A chromosome is described as a long, coiled molecule of DNA that carries genetic information in the form of genes. Explain how the structure of DNA allows genetic information to be copied accurately during DNA replication.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Before a cell can divide by mitosis, the genetic material in the nucleus must be replicated so that each daughter cell receives an identical copy of the DNA. DNA replication relies on the specific structure of the DNA molecule.
Model answer (5 marks)
DNA is a polymer of nucleotides forming two strands twisted into a double helix. The strands are held together by complementary base pairs – adenine with thymine and cytosine with guanine. During replication the strands separate, or unzip, creating two single‑stranded templates. Each original strand directs synthesis of a new complementary strand by the enzyme DNA polymerase, which adds nucleotides in the 5’→3’ direction. The result is two identical DNA molecules, each consisting of one original strand and one newly synthesised strand.
Examiner tips
- Show the double‑helix structure and base‑pairing first; this earns 1 mark. Explain strand separation and template use; each step earns 1 mark. State that two identical molecules are produced; this gives the final mark.
- common_mistakes
- :
- Using the wrong base‑pairing (e.g. A‑C or G‑T). Omitting the 5’→3’ direction of synthesis or the role of DNA polymerase.
Mark scheme (5 marks)
- DNA is a polymer made of two strands twisted into a double helix
- The two strands are held together by base pairs / complementary base pairing
- The two strands separate / unzip during replication
- Each original strand acts as a template for a new complementary strand
- Two identical DNA molecules are produced, each containing one original strand and one new strand
Key terms in this question
Related
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- Decode the mark scheme abbreviations →
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