Cell Cycle and Cell Division Class 11 Notes

The Cell Cycle

• G₁ phase (Gap 1): cell grows, organelles duplicate, proteins synthesised. The cell decides whether to divide or enter G₀ (quiescent phase — non-dividing, e.g. neurons, cardiac muscle cells).
• S phase (Synthesis): DNA replication occurs. Each chromosome is duplicated to form two sister chromatids joined at the centromere. The centrosome also duplicates. DNA content doubles (2C → 4C) but chromosome number stays the same.
• G₂ phase (Gap 2): final preparations — tubulin and other proteins for the mitotic spindle are synthesised.
• M phase (Mitosis + Cytokinesis): nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).

Mitosis

Mitosis is equational division — each daughter cell receives the same number and kind of chromosomes as the parent cell (2n → 2n). Mitosis is responsible for growth, tissue repair, and asexual reproduction. It produces two genetically identical daughter cells.

Stages of mitosis

• Prophase: chromatin condenses into visible chromosomes (each with two sister chromatids), the nuclear envelope breaks down, the nucleolus disappears, and the mitotic spindle begins to form from centrosomes.
• Metaphase: chromosomes align at the metaphase plate (equatorial plane). Spindle fibres from opposite poles attach to the kinetochores of sister chromatids.
• Anaphase: centromeres split; sister chromatids are pulled to opposite poles by shortening of spindle fibres. Each chromatid is now an independent chromosome.
• Telophase: chromosomes decondense, nuclear envelopes re-form, nucleoli reappear, and the spindle disassembles. This is essentially the reverse of prophase.

Cytokinesis follows karyokinesis. In animal cells a cleavage furrow pinches the cell in two; in plant cells a cell plate forms from the centre outward because of the rigid cell wall.

Meiosis

Meiosis is a special type of cell division that reduces the chromosome number by half (2n → n) to produce gametes (spores in plants). It involves two successive divisions — Meiosis I and Meiosis II — with no DNA replication between them. A single diploid cell produces four haploid daughter cells.

Meiosis I — Reduction division

Meiosis I separates homologous chromosomes (not sister chromatids) and halves the chromosome number. Prophase I is the longest and most complex phase of meiosis, divided into five substages:

• Leptotene: chromosomes begin to condense and become visible as thin threads.
• Zygotene: homologous chromosomes pair up (synapsis) to form bivalents (tetrads). The synaptonemal complex holds them together.
• Pachytene: crossing over (recombination) occurs — non-sister chromatids of a bivalent exchange segments at chiasmata. This generates genetic variation.
• Diplotene: the synaptonemal complex dissolves; homologues begin to separate but remain attached at chiasmata (visible as X-shaped intersections).
• Diakinesis: chromosomes are fully condensed; chiasmata terminalise; the nuclear envelope breaks down.

In metaphase I, bivalents align at the metaphase plate (not individual chromosomes). The orientation of each bivalent is random — independent assortment. In anaphase I, homologous chromosomes separate to opposite poles; sister chromatids remain joined. In telophase I, two haploid cells form.

Meiosis II — Equational division

Meiosis II resembles mitosis: sister chromatids separate. In prophase II the chromosomes condense again; in metaphase II individual chromosomes align at the plate; in anaphase II centromeres split and chromatids move to opposite poles; in telophase II four haploid nuclei are formed, followed by cytokinesis.

Significance of Meiosis

• Reduction: halves the chromosome number from diploid (2n) to haploid (n), ensuring that when two gametes fuse at fertilisation, the zygote restores the diploid number.
• Crossing over in prophase I creates new combinations of alleles on a single chromosome — genetic recombination.
• Independent assortment of bivalents at metaphase I means each gamete gets a unique mix of maternal and paternal chromosomes.
• Together, crossing over and independent assortment are the basis of genetic variation in sexually reproducing organisms — the raw material for evolution.

Mitosis vs Meiosis — Key Differences

• Mitosis: one division → two daughter cells; Meiosis: two divisions → four daughter cells.
• Mitosis: daughter cells are diploid (2n) and genetically identical to parent; Meiosis: daughter cells are haploid (n) and genetically different.
• Mitosis: no crossing over; Meiosis: crossing over occurs in prophase I.
• Mitosis: occurs in somatic cells for growth and repair; Meiosis: occurs in germ cells to produce gametes/spores.
• Mitosis: homologous chromosomes do not pair; Meiosis I: homologues pair as bivalents (synapsis).

Ploidy and Chromosome Number — Quick Reference

Ploidy refers to the number of sets of chromosomes. Diploid (2n) cells have two sets — one from each parent. Haploid (n) cells have one set. In humans 2n = 46, so n = 23. After S phase the DNA content doubles (4C) but the chromosome count remains 2n because sister chromatids share a centromere. In anaphase of mitosis, chromatids separate and the transient cell is temporarily 4n.

Solved Examples

Example: A cell with 2n = 14 chromosomes enters meiosis. How many chromosomes and chromatids are present in each daughter cell?

Solution: After meiosis I, each cell is haploid — n = 7 chromosomes, each with 2 chromatids (14 chromatids per cell). After meiosis II, sister chromatids separate, giving four daughter cells each with n = 7 chromosomes and 7 chromatids (since each chromosome is now a single chromatid). Total DNA content per cell is 1C.

Example: If a diploid organism has 2n = 8 chromosomes, how many bivalents form during meiosis I and how many tetrads are visible at pachytene?

Solution: With 2n = 8, there are 4 homologous pairs. Each pair forms one bivalent, so 4 bivalents form. Each bivalent is a tetrad (4 chromatids), so 4 tetrads are visible at pachytene. Crossing over between non-sister chromatids of each bivalent generates recombinant chromosomes.