Cell Cycle

**Cell division** is a crucial process in all living organisms. It involves not only the division of the cell itself but also important events like **DNA replication** and **cell growth**. These processes must occur in a coordinated sequence to ensure that the daughter cells formed receive intact genomes (complete sets of chromosomes) and can function properly.

The sequence of events by which a cell duplicates its genome, synthesises other cell constituents, and eventually divides into two daughter cells is called the **cell cycle**. While cell growth (increase in cytoplasm) is a continuous process throughout the cell cycle, DNA synthesis (replication) happens only during a specific stage.

The replicated chromosomes are then accurately distributed to the daughter nuclei through a complex series of events during cell division. These events are regulated by genetic control mechanisms.

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Phases Of Cell Cycle

A typical eukaryotic cell cycle (e.g., human cells in culture) lasts about 24 hours, although this duration can vary significantly among different organisms and cell types (e.g., yeast cell cycle is about 90 minutes).

The cell cycle is divided into two main phases:

1. **Interphase:** The phase between two successive M phases. It is a period of preparation for cell division. Although sometimes called the 'resting phase', the cell is metabolically active, growing, and replicating its DNA during interphase. Interphase lasts for more than 95% of the cell cycle duration in human cells.
2. **M Phase (Mitosis phase):** The phase during which actual cell division (mitosis) occurs. This phase is relatively short, lasting about an hour in human cells.

Interphase is further divided into three sub-phases:

• **G$_1$ phase (Gap 1):** Corresponds to the interval between mitosis (M phase) and the initiation of DNA replication (S phase). During G$_1$, the cell is metabolically active, grows continuously, and synthesises proteins and organelles, but it does not replicate its DNA.
• **S phase (Synthesis):** This is the period when **DNA synthesis or replication** takes place. The amount of DNA per cell doubles during this phase. If the initial amount of DNA is denoted as 2C in G$_1$, it becomes 4C in S. However, the number of chromosomes does not increase. If the cell is diploid (2n) in G$_1$, it remains 2n after S phase (each chromosome now has two sister chromatids). In animal cells, centriole duplication also occurs in the cytoplasm during the S phase.
• **G$_2$ phase (Gap 2):** Follows the S phase and precedes the M phase. During G$_2$, the cell continues to grow, and proteins are synthesized in preparation for mitosis (like spindle proteins).

The M phase involves **karyokinesis** (nuclear division and separation of chromosomes) followed by **cytokinesis** (cytoplasmic division). Karyokinesis is usually divided into four stages: Prophase, Metaphase, Anaphase, and Telophase.

Some cells in adult animals (e.g., heart cells) do not divide, while others divide only occasionally for repair or replacement. These cells exit the G$_1$ phase and enter an inactive stage called the **quiescent stage (G$_0$)**. Cells in G$_0$ are metabolically active but do not proliferate unless signaled to do so.

In animals, mitotic division is typically seen only in diploid somatic cells. However, in lower plants and some social insects, haploid cells can also divide by mitosis. In plants, mitotic divisions can occur in both haploid and diploid cells.

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M Phase (Mitosis)

The **M phase** includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). Mitosis is an **equational division** because the number of chromosomes in the parent cell and the daughter cells is the same (e.g., 2n $\to$ 2n). Mitosis is a continuous process, but it is conveniently divided into four stages of nuclear division (karyokinesis): Prophase, Metaphase, Anaphase, and Telophase.

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Prophase

The first stage of mitosis. It follows the S and G$_2$ phases of interphase. The chromosomal material, which was indistinct and intertwined chromatin fibers, begins to condense and become compact. The duplicated chromosomes become visible as distinct structures, each composed of two sister chromatids attached at the centromere. The centrosome (duplicated during S phase) begins to move towards opposite poles of the cell, radiating microtubules (asters) that form the spindle fibers (mitotic apparatus). By the end of prophase, organelles like Golgi complexes, ER, nucleolus, and the nuclear envelope disappear.

