Cell Cycle and Cell Division

Given/Concept: The cell cycle is the sequence of events from one cell division to the next.

Answer: The average cell cycle span for a mammalian cell is approximately 24 hours.

- $G_1$ phase: ~11 hours
- S phase: ~8 hours
- $G_2$ phase: ~4 hours
- M phase (Mitosis): ~1 hour

Thus, the total duration ≈ 24 hours. However, this duration can vary greatly depending on the cell type and physiological conditions.

Concept: Both are phases of cell division but refer to different events.

| Feature | Karyokinesis | Cytokinesis |
|---|---|---|
| Definition | Division of the nucleus | Division of the cytoplasm |
| Occurrence | Occurs first, during M phase (prophase to telophase) | Follows karyokinesis |
| Result | Two daughter nuclei are formed | Two daughter cells are formed |
| DNA | Involves separation of chromosomes/DNA | Does not directly involve DNA separation |
| Mechanism | Involves spindle apparatus, centromeres | In animals: cleavage furrow; in plants: cell plate formation |
| Can it occur alone? | Yes — karyokinesis can occur without cytokinesis (e.g., in syncytia) | No — cytokinesis generally follows karyokinesis |

Conclusion: Karyokinesis is nuclear division, while cytokinesis is cytoplasmic division. Together they complete cell division (M phase).

Concept: Interphase is the preparatory phase of the cell cycle during which the cell prepares itself for division. It is the longest phase of the cell cycle.

Interphase is divided into three sub-phases:

1. $G_1$ Phase (Gap 1 / Post-mitotic gap):
- The cell grows in size.
- Normal metabolic activities continue.
- Proteins, RNA, and other macromolecules are synthesised.
- Most organelle duplication occurs.
- The cell prepares for DNA replication.
- Duration: longest sub-phase (~11 hours in mammalian cells).

2. S Phase (Synthesis Phase):
- DNA replication (synthesis) takes place.
- Each chromosome is duplicated → each chromosome now consists of two sister chromatids joined at the centromere.
- The amount of DNA per cell doubles (from $2C$ to $4C$), but the chromosome number remains the same ($2N$).
- Histone proteins are also synthesised during this phase.

3. $G_2$ Phase (Gap 2 / Pre-mitotic gap):
- Cell continues to grow.
- Proteins required for cell division (e.g., tubulin for spindle fibres) are synthesised.
- Organelles such as mitochondria and chloroplasts duplicate.
- The cell prepares to enter mitosis.

Summary: Interphase is NOT a resting phase; it is a metabolically active phase essential for successful cell division.

Concept: Not all cells in the body continue to divide actively.

$G_0$ (Quiescent Phase):
- Some cells in the adult body exit the cell cycle after $G_1$ phase and enter a non-dividing, inactive state called the $G_0$ phase or quiescent phase.
- In this phase, cells are metabolically active but do not proliferate unless they receive an appropriate signal.
- These cells have indefinitely stopped dividing.

Examples of cells in $G_0$:
- Neurons (nerve cells) — permanently in $G_0$.
- Cardiac muscle cells.
- Some liver cells (can re-enter the cycle on stimulation).

Significance: $G_0$ allows the organism to maintain a stable population of differentiated, non-dividing cells. Cells in $G_0$ can sometimes be stimulated to re-enter the cell cycle (e.g., liver cells after partial hepatectomy).

Concept: Mitosis is the type of cell division that occurs in somatic (body) cells.

Reason mitosis is called equational division:

During mitosis, the chromosome number of the parent cell is exactly conserved in each of the two daughter cells.

- A diploid parent cell ($2N$) undergoes mitosis to produce two diploid daughter cells ($2N$ each).
- The DNA is first replicated during S phase, and then during mitosis, the sister chromatids separate equally so that each daughter cell receives the same number and same type of chromosomes as the parent cell.
- There is no reduction in chromosome number.

$$\text{Parent cell } (2N) \xrightarrow{\text{Mitosis}} 2 \text{ daughter cells } (2N \text{ each})$$

Since the chromosome number remains equal (unchanged) in parent and daughter cells, mitosis is called equational division.

