Mitosis and the cell cycle

Chromosomes are long molecules of DNA in the nucleus, and in body cells they come in pairs. In the cell cycle a cell grows, copies its DNA, then divides by mitosis: one copy of each chromosome goes to each end, and the cell splits in two. The two new cells are genetically identical to the parent cell. Mitosis is used for growth, repair and asexual reproduction.

Key facts

  • A chromosome is a long molecule of DNA carrying many genes. In body cells chromosomes come in pairs: humans have 23 pairs, 46 chromosomes in all.
  • The cell cycle: the cell grows and makes more ribosomes and mitochondria; the DNA replicates, so each chromosome has two copies; in mitosis one set of chromosomes is pulled to each end of the cell; then the cytoplasm and cell membrane divide, making two identical cells.
  • Mitosis is used for growth, repair and asexual reproduction. The new cells are genetically identical to the parent cell.
  • The stages of mitosis: prophase (the chromosomes condense), metaphase (they line up across the middle), anaphase (the two copies of each chromosome are pulled to opposite ends) and telophase (a new nucleus forms at each end). Then the cell splits in two.
  • The time a cell spends in a stage is proportional to how many cells are seen in it: time = (cells in the stage ÷ total cells) × length of the cycle. Mitotic index = cells in mitosis ÷ total cells.

Worked examples

Example 1

A human body cell has 46 chromosomes. How many pairs is that, and how many chromosomes are in each cell made when it divides by mitosis?

  1. Chromosomes in body cells come in pairs. 46 ÷ 2 = 23 pairs
  2. Before mitosis each chromosome is copied, so each new cell gets a full set.
  3. So each new cell has 46 chromosomes, the same as the parent cell.

Answer: 23 pairs; 46 in each new cell

Example 2

Put the stages of the cell cycle in order: the cytoplasm and cell membrane divide; the cell grows and copies its DNA; mitosis.

  1. First the cell grows, makes more organelles such as ribosomes and mitochondria, and copies its DNA.
  2. In mitosis, one set of chromosomes is pulled to each end of the cell and new nuclei form.
  3. Finally the cytoplasm and cell membrane divide to make two identical cells.

Answer: the cell grows and copies its DNA → mitosis → the cytoplasm and cell membrane divide

Example 3

Give two reasons why cells in your body divide by mitosis.

  1. A child grows because cells divide to make more cells.
  2. When skin is cut, nearby cells divide to replace the damaged ones.
  3. The new cells are identical to the old ones, so they do the same job.

Answer: growth, and repairing damaged tissue (for example, healing a cut)

Example 4

A bacterium-like cell divides every 20 minutes. Starting from one cell, how many cells are there after 2 hours?

  1. 2 hours = 120 minutes. 120 ÷ 20 = 6 divisions
  2. The number doubles at every division. 2^6 = 64
  3. So there are 64 cells.

Answer: 64

Example 5

Why is the DNA copied before a cell divides by mitosis?

  1. Each chromosome is copied.
  2. One copy goes into each new cell, so both have a full set.

Answer: so that each new cell gets a full set of chromosomes

Example 6

In which stage of mitosis does this happen: the chromosomes line up across the middle of the cell?

  1. In metaphase, the chromosomes line up across the middle of the cell.

Answer: metaphase

Example 7

The diagram shows a cell dividing by mitosis. Which stage is it, and what happens next?

  1. The chromosomes are lined up across the middle of the cell.
  2. So this is metaphase.
  3. Next the spindle fibres pull the two copies of each chromosome apart (anaphase).
  4. Then a nucleus forms at each end and the cell divides into two identical cells.

Answer: metaphase; next, in anaphase, the copies of each chromosome are pulled to opposite ends

Example 8

In a root tip squash, 40 of the 500 cells seen are in mitosis. Calculate the mitotic index.

  1. Mitotic index = cells in mitosis ÷ total number of cells. 40 ÷ 500 = 0.08
  2. As a percentage, 8% of the cells are dividing.

Answer: 0.08 (8%)

Example 9

In a root tip, 200 cells were counted and 10 were in metaphase. The cell cycle lasts 24 hours. Estimate how long metaphase lasts.

  1. Fraction of cells in metaphase: 10 ÷ 200 = 0.05
  2. The cycle in minutes: 24 × 60 = 1440
  3. Multiply. 0.05 × 1440 = 72
  4. So metaphase lasts about 72 minutes.

Answer: 72 minutes

Example 10

The cell cycle in some root tip cells lasts 20 hours. In a sample, 25 of 200 cells are in prophase. Estimate how long prophase lasts, in minutes.

  1. The fraction of cells in a stage shows the fraction of the cycle spent in it. 25 ÷ 200 = 0.125
  2. Multiply by the length of the cycle. 0.125 × 20 = 2.5 hours
  3. Change to minutes. 2.5 × 60 = 150 minutes

Answer: 150 minutes

Example 11

When preparing a root tip squash to see mitosis, why do you use only the tip, why do you add a stain, and why do you squash it?

  1. Mitosis happens in the meristem, just behind the very tip of the root.
  2. A stain such as toluidine blue colours the chromosomes, which are otherwise almost invisible.
  3. Squashing makes a layer one cell thick, so the cells don't overlap.

Answer: only the tip has dividing (meristem) cells; the stain makes the chromosomes visible; squashing spreads the cells into one thin layer so light passes through