Mitosis and the cell cycle · Grade 11
In Grade 11 you learn: timing 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?
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
Example 6
In which stage of mitosis does this happen: 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?
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.
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.
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.
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?
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