OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.1.7 DNA, Genes and Chromosomes Exam Questions
Free OxfordAQA International A Level Biology revision for 3.1.7 DNA, Genes and Chromosomes: what you need to know, common mistakes and exam-style questions with answers. Covers nucleotide structure, DNA and RNA, histones and chromosomes, genes, loci, introns and exons, the genome, and semi-conservative DNA replication.
Practise 3.1.7 DNA, Genes and Chromosomes with past papers
Once you have learnt this topic, find the 3.1.7 DNA, Genes and Chromosomes questions in real OxfordAQA International A Level Biology past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Biology Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About OxfordAQA International A Level Biology 3.1.7 DNA, Genes and Chromosomes
This Unit 1 topic introduces the structure of nucleic acids and how DNA is copied. You need to describe nucleotides, compare DNA with RNA (including the structure of mRNA and tRNA), explain how DNA is packaged with histones in eukaryotic nuclei, and contrast this with the DNA in mitochondria, chloroplasts and prokaryotes. You also learn what a gene is and where it is found, and explain semi-conservative replication. It leads straight into protein synthesis, meiosis, mutation and gene technology. Examiners often ask for complementary base pairing calculations, comparisons of DNA and RNA, and the roles of DNA helicase and DNA polymerase.
What you need to know
- Nucleotides A nucleotide is a pentose sugar, a phosphate group and a nitrogen-containing base. Nucleotides join by condensation reactions to form a sugar-phosphate backbone.
- DNA and RNA DNA contains deoxyribose and the bases A, T, C and G, and is a double helix with complementary base pairs (A with T, C with G) held by hydrogen bonds. RNA contains ribose and uracil instead of thymine, and is a short single chain. mRNA is a single linear strand; tRNA is folded into a clover-leaf shape with an anticodon and an amino acid binding site.
- Chromosomes In eukaryotic nuclei, DNA is long, linear and associated with histone proteins to form chromosomes. Mitochondria, chloroplasts and prokaryotes have short, circular DNA not associated with proteins.
- Genes A gene is a base sequence at a fixed position (locus) on DNA that codes for a polypeptide. In eukaryotes genes contain exons (coding) and introns (non-coding), and much DNA between genes is non-coding repeats. The genome is the complete set of genes in a cell.
- DNA replication DNA helicase breaks hydrogen bonds between bases and unwinds the helix. Free nucleotides pair with exposed bases on each template strand, and DNA polymerase joins adjacent nucleotides by condensation. Each new molecule has one original and one new strand, which is semi-conservative replication.
Common mistakes in 3.1.7 DNA, Genes and Chromosomes
- Saying DNA polymerase forms hydrogen bonds between bases. Hydrogen bonds form between complementary bases; DNA polymerase joins nucleotides in the new strand.
- Writing that RNA has thymine. RNA has uracil, which pairs with adenine.
- Saying helicase breaks the DNA strand. Helicase breaks the hydrogen bonds between the two strands, not the sugar-phosphate backbone.
- Stating that prokaryotic DNA is wound around histones. It is circular and not associated with histone proteins.
Try three 3.1.7 DNA, Genes and Chromosomes exam-style questions
1. A DNA sample contains 22% adenine. Calculate the percentage of guanine. (2 marks)
Show the mark scheme answer
Thymine = 22%, so A + T = 44%; C + G = 56%, so guanine = 28%.
2. Give three differences between DNA and mRNA. (3 marks)
Show the mark scheme answer
DNA has deoxyribose, mRNA has ribose; DNA has thymine, mRNA has uracil; DNA is double-stranded, mRNA single-stranded; DNA is much longer; DNA has hydrogen bonds between strands. (Any 3)
3. Explain what is meant by semi-conservative replication. (2 marks)
Show the mark scheme answer
Each new DNA molecule contains one strand from the original molecule; and one newly synthesised strand.
3.1.7 DNA, Genes and Chromosomes FAQs
What does OxfordAQA International A Level Biology 3.1.7 DNA, Genes and Chromosomes cover?
It covers nucleotide structure, DNA and RNA, histones and chromosomes, genes, loci, introns and exons, the genome, and semi-conservative DNA replication.
Which unit is 3.1.7 DNA, Genes and Chromosomes examined in?
It is part of Unit 1, The diversity of living organisms, which is examined at International AS. The Unit 5 synoptic paper at A2 can also draw on this content.
What is the difference between an intron and an exon?
Exons are the parts of a gene that code for amino acids. Introns are non-coding sections within a gene that are removed from pre-mRNA by splicing before translation.
