Lesson 5 of 9 / Mutation, cell division and cancer
Copy DNA once, then divide
When do DNA amount and chromosome number change?
In this lesson: Describe interphase and the mitotic cell cycle and explain its significance.
About 5 min
The key ideaInterphase includes growth, DNA replication and preparation. Mitosis separates duplicated genetic material and cytokinesis divides the cell.
Explore the idea
Track DNA amount through the cycle
Growth and ordinary activity. In the diploid model, chromosome number is 2n and DNA amount is 2C.
C is a relative DNA-content unit. This diagram distinguishes DNA amount from chromosome number; it does not depict the brief anaphase count within an undivided cell.
Explanation
The mitotic cell cycle includes interphase and division. During G1 the cell grows and carries out its activities. In S phase, DNA is replicated. During G2 the cell prepares for division and checks that required processes have been completed.
After replication, each chromosome consists of two sister chromatids joined at a centromere. DNA amount has doubled, but chromosome number, counted by centromeres before chromatid separation, has not. A replicated chromosome is not automatically a homologous pair.
Mitosis distributes corresponding sister-chromatid copies to two daughter nuclei. Cytokinesis divides the cytoplasm and cell boundary. The resulting cells normally retain the parental chromosome complement and closely matching genetic information, apart from mutation.
The cycle supports growth, replacement and repair, and asexual reproduction. Its timing must be regulated so that damaged or incompletely replicated DNA is not simply passed on and cell number remains appropriate to the tissue.
Step by step
- 1
Grow and replicate
G1 supports cell activity; S duplicates DNA; G2 prepares for division.
- 2
Distribute copies
Mitosis separates sister chromatids into corresponding daughter nuclei.
- 3
Divide the cell
Cytokinesis produces separate daughter cells.
Worked example
A diploid cell before mitosis
A cell has four chromosomes in G1. Immediately after S phase, how many chromosomes and chromatids does it have?
One way to explain it
Four chromosomes, each with two sister chromatids: eight chromatids in total. DNA amount has doubled, but each joined pair is still counted as one replicated chromosome at this point.
Why this answer works
- Specify the point in the cycle.
- Count centromeres and DNA copies separately.
Is this true? "Interphase is a period when the cell does nothing."
Interphase includes active metabolism, growth, DNA replication and preparation. It occupies much of the cycle.