Unit 5 Mitosis and meiosis
Key unit competence: Explain the cell cycle and compare mitosis and meiosis and their significance.
Key ideas
- Chromosomes are thread-like structures in the nucleus made of DNA and protein; they carry genes. Human body cells have 46 chromosomes (23 pairs): they are diploid (2n). Gametes have 23: they are haploid (n). Chromosomes of a pair are homologous.
- The cell cycle is the sequence of events from one cell division to the next: interphase (the longest stage: the cell grows, makes organelles and proteins, and replicates its DNA, so each chromosome becomes two chromatids joined at a centromere), then mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).
- Mitosis produces two genetically identical diploid daughter cells. Stages (PMAT): Prophase: chromosomes condense and become visible, the nuclear membrane breaks down, spindle forms. Metaphase: chromosomes line up on the equator, attached to spindle fibres by centromeres. Anaphase: centromeres split, sister chromatids are pulled to opposite poles. Telophase: nuclear membranes re-form around two sets of chromosomes; cytokinesis follows.
- Cytokinesis: in animal cells the membrane pinches in (cleavage furrow); in plant cells a cell plate forms in the middle and becomes a new cell wall.
- Importance of mitosis: growth, repair of damaged tissues (wounds, broken bones), replacement of worn-out cells (skin, blood), and asexual reproduction (e.g. binary fission, vegetative propagation). In plants it occurs in meristems (root and shoot tips).
- Meiosis is a reduction division that produces four haploid cells, each genetically different, used as gametes (sperm and eggs in humans; pollen and ovules in plants). It occurs in the testes and ovaries (anthers and ovules in flowers).
- Meiosis has two divisions: meiosis I separates homologous chromosomes (halves the chromosome number); meiosis II separates sister chromatids (like mitosis). In prophase I homologous chromosomes pair and may exchange segments (crossing over).
- Importance of meiosis: keeps the chromosome number constant from generation to generation (n + n = 2n at fertilisation), and produces genetic variation through crossing over and independent assortment of chromosomes.
- Uncontrolled mitosis forms a tumour; malignant tumours (cancer) invade other tissues. Risk factors: tobacco, radiation, some chemicals and viruses (e.g. HPV).
Common misconceptions
- "Mitosis halves the chromosome number." No: mitosis keeps the number the same; meiosis halves it.
- "Interphase is a resting stage." No: the cell is very active, growing and copying its DNA.
- "Meiosis happens in all body cells." No: meiosis occurs only in reproductive organs to make gametes.
- "Daughter cells from meiosis are identical." No: crossing over and independent assortment make them genetically different.
Worked examples
Example 1. A cell with 8 chromosomes divides by mitosis and another by meiosis. State the number and type of daughter cells in each case.
Solution
Mitosis: 2 daughter cells, each with 8 chromosomes (diploid), genetically identical. Meiosis: 4 daughter cells, each with 4 chromosomes (haploid), genetically different.
Example 2. Explain why meiosis is necessary for sexual reproduction.
Solution
Gametes must be haploid so that when a sperm (n) fuses with an egg (n), the zygote has the normal diploid number (2n). Without meiosis the chromosome number would double every generation. Meiosis also creates variation.
Practice questions
Define: (a) chromosome; (b) diploid; (c) haploid; (d) homologous chromosomes.
Show answer
(a) A thread-like structure of DNA and protein in the nucleus carrying genes. (b) Having two sets of chromosomes (2n). (c) Having one set (n). (d) A pair of chromosomes of the same size and shape carrying genes for the same characters, one from each parent.
Common mistake: Saying homologous chromosomes are identical copies.
Describe the events of each stage of mitosis.
Show answer
Prophase: chromosomes condense, nuclear membrane breaks, spindle forms. Metaphase: chromosomes line up at equator. Anaphase: chromatids pulled to poles. Telophase: nuclear membranes re-form; cytokinesis divides the cell.
Common mistake: Putting the stages in the wrong order.
Give three differences between mitosis and meiosis.
Show answer
Mitosis: one division, two daughter cells, diploid, identical, for growth/repair. Meiosis: two divisions, four cells, haploid, different, for gametes; crossing over occurs.
Common mistake: Saying mitosis produces gametes.
State three importances of mitosis.
Show answer
Growth of organisms; repair of tissues; replacement of worn-out cells; asexual reproduction.
Common mistake: Listing variation as a role of mitosis.
State two importances of meiosis.
Show answer
Keeps the chromosome number constant across generations; produces genetic variation; produces gametes.
Common mistake: Saying meiosis is for growth.
Explain what happens during interphase.
Show answer
The cell grows, makes proteins and organelles, stores energy, and replicates its DNA so each chromosome has two sister chromatids.
Common mistake: Calling interphase a resting stage.
Explain how cytokinesis differs in plant and animal cells.
Show answer
Animal cells: the cell membrane pinches inward (cleavage furrow). Plant cells: a cell plate forms in the middle and becomes a new cell wall.
Common mistake: Saying plant cells pinch in.
State three factors that increase the risk of cancer.
Show answer
Tobacco smoke; radiation (UV, X-rays); some chemicals (asbestos, benzene); some viruses (HPV, hepatitis B); unhealthy diet.
Common mistake: Saying cancer is caused by mosquito bites.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Cells with two sets of chromosomes are: a. two identical diploid cells 2. Mitosis produces: b. telophase 3. The nuclear membrane re-forms during: c. 46 4. The number of chromosomes in a normal human body cell is: d. diploid Show answer
1 → d (diploid) · 2 → a (two identical diploid cells) · 3 → b (telophase) · 4 → c (46)
- Body cells are 2n.
- Number and genes stay the same.
- Two new nuclei form.
- 23 pairs.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
making gametes · crossing over · growth and repair of tissues · interphase is the longest stage- An onion root tip is stained and viewed. Most cells are in interphase because .
- Mitosis is important for .
- Exchange of segments between homologous chromosomes is .
Show answer
1: interphase is the longest stage · 2: growth and repair of tissues · 3: crossing over
- Fewer cells are caught in the short stages.
- It produces identical cells.
- It happens in prophase I.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. In plant cells, cytokinesis takes place by forming a: a. cell plate 2. Chromatids are pulled to opposite poles during: b. cytokinesis 3. Division of the cytoplasm is: c. diploid 4. A zygote formed at fertilisation is: d. anaphase Show answer
1 → a (cell plate) · 2 → d (anaphase) · 3 → b (cytokinesis) · 4 → c (diploid)
- The rigid wall cannot pinch in.
- Centromeres split.
- It follows telophase.
- n + n = 2n.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
homologous chromosomes · interphase · four different haploid cells · sister chromatids- Meiosis produces .
- The longest stage of the cell cycle is .
- Two identical copies of a chromosome joined at a centromere are .
Show answer
1: four different haploid cells · 2: interphase · 3: sister chromatids
- It is a reduction division.
- The cell grows and copies DNA.
- They separate in anaphase of mitosis.