Unit 4 Cell divisions
Key unit competence: Assess the stages of the cell cycle and their significance in cell and nuclear division.
Key ideas
- The cell cycle is the sequence of events from one division to the next: interphase (G1: growth, protein and organelle synthesis; S phase: DNA replication, each chromosome becomes two chromatids; G2: further growth, energy stores, preparation for division) followed by mitosis (nuclear division) and cytokinesis. Some cells leave the cycle into G0 (non-dividing, e.g. nerve cells). Checkpoints (G1, G2, metaphase) check DNA and chromosome attachment before continuing.
- Diploid (2n) cells have two sets of chromosomes (human 46); haploid (n) cells have one set (gametes, 23). Homologous chromosomes are pairs of the same size, centromere position and gene loci, one maternal and one paternal.
- Mitosis gives two genetically identical diploid nuclei. Prophase: chromosomes condense; nucleolus disappears; centrioles move to poles and the spindle forms; nuclear envelope breaks down. Metaphase: chromosomes line up at the equator (metaphase plate), spindle fibres attached at kinetochores of centromeres. Anaphase: centromeres split; sister chromatids pulled to opposite poles by shortening spindle fibres. Telophase: chromatids (now chromosomes) reach poles and decondense; nuclear envelopes and nucleoli re-form. Cytokinesis: cleavage furrow (animals) or cell plate (plants).
- Roles of mitosis: growth, repair and replacement of tissues, asexual reproduction and vegetative propagation, clonal expansion of lymphocytes in immunity, keeping chromosome number and genetic information constant.
- Abnormal mitosis: when checkpoints fail or proto-oncogenes mutate into oncogenes or tumour-suppressor genes (e.g. p53) are inactivated, cells divide uncontrollably, forming a tumour. Benign tumours stay in one place; malignant tumours (cancer) invade tissues and spread (metastasis). Causes: carcinogens (tobacco tar, asbestos, aflatoxin), radiation (UV, X-rays), some viruses (HPV, hepatitis B), inherited mutations. Treatments (chemotherapy, radiotherapy) target dividing cells.
- Meiosis has two divisions and produces four haploid, genetically different cells. Meiosis I (reduction division): prophase I: homologous chromosomes pair (synapsis) to form bivalents; non-sister chromatids exchange segments at chiasmata (crossing over); metaphase I: bivalents line up at the equator with random orientation (independent assortment); anaphase I: homologous chromosomes separate (centromeres do not split); telophase I: two haploid nuclei. Meiosis II is like mitosis: sister chromatids separate, giving four haploid cells.
- Significance of meiosis: halves the chromosome number so it stays constant after fertilisation; creates genetic variation by crossing over, independent assortment, and random fertilisation, which allows adaptation and evolution. Meiosis occurs in testes and ovaries (spermatogenesis and oogenesis) and in anthers and ovules.
- Errors in meiosis: non-disjunction (chromosomes fail to separate) gives gametes with an extra or missing chromosome, e.g. Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY).
- Comparison: mitosis – one division, two diploid identical cells, no pairing of homologues, no crossing over, in somatic cells. Meiosis – two divisions, four haploid different cells, homologues pair and cross over, in reproductive organs.
Common misconceptions
- "DNA is replicated during prophase." No: DNA is replicated in the S phase of interphase.
- "In anaphase I sister chromatids separate." No: in anaphase I homologous chromosomes separate; sister chromatids separate in anaphase II (and in mitosis).
- "All tumours are cancer." No: benign tumours do not invade or spread; only malignant tumours are cancer.
- "Meiosis II halves the chromosome number." No: the number is halved in meiosis I; meiosis II separates chromatids.
Worked examples
Example 1. A cell with 2n = 8 undergoes meiosis. How many chromosomes and chromatids are in each cell at the end of meiosis I and meiosis II?
Solution
End of meiosis I: 4 chromosomes, each still with 2 chromatids (8 chromatids per cell). End of meiosis II: 4 chromosomes, each a single chromatid (4 chromatids per cell).
Example 2. Explain how crossing over and independent assortment produce variation.
Solution
In crossing over (prophase I), non-sister chromatids of a bivalent exchange segments at chiasmata, producing new combinations of alleles on each chromatid. In independent assortment (metaphase I), each bivalent orients randomly, so maternal and paternal chromosomes are mixed in many combinations (2²³ in humans). Together they make every gamete genetically unique.
