Learning Objectives
- Understand the phases of the cell cycle
- Learn the stages and significance of mitosis
- Learn the stages and significance of meiosis
- Differentiate between mitosis and meiosis
Key Concepts
Cell Cycle
The sequence of events from one cell division to the next. Duration varies: yeast ~90 min, human cells ~24 hours. Two main phases: Interphase (95% of cell cycle) and M phase (Mitotic phase — 5%).
Interphase: (1) G1 phase (Gap 1): Cell growth, protein synthesis, organelle production. (2) S phase (Synthesis): DNA replication occurs; chromosome number remains same but DNA content doubles (2n → 2n, but 2C → 4C). (3) G2 phase (Gap 2): Preparation for division, continued protein synthesis. Cells that do not divide enter G0 phase (quiescent phase) — e.g., neurons, mature RBCs.
Mitosis
Equational division — daughter cells have the same chromosome number as parent. Occurs in somatic cells. Four stages:
Prophase: Chromatin condenses into chromosomes (each with 2 sister chromatids joined at centromere). Centrioles move to opposite poles. Nucleolus and nuclear membrane start disappearing. Spindle fibres begin to form.
Metaphase: Chromosomes align at the equatorial plate (metaphase plate). Spindle fibres attach to kinetochores at centromeres. Chromosomes are most visible — best stage for karyotyping.
Anaphase: Centromeres split. Sister chromatids separate and move to opposite poles (shortest phase). Chromosomes appear V-shaped (metacentric), L-shaped (submetacentric), or J-shaped (acrocentric).
Telophase: Chromosomes decondense. Nuclear membrane and nucleolus reappear. Spindle fibres disappear. Two nuclei formed.
Cytokinesis: Division of cytoplasm. In animal cells: cleavage furrow (centripetal). In plant cells: cell plate formation (centrifugal, from centre outward).
Meiosis
Reductional division — chromosome number halved (2n → n). Occurs in reproductive cells (gamete formation). Two successive divisions: Meiosis I (reductional) and Meiosis II (equational).
Meiosis I — Prophase I (longest phase of meiosis): Five substages: Leptotene (chromosomes visible as thin threads), Zygotene (homologous chromosomes pair up — synapsis, forming bivalents), Pachytene (crossing over occurs between non-sister chromatids — recombinant chromatids formed; synaptonemal complex visible), Diplotene (synaptonemal complex dissolves, chiasmata visible — points where crossing over occurred), Diakinesis (bivalents fully condensed, nuclear membrane breaks, terminalisation of chiasmata).
Metaphase I: Bivalents align at equatorial plate. Anaphase I: Homologous chromosomes separate (not sister chromatids) — reductional. Telophase I: Two haploid nuclei formed. Meiosis II: Similar to mitosis — sister chromatids separate. Result: 4 haploid cells.
Significance
Mitosis: Growth, repair, asexual reproduction, maintains chromosome number. Meiosis: Produces gametes, reduces chromosome number, crossing over introduces genetic variation (important for evolution).
Summary
The cell cycle includes interphase (G1, S, G2) and M phase. Mitosis produces two identical diploid cells for growth and repair. Meiosis produces four haploid gametes with genetic variation through crossing over and independent assortment. Prophase I of meiosis with its five substages is crucial for recombination.
Important Terms
- Synapsis: Pairing of homologous chromosomes in zygotene of meiosis I
- Crossing over: Exchange of genetic material between non-sister chromatids in pachytene
- Chiasmata: X-shaped points where crossing over has occurred, visible in diplotene
- Bivalent: Pair of homologous chromosomes (also called tetrad — 4 chromatids)
- Kinetochore: Protein structure at centromere where spindle fibres attach
- Karyotype: Organized visual profile of chromosomes; best studied at metaphase
- Cytokinesis: Division of cytoplasm following nuclear division
- G0 phase: Quiescent stage where cells exit the cell cycle
Quick Revision
- Cell cycle: G1 → S → G2 → M (Mitosis)
- S phase: DNA replication (2C → 4C); chromosome number unchanged
- Mitosis: equational; 2 diploid cells; somatic cells
- Meiosis: reductional then equational; 4 haploid cells; germ cells
- Prophase I substages: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis
- Crossing over in Pachytene; Chiasmata visible in Diplotene
- Metaphase: best for karyotyping (chromosomes most condensed)
- Cytokinesis: furrow (animal) vs cell plate (plant)