Universal means codons. A common answer that didn't gain credit was “the genetic code is the same in all organisms”, and “DNA is universal” on its own was not accepted. The answer had to mention codons or triplets.
✔ Gains credit: the same codon (triplet of bases) always codes for the same amino acid, in all species.
Degenerate code, not DNA. Writing that “DNA is degenerate” wasn't credited, and nor was writing about bases rather than codons, or saying triplets “produce” amino acids. Some also said the code is universal because it is degenerate.
✔ Gains credit: the genetic code is degenerate: more than one codon (triplet) codes for each amino acid.
Compare the DNA, not cells. Many students compared whole cells, e.g. “DNA floats in the cytoplasm”, or compared DNA with RNA. Lists about each type of DNA without clear comparisons gained no marks, and only 0.3% scored five out of five.
✔ Gains credit: a direct statement about the DNA itself, with the two types side by side, e.g. eukaryotic DNA is longer and contains introns, whereas prokaryotic DNA is shorter and has no introns.
Prokaryotic DNA misconceptions. Common misconceptions were that prokaryotic DNA is single-stranded, does not form a double helix, or exists only as plasmids. These limited the marks for otherwise good answers.
✔ Gains credit: prokaryotic DNA is double-stranded, like eukaryotic DNA, and is present as one molecule of DNA, which may be found alongside plasmids.
Keep practising
The other AQA 3.4 self-marking quizzes: Protein synthesis · Mutation and meiosis · Natural selection · Species and biodiversity
Topic 3.3 quizzes start with surface area to volume ratio
DNA, genes and chromosomes: common questions and mark scheme insights
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.