Definition
Interphase has three sub-phases: (cell grows, carries out normal functions), (DNA/chromosomes duplicate), (final growth and preparation before division). Most of a cell's life is spent in interphase, not actually dividing.
Rule
Prophase: chromatin condenses into visible chromosomes; spindle fibres begin to form from the centrosomes.
Metaphase: chromosomes line up at the cell's centre (equator).
Anaphase: sister chromatids separate and are pulled to opposite poles by spindle fibres.
Telophase: nuclear membranes reform around each set of chromosomes, which uncoil back into chromatin.
Cytokinesis (division of the cytoplasm) follows telophase — in animal cells the membrane pinches inward; in plant cells a rigid cell plate forms down the middle (since the cell wall can't simply pinch).
Remember
Mitosis produces two genetically identical cells — it is the basis of growth and of regeneration (replacing damaged or worn-out cells), which is why skin cells divide continuously but highly specialized nerve and muscle cells rarely do.
Definition
Meiosis produces sex cells (gametes) with half the chromosome number of the parent cell, through two successive divisions (Meiosis I and Meiosis II).
Crossing over (in prophase I) exchanges segments between homologous chromosomes — the main source of genetic variation between offspring.
Example 1 — chromosome number after meiosis I
An organism has pairs of chromosomes ( total). How many chromosomes and chromatids are in each cell right after Meiosis I?
- Meiosis I separates homologous pairs, halving the chromosome number: chromosomes per cell.
- Each of those chromosomes still consists of sister chromatids (they don't separate until Meiosis II): chromatids.
Answer: chromosomes, chromatids per cell
Remember
Meiosis II is needed because Meiosis I only separates homologous pairs — the sister chromatids of each chromosome are still joined, and Meiosis II (which proceeds much like mitosis) separates them.
Quick Recap
- ●Interphase: (growth), (DNA duplicates), (final growth)
- ●Mitosis: Prophase (chromosomes condense) Metaphase (line up) Anaphase (chromatids separate) Telophase (nuclei reform)
- ●Cytokinesis: membrane pinches in animals; a cell plate forms in plants
- ●Mitosis: 2 identical cells, growth/repair. Meiosis: gametes with half the chromosomes, via two divisions
- ●Crossing over in Meiosis I creates genetic variation; Meiosis II separates sister chromatids
