Study sheet: Cell Division: Mitosis and Meiosis

📋 Course Outline

1. Cell Cycle and Genetic Material
2. Chromosome and Chromatid Counting
3. Mitotic Phases and Cytokinesis
4. Meiotic Divisions
5. Meiotic Genetic Mixing
6. Mitosis and Meiosis Compared
7. Cellular Contexts of Meiosis

📖 1. Cell Cycle and Genetic Material

🔑 Key Concepts & Definitions

• Cell cycle : The sequence of events by which a mother cell grows, duplicates its genetic material, and distributes it into forming daughter cells.
• Semi-conservative replication : Changes each chromosome from one to two chromatids without changing the chromosome number.

★ Must-know

📌 The cell cycle comprises interphase and phase M, with interphase including G1, S, and G2, while phase M includes mitosis and cytokinesis.

Further detail

• G0 is a quiescent phase in which the cell does not divide but may carry out other cellular activities.

💡 Memory Hook

Growth → duplication → distribution

📖 2. Chromosome and Chromatid Counting

★ Must-know

📌 Homologous chromosomes are the paired chromosomes of the same type, whereas sister chromatids are the two copies of one replicated chromosome.

• In humans, mitosis produces two cells with 46 chromosomes carrying one chromatid each, whereas meiosis produces four cells with 23 chromosomes carrying one chromatid each.

Further detail

📐 Formula — In the notation $An = B$, A represents the number of homologous chromosome sets and B represents the total number of chromatids.

• Chromosome and chromatid numbers can be followed at every stage by identifying whether chromosomes are replicated and whether homologues or sister chromatids have been separated.

💡 Memory Hook

Homologues are chromosome pairs; sister chromatids are copied halves

📖 3. Mitotic Phases and Cytokinesis

★ Must-know

• 🔄 Mitosis proceeds through:

  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase
  6. Cytokinesis

• During prometaphase, the nuclear envelope disassembles through lamin phosphorylation and kinetochores capture chromosomes.

• During metaphase, chromosomes align on the equatorial plate at equal distance from the two spindle poles.

• During anaphase, sister chromatids separate and migrate toward opposite poles.

• During telophase and cytokinesis, chromosomes decondense, nuclear envelopes reform, and an actin–myosin contractile ring separates the cytoplasm.

Further detail

• During prophase, chromatin condenses, centrosomes migrate, and the microtubule spindle begins to form.

💡 Memory Hook

Prophase → prometaphase → metaphase → anaphase → telophase

📖 4. Meiotic Divisions

🔑 Key Concepts & Definitions

• Meiosis : A process specific to germinal cells that consists of two successive divisions preceded by a single replication.

★ Must-know

📌 Meiosis I is reductional because it separates homologous chromosomes and changes 2n to n, whereas meiosis II is equational because it separates sister chromatids.

• Meiosis produces four haploid cells from one diploid mother cell after one replication and two divisions.

Further detail

• During prophase I, homologous chromosomes pair into bivalents before the later meiotic divisions.

💡 Memory Hook

Meiosis I separates homologues; meiosis II separates sister chromatids

📖 5. Meiotic Genetic Mixing

🔑 Key Concepts & Definitions

• Intrachromosomal mixing : Results from crossing-over between homologous chromosomes during prophase I.
• Interchromosomal mixing : Results from the random distribution of homologous chromosomes during anaphase I.

★ Must-know

• Crossing-over involves exchange between homologous chromosomes of the same pair and is associated with chiasmata and genetic alleles.

Further detail

• Intrachromosomal and interchromosomal mixing both contribute to the genetic diversity of gametes.

💡 Memory Hook

Crossing-over and random assortment → gamete diversity

📖 6. Mitosis and Meiosis Compared

📝 Essential Points

📌 Mitosis occurs in somatic cells and produces two daughter cells, whereas meiosis occurs in germinal cells and produces four daughter cells.

📌 Mitosis preserves the diploid chromosome number in daughter cells, whereas meiosis produces haploid daughter cells.

• A human mitotic cycle changes one cell with 46 replicated chromosomes into two cells with 46 chromosomes carrying one chromatid each.

• A human meiotic process changes one cell with 46 replicated chromosomes into four cells with 23 chromosomes carrying one chromatid each.

💡 Memory Hook

Mitosis preserves chromosome number; meiosis halves it

📖 7. Cellular Contexts of Meiosis

★ Must-know

• Meiosis is associated with the formation of gametes, and fusion of two haploid gametes restores a diploid zygote.

Further detail

• The meiotic products are incorporated into gamete formation in spermatogenesis and oogenesis.

📊 Synthesis Tables

Mitosis versus meiosis

Dimension	Mitosis	Meiosis
Cell type	Somatic cells	Germinal cells
Replications and divisions	One replication and one division	One replication and two divisions
Daughter cells	Two	Four
Chromosome number	Maintained	Reduced from 2n to n
Specific events	Sister-chromatid separation	Homologue separation, crossing-over, and random assortment