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Metaphase

Starts with the complete disintegration of the nuclear envelope, allowing chromosomes to spread throughout the cytoplasm. Chromosome condensation is complete, making them clearly visible for morphological study. At metaphase, each chromosome (with two sister chromatids held by the centromere) has disc-shaped structures called **kinetochores** at the surface of the centromeres. Spindle fibers attach to these kinetochores.

Characteristic event: All chromosomes align at the center of the cell on a plane called the **metaphase plate**. One chromatid of each chromosome is attached by its kinetochore to spindle fibers from one pole, and the sister chromatid is attached to spindle fibers from the opposite pole.

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Anaphase

Begins with the simultaneous **splitting of the centromere** of each chromosome at the metaphase plate. This separates the two sister chromatids. These separated chromatids are now considered **daughter chromosomes**. They begin to migrate rapidly towards the two opposite poles of the cell, pulled by the shortening spindle fibers attached to their kinetochores. The centromere leads the movement towards the pole, with the chromosome arms trailing behind.

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Telophase

The final stage of karyokinesis. Daughter chromosomes reach their respective poles and begin to **decondense**, losing their distinct individuality. The chromatin material collects at each pole. Key events: Nuclear envelope reassembles around each set of chromosomes at the poles, forming two daughter nuclei. Nucleolus, Golgi complex, and ER also reform. This stage marks the end of nuclear division.

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Cytokinesis (Cytoplasmic Division)

**Cytokinesis** is the division of the cytoplasm of the parent cell into two daughter cells, following karyokinesis. Mitosis (karyokinesis) is incomplete without cytokinesis.

• In **animal cells**, cytokinesis is achieved by the formation of a **cleavage furrow** in the plasma membrane. The furrow gradually deepens from the periphery inwards, eventually pinching the cell into two.

• In **plant cells**, which have a rigid cell wall, cytokinesis occurs by **cell plate formation**. Wall formation starts in the center of the cell with the formation of a precursor called the cell plate (representing the future middle lamella). The cell plate grows outwards towards the lateral walls, eventually dividing the cell into two.

During cytokinesis, organelles like mitochondria and plastids are distributed between the two daughter cells. In some organisms, karyokinesis occurs without cytokinesis, resulting in a multinucleate condition (syncytium), e.g., liquid endosperm in coconut.

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Significance Of Mitosis

**Mitosis** is an equational division that is primarily responsible for **growth** and **repair** in multicellular organisms. It is usually restricted to diploid cells, but can occur in haploid cells in some lower plants and social insects. Mitosis produces diploid daughter cells that are genetically identical to the parent cell.

Significance of mitosis:

• **Growth:** Mitosis is the basis for the growth of multicellular organisms (increase in cell number). Mitotic divisions in meristematic tissues (apical and lateral cambium) cause continuous growth in plants.
• **Cell Repair:** Mitosis is essential for replacing old, dead, or injured cells. For example, cells of the epidermis, gut lining, and blood cells are constantly replaced through mitotic divisions.
• **Maintenance of Nucleo-cytoplasmic Ratio:** Cell growth increases the size of the cytoplasm relative to the nucleus. Cell division through mitosis helps restore the optimal ratio between the nucleus and cytoplasm.
• **Asexual Reproduction:** In some organisms, mitosis is the mode of asexual reproduction (e.g., in unicellular eukaryotes like Amoeba, it is binary fission).

Meiosis

**Meiosis** is a specialized type of cell division that occurs in diploid cells (germ cells) to produce **haploid gametes** (sex cells). It is called **reductional division** because the number of chromosomes is reduced by half in the daughter cells compared to the parent cell (e.g., 2n $\to$ n).

Meiosis is essential for sexual reproduction. It ensures the production of haploid gametes. When two haploid gametes (male and female) fuse during fertilisation, the diploid chromosome number of the species is restored in the zygote. Meiosis ensures the conservation of the specific chromosome number across generations in sexually reproducing organisms.

Meiosis consists of two sequential cycles of nuclear and cell division, **meiosis I** and **meiosis II**, but only a single cycle of DNA replication (which occurs during the S phase before meiosis I).

