Mitosis and the cell cycle · Year 11
In Year 11 you get fluent with: chromosomes and mitosis.
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.
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?
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.
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.
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?
Answer: 64
Example 5
Why is the DNA copied before a cell divides by mitosis?
Answer: so that each new cell gets a full set of chromosomes