This is in contrast to meiosis, which is called reductional division because it halves the chromosome number.

Concept: Different events of cell division occur at specific, well-defined stages.

(i) Chromosomes are moved to spindle equator:
→ Metaphase (Metaphase I in meiosis / Metaphase in mitosis)
- During metaphase, chromosomes align at the metaphase plate (equatorial plate) due to equal pulling forces of spindle fibres from both poles.

(ii) Centromere splits and chromatids separate:
→ Anaphase (Anaphase in mitosis / Anaphase II in meiosis)
- During anaphase, the centromere of each chromosome splits, and the two sister chromatids are pulled to opposite poles by spindle fibres.

(iii) Pairing between homologous chromosomes takes place:
→ Zygotene stage of Prophase I of Meiosis I
- The pairing of homologous chromosomes is called synapsis, and it occurs during the zygotene sub-stage of prophase I.

(iv) Crossing over between homologous chromosomes takes place:
→ Pachytene stage of Prophase I of Meiosis I
- Crossing over (exchange of segments between non-sister chromatids of homologous chromosomes) occurs during the pachytene sub-stage of prophase I.

Concept: These terms are associated with Prophase I of Meiosis I.

(a) Synapsis:
- Synapsis is the pairing of homologous chromosomes that occurs during the zygotene stage of prophase I of meiosis.
- The two homologous chromosomes come together and align precisely along their entire length, gene by gene.
- This pairing is facilitated by a protein structure called the synaptonemal complex.
- Synapsis ensures that homologous chromosomes are properly aligned for crossing over.

(b) Bivalent (Tetrad):
- A bivalent (also called a tetrad) is the structure formed by the pairing of two homologous chromosomes during zygotene–pachytene of prophase I.
- Since each homologous chromosome consists of two sister chromatids (after S phase replication), a bivalent contains 4 chromatids in total.
- The number of bivalents = haploid number ($N$) of the organism.
- Example: In humans ($2N = 46$), there are 23 bivalents during meiosis I.

(c) Chiasmata (singular: Chiasma):
- Chiasmata are the X-shaped structures visible during diplotene stage of prophase I, at the points where crossing over has occurred between non-sister chromatids of homologous chromosomes.
- They represent the physical sites of exchange of chromosomal segments.
- Chiasmata hold the homologous chromosomes together until anaphase I, when they are resolved (terminalized) and the homologs separate.
- They are the cytological evidence of crossing over.

Diagram (described):

A bivalent showing synapsis and chiasmata can be illustrated as:

$$\underbrace{\text{Homologous chromosome 1 (2 chromatids)} + \text{Homologous chromosome 2 (2 chromatids)}}_{\text{Bivalent / Tetrad}}$$

At the point of crossing over, the two non-sister chromatids form an X-shaped chiasma.

*(Note: A diagram showing two homologous chromosomes paired with their chromatids and an X-shaped chiasma at the crossing-over point should be drawn in the answer sheet.)*

Concept: Cytokinesis is the division of cytoplasm following karyokinesis. The mechanism differs in plant and animal cells due to structural differences (presence/absence of cell wall).

| Feature | Cytokinesis in Animal Cells | Cytokinesis in Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow formation | Cell plate formation |
| How it occurs | The cell membrane constricts inward from the periphery toward the centre due to a contractile ring of actin and myosin filaments | Vesicles derived from the Golgi apparatus fuse at the centre (equatorial plate) and grow outward toward the periphery |
| Direction | Outside → inward (centripetal) | Inside → outward (centrifugal) |
| Cell wall | No cell wall; only plasma membrane involved | New cell wall (cell plate) is laid down between the two daughter cells |
| Centrioles | Centrioles help organise the cleavage | No centrioles involved |
| End result | Two daughter cells separated by plasma membrane | Two daughter cells separated by a new cell wall (cell plate) |

Reason for difference: Plant cells have a rigid cell wall that cannot be pinched inward, so they form a cell plate instead of a cleavage furrow.

