8876 / 2027

Lesson 5 of 9 / Mutation, cell division and cancer

Copy DNA once, then divide

When do DNA amount and chromosome number change?

I can describe interphase and the mitotic cell cycle and explain its significance.

  • I can describe the mitotic cycle and principal mitotic stages.Syllabus

    Syllabus 8876, Core 2(g). Describe the mitotic cycle and principal mitotic stages.

  • I can explain mitotic-cycle significance and the need for tight regulation.Syllabus

    Syllabus 8876, Core 2(h). Explain mitotic-cycle significance and the need for tight regulation.

About 5 min

Make a guess. You are not marked.

Interphase is often called the "resting" phase. Is the cell resting?

  1. It is; nothing happens until mitosis.
  2. It grows and copies its DNA.
  3. It rests apart from making organelles.
Show the answer

It grows and copies its DNA.

Interphase includes growth, DNA replication in S phase and preparation for division.

The key idea

Interphase 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

2CDNA amountPer cell at the selected point

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. 1

    Grow and replicate

    G1 supports cell activity; S duplicates DNA; G2 prepares for division.

  2. 2

    Distribute copies

    Mitosis separates sister chromatids into corresponding daughter nuclei.

  3. 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.

Watch out for this

A student says: "Interphase is a period when the cell does nothing." What is wrong with this?

Show the answer

Interphase includes active metabolism, growth, DNA replication and preparation. It occupies much of the cycle.

Check your understanding

Which event normally happens before mitosis?

  1. DNA replication during S phase.
  2. A second round of replication between every mitotic stage.
  3. Separation of sister chromatids during G1.
Show the answer

DNA replication during S phase.

The duplicated material is then distributed to daughter nuclei.

Quick recall

Card 1 of 3. Answer in your head, then check.

When do DNA amount and chromosome number change?

You can return to this lesson any time.