Mitosis and the cell cycle · Grade 12
In Grade 12 you get fluent with: 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
- 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
In a root tip squash, 40 of the 500 cells seen are in mitosis. Calculate the mitotic index.
Answer: 0.08 (8%)
Example 2
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 3
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 4
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