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.

| Feature | Anaphase of Mitosis | Anaphase I of Meiosis |
|---|---|---|
| What separates? | Sister chromatids separate (centromere splits) | Homologous chromosomes separate (centromere does NOT split) |
| Centromere | Centromere splits | Centromere does NOT split; each chromosome still has two chromatids |
| Number of chromosomes moving to each pole | Equal to the original chromosome number ($2N$) | Half the original chromosome number ($N$) |
| Nature of chromosomes at poles | Each pole receives single-chromatid chromosomes | Each pole receives double-chromatid (bivalent) chromosomes |
| Ploidy at poles | Diploid ($2N$) | Haploid ($N$), but each chromosome is still made of 2 chromatids |
| Result | Maintains chromosome number | Reduces chromosome number by half |
| Preceded by | Metaphase (chromosomes aligned individually) | Metaphase I (bivalents aligned) |

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.

| Feature | Mitosis | Meiosis |
|---|---|---|
| Occurrence | Somatic (body) cells | Reproductive cells (gonads); cells destined to form gametes |
| Number of divisions | One (single division) | Two (meiosis I + meiosis II) |
| Number of daughter cells | Two | Four |
| Chromosome number in daughter cells | Same as parent ($2N → 2N$) | Half of parent ($2N → N$) |
| Ploidy | Diploid → Diploid | Diploid → Haploid |
| DNA replication | Once before division | Once before meiosis I; no replication before meiosis II |
| Synapsis | Does not occur | Occurs during zygotene of prophase I |
| Crossing over | Does not occur | Occurs during pachytene of prophase I |
| Bivalent/Tetrad formation | Absent | Present during prophase I |
| Prophase duration | Short | Very long (especially prophase I) |
| Chiasmata | Absent | Present |
| Genetic composition of daughter cells | Genetically identical to parent | Genetically different (due to crossing over and independent assortment) |
| Purpose | Growth, repair, asexual reproduction | Formation of gametes; sexual reproduction |
| Type of division | Equational division | Reductional division (meiosis I) + Equational division (meiosis II) |

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:

| Stage | Chromosome Number (N) | DNA Content (C) | Explanation |
|---|---|---|---|
| $G_1$ phase | $2N$ | $2C$ | Normal diploid state; no replication yet |
| S phase | $2N$ | $2C → 4C$ | DNA replication occurs; chromosome number unchanged (chromatids joined at centromere), but DNA doubles |
| $G_2$ phase | $2N$ | $4C$ | DNA replication complete; chromosomes are now double-stranded (2 chromatids each) |
| Mitosis | | | |
| Prophase | $2N$ | $4C$ | Chromosomes condense; no change in N or C |
| Metaphase | $2N$ | $4C$ | Chromosomes align; no change |
| Anaphase | $4N$ (chromatids counted as chromosomes) | $4C$ | Centromeres split; sister chromatids separate and move to poles; if chromatids are counted as chromosomes, N appears to double |
| Telophase | $2N$ (per daughter nucleus) | $2C$ (per daughter cell after cytokinesis) | Two nuclei form, each with $2N$ chromosomes and $2C$ DNA |
| After Cytokinesis | $2N$ | $2C$ | Two daughter cells, each identical to parent |
| Meiosis I | | | |
| Prophase I | $2N$ | $4C$ | Bivalents form; crossing over occurs |
| Metaphase I | $2N$ | $4C$ | Bivalents at equatorial plate |
| Anaphase I | $2N$ (homologs separate) | $4C$ | Homologous chromosomes move apart; centromere does NOT split |
| Telophase I / After Meiosis I | $N$ (per cell) | $2C$ (per cell) | Each cell has half the chromosomes, each still with 2 chromatids |
| Meiosis II | | | |
| Prophase II | $N$ | $2C$ | No replication |
| Metaphase II | $N$ | $2C$ | Chromosomes align |
| Anaphase II | $2N$ (chromatids separate) | $2C$ | Centromeres split; sister chromatids separate |
| Telophase II / After Meiosis II | $N$ (per cell) | $C$ (per cell) | Four haploid cells, each with $N$ chromosomes and $C$ DNA |

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).