Practice questions
Describe the events of the three stages of interphase.
Show answer
G1: cell grows, makes proteins and organelles. S: DNA replicates; each chromosome forms two sister chromatids. G2: further growth, energy stores built, spindle proteins made; checks before mitosis.
Common mistake: Saying interphase is a resting phase.
Describe the behaviour of chromosomes in each stage of mitosis.
Show answer
Prophase: condense, become visible, spindle forms, envelope breaks down. Metaphase: line up at equator attached at centromeres. Anaphase: centromeres split, chromatids pulled to opposite poles. Telophase: chromosomes at poles decondense, envelopes form.
Common mistake: Saying homologous chromosomes separate in mitosis.
State four roles of mitosis.
Show answer
Growth; repair of tissues; replacement of worn-out cells; asexual reproduction; clonal expansion of lymphocytes; maintains genetic stability.
Common mistake: Including gamete formation.
Explain how cancer develops and name three carcinogens.
Show answer
Mutations in proto-oncogenes (to oncogenes) and tumour-suppressor genes (p53) remove control of the cell cycle; cells divide repeatedly, forming a tumour; malignant tumours invade and spread. Carcinogens: tobacco tar, UV/X-rays, asbestos, aflatoxin, HPV.
Common mistake: Saying cancer is infectious.
Describe the events of prophase I and metaphase I of meiosis.
Show answer
Prophase I: chromosomes condense; homologues pair (synapsis) forming bivalents; crossing over at chiasmata; spindle forms; envelope breaks. Metaphase I: bivalents line up at the equator with random orientation.
Common mistake: Saying chromatids separate in metaphase I.
Give five differences between mitosis and meiosis.
Show answer
One vs two divisions; two vs four daughter cells; diploid vs haploid; identical vs different; no synapsis/crossing over vs synapsis and crossing over; somatic cells vs reproductive organs.
Common mistake: Saying both form gametes.
Explain the significance of meiosis.
Show answer
Halves the chromosome number so it remains constant after fertilisation; produces genetic variation through crossing over and independent assortment, allowing adaptation and evolution.
Common mistake: Saying meiosis is for growth.
Explain how non-disjunction can cause Down syndrome.
Show answer
In meiosis the chromosome 21 pair (or chromatids) fails to separate; one gamete gets two copies of chromosome 21; fertilisation by a normal gamete gives a zygote with three copies (47 chromosomes).
Common mistake: Saying Down syndrome is caused by an infection.
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. Mitosis is important for: a. G0 2. Tar in tobacco smoke is an example of a: b. growth, repair and asexual reproduction 3. Cells that permanently stop dividing enter: c. carcinogen 4. Points where non-sister chromatids exchange segments are: d. chiasmata Show answer
1 → b (growth, repair and asexual reproduction) · 2 → c (carcinogen) · 3 → a (G0) · 4 → d (chiasmata)
- It keeps genetic material constant.
- It causes mutations.
- Example: mature nerve cells.
- Crossing over happens there.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
oncogenes · loss of the X chromosome · trisomy of chromosome 21 · non-disjunction- Genes that cause cancer when mutated from normal growth genes are .
- Failure of chromosomes to separate in meiosis is .
- Down syndrome results from .
Show answer
1: oncogenes · 2: non-disjunction · 3: trisomy of chromosome 21
- They come from proto-oncogenes.
- It causes trisomies.
- Cells have 47 chromosomes.
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. Spindle fibres attach to chromosomes at the: a. sister chromatids move to opposite poles 2. DNA replication takes place during: b. kinetochores of centromeres 3. Chromosomes line up at the equator during: c. metaphase 4. In anaphase of mitosis: d. S phase of interphase Show answer
1 → b (kinetochores of centromeres) · 2 → d (S phase of interphase) · 3 → c (metaphase) · 4 → a (sister chromatids move to opposite poles)
- They pull chromatids apart.
- Each chromosome then has two chromatids.
- The metaphase plate.
- Centromeres split.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
chiasma · p53 · 47 chromosomes (trisomy) · cell plate- The tumour-suppressor gene often mutated in cancers is .
- In plant cells, cytokinesis forms a .
- A gamete with 24 chromosomes fuses with a normal human gamete. The zygote has .
Show answer
1: p53 · 2: cell plate · 3: 47 chromosomes (trisomy)
- It stops damaged cells dividing.
- It becomes the new wall.
- 24 + 23 = 47.