1. During which cell-cycle stage is DNA replicated?
- Cytokinesis
- Every stage equally
- S phase
Check the answer and reasoning
S phase
DNA synthesis occurs during the S phase of interphase.
Biology / AS level
The mitotic cell cycle
The cell cycle includes growth, DNA replication and division.
Learning objectives
- Trace DNA replication and mitotic division.
- Distinguish chromosome number from DNA amount.
- Connect checkpoints with controlled growth.
The big idea
The cell cycle includes growth, DNA replication and division. After replication each chromosome has two sister chromatids; mitosis separates these into daughter nuclei, normally preserving the chromosome complement.
Interphase is metabolically active rather than a resting stage. Checkpoints coordinate division with DNA condition and cell signals. Stem cells retain the capacity to divide and differentiate, while uncontrolled proliferation can result when growth regulation fails.
Worked example
A diploid cell has 12 chromosomes before replication. How many chromosomes does each normal mitotic daughter nucleus receive?
Follow the solution
Replication copies DNA; mitosis allocates one equivalent chromosome set to each daughter nucleus, retaining 12.
Investigate and explain
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
No exact PhET model is claimed for this topic. Use diagrams and supplied-data reasoning instead.
Check your understanding
2. A diploid cell has 12 chromosomes before replication. How many chromosomes does each normal mitotic daughter nucleus receive?
- 12
- 6
- 24
Check the answer and reasoning
12
Replication copies DNA; mitosis allocates one equivalent chromosome set to each daughter nucleus, retaining 12.
3. Does DNA replication immediately double the number of chromosomes counted by centromeres?
- Yes
- No
- Only in plants
Check the answer and reasoning
No
It doubles DNA amount and creates sister chromatids; chromosome number by centromere count is initially unchanged.