AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
DNA replication occurs during the interphase of the cell cycle.
Full specification wording
All cells arise from other cells: DNA replication occurs during the interphase of the cell cycle.
DNA replication occurs during the S (synthesis) phase of interphase, strictly before mitosis begins.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 3 of 14
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
DNA replication occurs during the S (synthesis) phase of interphase, strictly before mitosis begins. During this phase, the mass of DNA in the cell doubles, but the chromosome number remains the same. Each chromosome replicates to form two identical sister chromatids joined at a centromere. This ensures that when the cell divides during mitosis, each daughter cell receives an exact copy of the genetic material, maintaining genetic stability. For example, a human cell entering interphase has 46 chromosomes; after S phase, it still has 46 chromosomes, but comprises 92 chromatids. If replication is inhibited, for instance by chemotherapy drugs, the cell cannot progress to mitosis.
What earns the marks
- State that DNA replication occurs during interphase, specifically the S (synthesis) phase, before mitosis.
- Explain that the mass or quantity of DNA doubles during interphase, but the chromosome number remains constant.
- Describe the product of replication as a chromosome consisting of two identical sister chromatids joined by a centromere.
- Explain that this process ensures each daughter cell receives genetically identical copies of DNA following cell division.
Common mistakes and how to put them right
Stating that the chromosome number doubles during interphase.
Put it right: The mass of DNA doubles, but the chromosome number remains the same until the chromatids separate in anaphase.
Placing DNA replication in prophase.
Put it right: Replication occurs strictly during interphase, before the stages of mitosis begin.
Confusing chromatids with homologous chromosomes.
Put it right: Sister chromatids are identical copies joined at a centromere, whereas homologous chromosomes are pairs of chromosomes (one maternal, one paternal) that are not identical.
Exam tips
- When interpreting graphs of DNA mass during the cell cycle, look for the doubling of DNA mass to identify the S phase of interphase.
- Always distinguish clearly between 'chromosomes' and 'chromatids' in your answers to avoid losing marks on cell cycle questions.
Try the lesson
The start of the All cells arise from other cells: DNA replication occurs during the interphase of the cell cycle. lesson, made from checked teaching notes for this specification point.
Have a go first
During which phase of the cell cycle does DNA replication occur?
Listen
DNA replication occurs during the S (synthesis) phase of interphase, strictly before mitosis begins. During this phase, the mass of DNA in the cell doubles, but the chromosome number remains the same. Each chromosome replicates to form two identical sister chromatids joined at a centromere. This ensures that when the cell divides during mitosis, each daughter cell receives an exact copy of the genetic material, maintaining genetic stability. For example, a human cell entering interphase has 46 chromosomes; after S phase, it still has 46 chromosomes, but comprises 92 chromatids. If replication is inhibited, for instance by chemotherapy drugs, the cell cannot progress to mitosis. A strong answer earns marks for these points. State that DNA replication occurs during interphase, specifically the S (synthesis) phase, before mitosis. Explain that the mass or quantity of DNA doubles during interphase, but the chromosome number remains constant. Describe the product of replication as a chromosome consisting of two identical sister chromatids joined by a centromere. Explain that this process ensures each daughter cell receives genetically identical copies of DNA following cell division. A common slip: Stating that the chromosome number doubles during interphase. Instead, The mass of DNA doubles, but the chromosome number remains the same until the chromatids separate in anaphase. Exam tip: When interpreting graphs of DNA mass during the cell cycle, look for the doubling of DNA mass to identify the S phase of interphase.
See it
Key words
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Interphase and S phase
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Sister chromatids and centromeres
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DNA mass versus chromosome number
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See every definition
- Interphase and S phase
- The stage of the cell cycle during which DNA replication occurs, specifically in the S (synthesis) phase, strictly before mitosis begins.
- Sister chromatids and centromeres
- Each chromosome replicates to form two identical sister chromatids joined at a centromere.
- DNA mass versus chromosome number
- During the S phase the mass of DNA in the cell doubles, but the chromosome number remains the same.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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