Notes
The cell cycle
- Interphase: G1 (growth), S (DNA copied), G2 (prepare). Most of a cell's life.
- Mitosis (PMAT) then cytokinesis.
- Each copied chromosome has two sister chromatids joined at a centromere.
Mitosis phases
| Phase | What happens |
|---|---|
| Prophase | Chromosomes condense, nuclear envelope breaks down, spindle forms |
| Metaphase | Chromosomes line up in the middle |
| Anaphase | Sister chromatids pulled apart |
| Telophase | Two nuclei form |
Control and cancer
Checkpoints, cyclins and growth factors control division. Cancer is uncontrolled division caused by mutations. Malignant tumors can spread (metastasis).
Meiosis
- Makes gametes. Diploid ($2n$) to haploid ($n$). Humans: $2n = 46$, $n = 23$.
- Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
- Crossing over and independent assortment create variation.
Mitosis vs meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells made | 2 diploid | 4 haploid |
| Genetically | Identical | Different |
| Purpose | Growth, repair | Gametes |
Topics in this unit
Each topic has its own short notes and practice.
Practice
In anaphase, sister chromatids are pulled apart.
DNA is copied during S phase.
Meiosis halves the chromosome number.
Without spindle fibers, rapidly dividing cancer cells cannot complete mitosis.
Two haploid gametes ($n + n$) restore the diploid number $2n$.
Pairing and crossing over occur only in prophase I of meiosis.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A student looks at 200 cells from an onion root tip under a microscope and counts how many are in each stage: interphase 160, prophase 20, metaphase 8, anaphase 4, telophase 8.
(a) Which stage takes the longest? Use the data to explain.
Show the answer and scoring guide
Interphase takes the longest. 160 of 200 cells (80%) were in interphase. The more time a stage takes, the more cells you catch in that stage at one moment.
Check off the points you earned:
(b) Describe what happens to the chromosomes during metaphase and anaphase.
Show the answer and scoring guide
In metaphase, chromosomes line up along the middle of the cell, attached to spindle fibers. In anaphase, the sister chromatids separate and are pulled to opposite ends of the cell.
Check off the points you earned:
(c) Onion root tip cells were used instead of onion pollen cells. Explain why root tips are a good place to study mitosis, and why pollen would show meiosis results instead.
Show the answer and scoring guide
Root tips grow quickly, so many cells are dividing by mitosis to make new body cells. Pollen is formed by meiosis in the flower and carries the plant's sperm, so its cells are haploid rather than ordinary body cells.
Check off the points you earned: