All organisms, regardless of size, begin life as a single cell. Growth and reproduction, fundamental characteristics of life, occur at the cellular level. Cells reproduce by dividing into two, with each parent cell giving rise to two daughter cells.

These daughter cells can then grow and divide themselves, creating a new population of cells. This cycle of growth and division allows a single cell to form structures containing millions of cells, eventually leading to complex organisms.

Cell Cycle

Cell division is a vital process in all living organisms. It involves not just the physical splitting of a cell but also DNA replication and cell growth.

To ensure the correct division and the formation of healthy daughter cells with complete sets of genetic material (intact genomes), cell division, DNA replication, and cell growth must occur in a highly coordinated sequence of events.

This sequence of events by which a cell duplicates its genome, synthesizes other cell components, and finally divides into two daughter cells is termed the cell cycle.

Cell growth, specifically the increase in cytoplasmic volume, is a continuous process throughout the cell cycle. However, DNA synthesis (replication) occurs only during a specific period.

The replicated chromosomes are then precisely distributed to the daughter nuclei through a complex series of events during cell division. These events are under strict genetic control.

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Phases Of Cell Cycle

A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide approximately every 24 hours. The duration of the cell cycle can vary significantly depending on the organism and the specific cell type (e.g., yeast takes about 90 minutes).

The cell cycle is fundamentally divided into two major phases (Figure 10.1):

1. Interphase: This is the period between two successive M phases. It is a phase of preparation for cell division, characterized by cell growth and DNA replication. Although sometimes called the "resting phase," it is highly metabolically active. Interphase occupies the vast majority of the cell cycle duration (over 95% in a 24-hour human cell cycle).
2. M Phase (Mitosis phase): This is when the actual cell division (mitosis) occurs. It starts with nuclear division (karyokinesis) and usually ends with cytoplasmic division (cytokinesis). The M phase is relatively short, lasting about an hour in a typical human cell cycle.

Interphase is further subdivided into three phases:

• G1 phase (Gap 1): This is the interval between mitosis and the initiation of DNA replication. During G1, the cell is metabolically active, grows continuously, and synthesizes proteins and RNA, but the DNA does not replicate.
• S phase (Synthesis): This is the period when DNA synthesis or replication takes place. During the S phase, the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C, it increases to 4C. However, the number of chromosomes remains the same. If the cell was diploid (2n) in G1, it is still considered 2n after S phase (each chromosome now has two sister chromatids, but the number of centromeres is unchanged). In animal cells, DNA replication occurs in the nucleus, and centriole duplication occurs in the cytoplasm during the S phase.
• G2 phase (Gap 2): This phase follows the S phase and precedes the M phase. During G2, the cell continues to grow, and proteins necessary for mitosis are synthesized.

G0 phase (Quiescent stage): Some cells in adult animals (like heart cells) do not divide, while others divide only occasionally when needed (e.g., for repair). These cells exit the G1 phase and enter an inactive stage called the G0 phase. Cells in G0 are metabolically active but do not proliferate unless signaled to do so by the organism's needs.

Mitotic cell division usually occurs in diploid somatic cells in animals, but there are exceptions where haploid cells undergo mitosis (e.g., male honey bees). Plants, however, commonly exhibit mitotic divisions in both haploid and diploid cells (e.g., gametophyte generation is haploid and undergoes mitosis for gamete formation; sporophyte generation is diploid and undergoes mitosis for growth).

M Phase

The M phase is the most visible and dynamic part of the cell cycle, involving significant changes in cellular components. It results in the division of the parent cell into two daughter cells.

Mitosis is often called equational division because the number of chromosomes in the daughter cells is the same as in the parent cell.

Although mitosis is described in distinct stages, it is a continuous process. Karyokinesis (nuclear division) is typically divided into four main stages:

• Prophase
• Metaphase
• Anaphase
• Telophase

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Prophase

Prophase is the first stage of karyokinesis and follows the S and G2 phases of interphase. Key events include (Figure 10.2 a):

• Chromatin condensation: The chromosomal material, which was loosely intertwined after replication in S and G2, begins to condense. This process makes the chromosomes visible as distinct, compact structures composed of two sister chromatids held together at the centromere.
• Centrosome movement: The centrosome, which duplicated during interphase S phase, starts moving towards opposite poles of the cell.
• Formation of mitotic apparatus: Each centrosome begins to radiate microtubules called asters. The asters, along with the spindle fibers that connect them, form the mitotic apparatus (spindle).
• By the end of prophase, organelles like Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope are no longer visible.

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Metaphase

Metaphase is marked by the complete disintegration of the nuclear envelope, allowing chromosomes to spread throughout the cytoplasm (Figure 10.2 b).

Key events and characteristics:

• Chromosome condensation is complete: Chromosomes are clearly visible and their morphology is best studied at this stage. Each metaphase chromosome consists of two sister chromatids connected at the centromere.
• Kinetochores: Small, disc-shaped structures are present on the surface of the centromere of each chromatid. These are the attachment sites for spindle fibers.
• Alignment at metaphase plate: Spindle fibers from opposite poles attach to the kinetochores of the sister chromatids. Chromosomes are moved to the cell's equator and align along an imaginary plane called the metaphase plate. Each chromosome is positioned such that one sister chromatid is connected to spindle fibers from one pole, and the other sister chromatid is connected to fibers from the opposite pole.

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Anaphase

Anaphase is characterized by the separation of sister chromatids (Figure 10.2 c).

Key events:

• Centromeres split: The centromere holding the two sister chromatids together simultaneously divides.
• Chromatids separate: The now separated sister chromatids are considered individual chromosomes (daughter chromosomes). They begin to move towards opposite poles of the cell.
• Migration pattern: As the daughter chromosomes move towards the poles, their centromeres lead the way (closest to the pole), while the chromosome arms trail behind.

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Telophase

Telophase is the final stage of karyokinesis (Figure 10.2 d).

Key events:

• Chromosomes decondense: The chromosomes that have reached their respective poles begin to decondense and lose their distinct structure. The chromatin material collects at each pole.
• Nuclear envelope reforms: A nuclear envelope develops around each cluster of chromosomes at the poles, forming two distinct daughter nuclei.
• Nucleolus and other organelles reappear: The nucleolus, Golgi complex, and ER reform.

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Cytokinesis

Cytokinesis is the division of the cytoplasm of the cell, which usually follows karyokinesis and completes the process of cell division (Figure 10.2 e).

• In animal cells: Cytokinesis is achieved by the formation of a cell furrow in the plasma membrane. The furrow deepens and eventually pinches the cell into two daughter cells.
• In plant cells: Due to the rigid cell wall, cytokinesis occurs by the formation of a cell plate in the center of the cell. The cell plate forms from vesicles derived from the Golgi apparatus and grows outwards to meet the existing lateral cell walls, dividing the cytoplasm into two. The cell plate eventually develops into the new cell wall and middle lamella between the daughter cells.
• Organelle distribution: Organelles like mitochondria and plastids are distributed between the two daughter cells during cytoplasmic division.

Syncytium: In some organisms, karyokinesis (nuclear division) is not immediately followed by cytokinesis, resulting in a single cell containing multiple nuclei. This multinucleate condition is called a syncytium (e.g., the liquid endosperm in coconut).

Significance Of Mitosis

Mitosis, also known as equational division, primarily occurs in diploid cells, though some lower plants and social insects have haploid cells that divide by mitosis (e.g., male honey bees).

Key significance of mitosis:

• Production of genetically identical daughter cells: Mitosis results in two diploid daughter cells that are genetically identical to the parent cell. This ensures genetic stability.
• Growth: Mitotic divisions are responsible for the growth of multicellular organisms, increasing the number of cells.
• Restoration of nucleo-cytoplasmic ratio: As a cell grows during interphase, the ratio between the nucleus volume and cytoplasm volume can become unbalanced. Mitosis restores this ratio by dividing the cell.
• Cell repair and replacement: Mitosis is crucial for replacing worn-out or damaged cells, facilitating tissue repair (e.g., replacement of cells in the epidermis, gut lining, and blood).
• Continuous growth in plants: Mitotic divisions in meristematic tissues (apical and lateral cambium) enable plants to grow continuously throughout their life.

Meiosis

Sexual reproduction involves the fusion of two gametes (sex cells), each containing a haploid set of chromosomes. Gametes are produced from specialized diploid cells through a unique type of cell division called meiosis.

Meiosis is also known as reductional division because it reduces the chromosome number by half, producing haploid daughter cells (gametes) from a diploid parent cell.

Significance of Meiosis in life cycle: Meiosis establishes the haploid phase in sexually reproducing organisms. Fertilization, the fusion of haploid gametes, restores the diploid phase.

Meiosis occurs during gametogenesis (formation of gametes) in both plants and animals.

Key features of meiosis:

• Involves two sequential cycles of nuclear and cell division: Meiosis I and Meiosis II.
• However, there is only a single cycle of DNA replication, which occurs before Meiosis I (during the S phase of interphase).
• Meiosis I is initiated after the parental chromosomes have replicated in S phase, resulting in identical sister chromatids.
• A key event in Meiosis I is the pairing of homologous chromosomes and recombination (crossing over) between non-sister chromatids of homologous chromosomes.
• At the end of Meiosis II, four haploid cells are formed.

Meiotic events are grouped into phases:

• Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I
• Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II

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Meiosis I

Meiosis I is the reductional division where homologous chromosomes separate.

Prophase I: This is a long and complex stage compared to mitotic prophase. It is subdivided into five substages based on chromosomal behavior:

• Leptotene: Chromosomes become gradually visible as thin threads under the light microscope. Chromosome compaction continues.
• Zygotene: Chromosomes begin to pair up (synapsis). Paired homologous chromosomes are called homologous chromosomes. Synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The paired homologous chromosomes with their synaptonemal complex form a structure called a bivalent or tetrad (referring to the four chromatids).
• Pachytene: Bivalent chromosomes become more distinct, and the four chromatids of each bivalent are clearly visible as tetrads. This stage is characterized by the appearance of recombination nodules, which are the sites where crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over is the exchange of genetic material between homologous chromosomes. It is catalyzed by the enzyme recombinase and leads to genetic recombination. Recombination is completed by the end of pachytene.
• Diplotene: The synaptonemal complex dissolves. The recombined homologous chromosomes in the bivalents tend to separate but remain linked at the sites of crossing over, forming X-shaped structures called chiasmata (singular: chiasma). Diplotene can be very long (months or years) in oocytes of some vertebrates.
• Diakinesis: The final stage of Prophase I. Marked by the terminalisation of chiasmata (chiasmata move towards the ends of the chromosomes). Chromosomes become fully condensed. The meiotic spindle is assembled. The nucleolus disappears, and the nuclear envelope breaks down. Diakinesis is a transition to Metaphase I.

Metaphase I: Bivalent chromosomes (pairs of homologous chromosomes) align at the equatorial plate of the spindle (Figure 10.3).

Microtubules from opposite poles attach to the kinetochore of each homologous chromosome (not sister chromatids). Each homologous chromosome is pulled towards a different pole.

Anaphase I: The homologous chromosomes separate and move towards opposite poles (Figure 10.3). Each pole receives a haploid set of chromosomes, but each chromosome still consists of two sister chromatids attached at the centromere.

Telophase I: The chromosomes that have reached the poles decondense to some extent. Nuclear membrane and nucleolus may reappear. Cytokinesis usually follows, resulting in the formation of two haploid daughter cells called a dyad (Figure 10.3).

Interkinesis: The stage between Meiosis I and Meiosis II. Generally short-lived. Importantly, there is no DNA replication during interkinesis.

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Meiosis II

Meiosis II is essentially similar to a normal mitosis (equational division), where sister chromatids separate. It starts immediately after cytokinesis I, often before chromosomes are fully elongated.

Prophase II: Nuclear membrane (if reformed) disappears. Chromosomes (each with two chromatids) become compact again (Figure 10.4).

Metaphase II: Chromosomes align at the equatorial plate in each of the two daughter cells (Figure 10.4). Microtubules from opposite poles attach to the kinetochores of sister chromatids.

Anaphase II: This stage begins with the simultaneous splitting of the centromere of each chromosome (Figure 10.4). This allows the sister chromatids to separate and move towards opposite poles as individual chromosomes (daughter chromosomes).

Telophase II: Meiosis concludes with Telophase II (Figure 10.4). The two groups of chromosomes at each pole decondense. A nuclear envelope reforms around each group. Cytokinesis follows, dividing the cytoplasm of each cell.

The result of meiosis is the formation of a tetrad of cells, i.e., four haploid daughter cells.

Significance Of Meiosis

Meiosis plays a critical role in sexually reproducing organisms:

• Conservation of chromosome number: Although it halves the chromosome number during gamete formation, meiosis ensures that the specific chromosome number of a species is maintained across generations. When two haploid gametes fuse during fertilization, the diploid chromosome number characteristic of the species is restored.
• Increase in genetic variability: Crossing over (recombination) during Prophase I leads to the exchange of genetic material between homologous chromosomes. This shuffles genes and creates new combinations of alleles on chromosomes. Independent assortment of homologous chromosomes during Anaphase I further contributes to genetic variation in the resulting gametes. This genetic variability is the raw material for evolution.

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