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Meiosis I (Reductional Division)

Meiosis I is the first meiotic division, during which homologous chromosomes separate. It starts after DNA replication in the S phase, so parental chromosomes have replicated to produce identical sister chromatids.

Stages of Meiosis I:

1. **Prophase I:** Longest and most complex stage. Subdivided into five phases (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis):
   • Leptotene: Chromosomes become gradually visible and continue compaction.
   • Zygotene: Homologous chromosomes start pairing (synapsis), forming synaptonemal complexes. Paired homologous chromosomes are called bivalents or tetrads (each bivalent has 4 chromatids).
   • Pachytene: Bivalents/tetrads are clearly visible. **Crossing over** occurs – exchange of genetic material between non-sister chromatids of homologous chromosomes at recombination nodules. Crossing over is enzyme-mediated (recombinase). It leads to genetic recombination.
   • Diplotene: Synaptonemal complex dissolves. Recombined homologous chromosomes start separating but remain linked at points of crossing over (chiasmata - X-shaped structures). Diplotene can last for months/years in some cells (e.g., vertebrate oocytes).
   • Diakinesis: Final stage of Prophase I. Marked by terminalisation of chiasmata. Chromosomes are fully condensed. Meiotic spindle assembled. Nucleolus and nuclear envelope disappear. Transition to metaphase I.

2. **Metaphase I:** Bivalent chromosomes align on the equatorial plate. Microtubules from opposite poles attach to the kinetochores of **homologous chromosomes** (not sister chromatids).
3. **Anaphase I:** **Homologous chromosomes separate** and move towards opposite poles. Sister chromatids remain attached at their centromeres. Each pole receives a haploid set of chromosomes (each chromosome still composed of two chromatids).
4. **Telophase I:** Chromosomes reach poles, decondense. Nuclear membrane and nucleolus reappear. Cytokinesis follows, forming a **dyad of cells** (two haploid cells). A brief interkinesis may occur between meiosis I and II, without DNA replication.

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Meiosis II (Equational Division)

Meiosis II is the second meiotic division, similar to a normal mitosis. It separates the sister chromatids. Meiosis II starts immediately after cytokinesis I (or interkinesis).

Stages of Meiosis II:

1. **Prophase II:** Nuclear envelope disappears. Chromosomes become compact again.
2. **Metaphase II:** Chromosomes (each with two chromatids) align at the equatorial plate. Microtubules from opposite poles attach to the kinetochores of **sister chromatids**.

3. **Anaphase II:** **Simultaneous splitting of the centromere** of each chromosome (at the metaphase plate). Sister chromatids (now daughter chromosomes) separate and move towards opposite poles, pulled by spindle fibres.
4. **Telophase II:** Two groups of chromosomes at poles are enclosed by nuclear envelopes, forming two nuclei in each cell of the dyad. Cytokinesis follows (cytokinesis II), resulting in a **tetrad of cells** (four haploid daughter cells).

Thus, at the end of meiosis, four haploid cells are formed from a single diploid cell. These haploid cells are the gametes.

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Significance Of Meiosis

Meiosis is vital for sexually reproducing organisms due to two main reasons:

1. **Conservation of Chromosome Number:** Meiosis is a reductional division that reduces the chromosome number by half, producing haploid gametes. Fertilisation involves the fusion of two haploid gametes, restoring the original diploid chromosome number characteristic of the species in the zygote. This ensures that the chromosome number remains constant across generations.
2. **Increase in Genetic Variability:** Meiosis generates genetic variability in the population through two mechanisms:
   • **Crossing over:** Exchange of genetic material between homologous chromosomes during Prophase I creates new combinations of alleles on chromosomes.
   • **Independent assortment:** Homologous chromosomes align and separate randomly during Anaphase I, leading to different combinations of maternal and paternal chromosomes in the resulting daughter cells.
   This increased genetic variability is the raw material for evolution, allowing populations to adapt to changing environments and increasing the chances of species survival.

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