Cell Cycle and Cell Division — NCERT Solutions

Concept: Meiosis produces four haploid daughter cells, but their sizes may or may not be equal depending on the type of cell undergoing meiosis.

Equal-sized daughter cells from meiosis:

• Spermatogenesis (formation of sperms in males): Meiosis of a primary spermatocyte produces four equal-sized spermatids, which then differentiate into spermatozoa.
• Microspore formation in plants (pollen grains): The four microspores formed from a microspore mother cell are equal in size.

Unequal-sized daughter cells from meiosis:

• Oogenesis (formation of eggs in females): Meiosis of a primary oocyte produces one large egg cell (ovum) and three small polar bodies. The unequal cytokinesis ensures that the egg retains most of the cytoplasm and nutrients.
• Meiosis I → one secondary oocyte (large) + one first polar body (small)
• Meiosis II → one mature ovum (large) + one second polar body (small)
• Megasporogenesis in plants: The megaspore mother cell undergoes meiosis to form four megaspores, of which typically one is large and functional and the other three degenerate.

Conclusion: Equal-sized products → spermatogenesis; Unequal-sized products → oogenesis.

Concept: Both anaphase of mitosis and anaphase I of meiosis involve movement of chromosomes to opposite poles, but the nature of what moves is fundamentally different.

Key distinction: In anaphase of mitosis, centromeres split and sister chromatids move apart. In anaphase I of meiosis, centromeres do NOT split; instead, whole homologous chromosomes (each still consisting of two chromatids) move to opposite poles.

Concept: Mitosis and meiosis are two types of cell division with fundamentally different purposes and outcomes.

Concept: Meiosis is the special type of cell division that occurs in sexually reproducing organisms for the formation of gametes.

Significance of Meiosis:

1. Maintenance of chromosome number: Meiosis reduces the chromosome number by half (from $2N$ to $N$) in gametes. When two gametes fuse during fertilisation, the original diploid chromosome number ($2N$) is restored. This ensures that the chromosome number remains constant from generation to generation in a species.

$$2N \xrightarrow{\text{Meiosis}} N \xrightarrow{\text{Fertilisation}} 2N$$

2. Genetic variation: Meiosis introduces genetic variability through:

• Crossing over during pachytene of prophase I → new combinations of alleles (recombination).
• Independent assortment of homologous chromosomes during metaphase I → different combinations of maternal and paternal chromosomes in gametes.

3. Role in evolution: The genetic variations produced by meiosis provide the raw material for natural selection and evolution.

4. Formation of gametes: Meiosis is essential for the production of haploid gametes (sperms and eggs) required for sexual reproduction.

5. Repair of errors: The process of crossing over can sometimes correct genetic errors by providing new combinations.

Conclusion: Meiosis is crucial for both the conservation of species-specific chromosome numbers and the generation of genetic diversity, which is the basis of evolution.

Note: This question is discussion-based. The following points provide the scientific basis for the discussion.

(i) Haploid organisms/cells where cell division (mitosis) occurs:

Mitosis can occur in haploid cells. Examples include:

• Male honeybees (drones): Drones are haploid (develop from unfertilised eggs by parthenogenesis). All their somatic cells are haploid, yet they undergo mitosis for growth and development.
• Male ants and wasps: Similarly haploid males exist in these species.
• Lower plants (Bryophytes and Pteridophytes): The gametophyte generation is haploid. In mosses, the dominant gametophyte is haploid and undergoes mitosis to grow and develop.
• Fungi: Many fungi (e.g., Neurospora) exist predominantly in the haploid state and undergo mitosis for vegetative growth.
• Algae: Haploid algae undergo mitosis for vegetative reproduction.

(ii) Haploid cells in higher plants where cell division does NOT occur:

In higher plants (angiosperms), certain haploid cells are terminally differentiated and do not divide:

• Pollen grains (microspores): After the generative cell divides to form two male gametes, the vegetative cell does not divide further.
• Synergids and antipodal cells in the embryo sac: These are haploid cells that do not undergo further division (antipodals may degenerate).
• Mature egg cell (female gamete): The egg cell is haploid and does not divide until fertilisation.

Conclusion: Cell division is not exclusively linked to ploidy level. Both haploid and diploid cells can divide by mitosis; the ability to divide depends on the developmental programme of the cell.

Concept: DNA replication during S phase is a prerequisite for normal mitosis.

Answer: Normally, NO — mitosis cannot occur properly without prior DNA replication in S phase.

Reasoning:

• During S phase, each chromosome is duplicated to form two sister chromatids joined at the centromere. This doubles the DNA content from $2C$ to $4C$.
• During mitosis (specifically anaphase), the centromeres split and sister chromatids are pulled to opposite poles, so each daughter cell receives a complete set of chromosomes ($2N$, $2C$).
• If DNA replication does not occur, each chromosome would consist of only one chromatid. When the cell attempts mitosis, each daughter cell would receive only half the normal DNA content ($N$ chromosomes, $1C$ DNA), leading to genetically deficient daughter cells.

However, in exceptional/experimental situations:

• If DNA replication is artificially blocked but the cell is forced into mitosis, the chromosomes would separate but daughter cells would be aneuploid or have incomplete genetic information, which is lethal.
• In some organisms, amitosis (direct cell division without spindle formation) can occur, but this is not true mitosis.

Conclusion: Mitosis without DNA replication in S phase would result in daughter cells with an incomplete genome. Under normal physiological conditions, DNA replication is essential before mitosis.

Concept: DNA replication and cell division are two separate events that are normally coupled but can be uncoupled.

Answer: YES — DNA replication can occur without subsequent cell division.

Examples and Reasoning:

1. $G_0$ phase: Cells that enter the quiescent $G_0$ phase may have already replicated their DNA but do not proceed to divide.

2. Polytene chromosomes (Endoreduplication): In certain cells (e.g., salivary gland cells of Drosophila), DNA replication occurs repeatedly without cell division, resulting in polytene chromosomes with many copies of DNA strands.

3. Endopolyploidy: In some plant cells and animal cells, DNA replication occurs multiple times without mitosis or cytokinesis, resulting in cells with very high DNA content (polyploidy).

4. Liver cells: Hepatocytes can undergo DNA replication (becoming tetraploid or octaploid) without completing cell division.

5. Abortive cell cycle: If a cell replicates DNA but then receives signals to stop (e.g., DNA damage checkpoints), it may not proceed to divide.

Conclusion: DNA replication is a necessary prerequisite for cell division, but cell division is NOT a necessary consequence of DNA replication. DNA replication can occur independently of cell division in several biological contexts.

Concept: The chromosome number (N) and DNA content (C) change at specific stages of the cell cycle.

Starting point: A normal diploid cell has $2N$ chromosomes and $2C$ DNA content.

Changes during the Cell Cycle:

Summary:

• DNA content ($C$) doubles during S phase and halves at cytokinesis (in mitosis) or at the end of meiosis I and meiosis II.
• Chromosome number ($N$) remains constant through mitosis (daughter cells = $2N$) but is halved after meiosis I ($N$) and remains $N$ after meiosis II.
• The key events that change these parameters are: S phase (doubles DNA), anaphase of mitosis/anaphase II (centromere splits), and anaphase I (homologs separate).