Cell Cycle and Cell DivisionClass 11 Biology Notes

Cell Cycle

Have you ever wondered how a giant tree or a large animal can grow from a single, tiny cell? The answer lies in a fundamental process of life: cell division. All living organisms are made of cells, and these cells have the remarkable ability to grow and reproduce by dividing. When a parent cell divides, it creates two new daughter cells. These daughter cells can then grow and divide themselves, leading to a massive population of cells—millions of them—all originating from just one initial cell.

This orderly cycle of growth and division is called the cell cycle. It is a highly coordinated series of events that ensures a cell correctly duplicates its genetic material (genome), synthesizes all its other components, and then divides into two healthy daughter cells. While the cell grows continuously by increasing its cytoplasm, the crucial process of copying its DNA, known as DNA synthesis, happens only at a specific time during the cycle. After the DNA is copied, the replicated chromosomes are carefully distributed to the new daughter cells. The entire process is under precise genetic control.

Phases of Cell Cycle

The cell cycle is divided into two main phases:

1. Interphase: The period of preparation for cell division.
2. M Phase (Mitosis Phase): The period when the actual cell division occurs.

Interphase

Although sometimes called the "resting phase," Interphase is a time of intense activity where the cell grows and prepares for division. It is subdivided into three stages:

• G₁ phase (Gap 1): This is the stage between the end of the previous division (mitosis) and the start of DNA replication. During G₁, the cell is metabolically active and grows in size, but it does not copy its DNA.
• S phase (Synthesis): This is the crucial stage where the cell replicates its DNA. The amount of DNA in the cell doubles. For example, if the initial amount of DNA is represented as 2C, it increases to 4C. However, the number of chromosomes does not change. If the cell is diploid ($2n$), it remains $2n$ even after the S phase. In animal cells, the centriole also duplicates during this phase.
• G₂ phase (Gap 2): After DNA replication, the cell enters the G₂ phase. Here, it continues to grow and synthesizes proteins that will be needed for mitosis.

G₀ Phase (Quiescent Stage)

Some cells in adult animals, like heart cells, do not divide. Other cells only divide when needed to replace old or damaged cells. These cells exit the G₁ phase and enter an inactive state called the quiescent stage ($G_0$). Cells in the $G_0$ phase are metabolically active but do not divide unless the organism requires them to.

M Phase

The M Phase is the most dramatic part of the cell cycle, where the cell undergoes a major reorganization to divide. It consists of two main events:

1. Karyokinesis: The division of the nucleus, where the duplicated chromosomes are separated into two identical sets.
2. Cytokinesis: The division of the cytoplasm, which splits the parent cell into two daughter cells.

Because the number of chromosomes in the parent cell and the two daughter cells remains the same, mitosis is also known as equational division. Karyokinesis is a continuous process, but for ease of study, it is divided into four stages.

Prophase

Prophase is the first stage of nuclear division, following the G₂ phase of interphase.

• Chromosome Condensation: The long, intertwined DNA molecules begin to condense and untangle, forming compact, visible mitotic chromosomes. Each chromosome is now seen to be composed of two identical sister chromatids attached at a point called the centromere.
• Spindle Formation: The centrosomes, which duplicated during the S phase, start moving towards opposite ends (poles) of the cell. They radiate microtubules called asters. The asters, along with the spindle fibres that form between them, create the mitotic apparatus.
• Disappearance of Organelles: By the end of prophase, the Golgi complex, endoplasmic reticulum, nucleolus, and the nuclear envelope are no longer visible under a microscope.

Metaphase

The start of metaphase is marked by the complete breakdown of the nuclear envelope.

• Chromosome Alignment: The chromosomes, now fully condensed, spread throughout the cytoplasm. This is the best stage to study the morphology (shape and size) of chromosomes.
• Attachment of Spindle Fibres: Spindle fibres attach to small, disc-shaped structures on the surface of the centromeres called kinetochores.
• Metaphase Plate: The chromosomes are moved to the center of the cell and align along the equatorial plane. This alignment is called the metaphase plate. Each chromosome is connected to both poles; one chromatid's kinetochore is attached to a spindle fibre from one pole, and its sister chromatid's kinetochore is attached to a fibre from the opposite pole.

Anaphase

Anaphase begins with the simultaneous splitting of the centromere of each chromosome.

• Separation of Chromatids: The sister chromatids separate and are now considered individual daughter chromosomes.
• Movement to Poles: The newly separated daughter chromosomes begin to migrate towards the two opposite poles of the cell. The centromere of each chromosome leads the way, with the arms of the chromosome trailing behind.

Telophase

Telophase is the final stage of nuclear division.

• Chromosome Decondensation: The chromosomes arrive at their respective poles, decondense, and lose their distinct structure. They form clusters of chromatin material at each pole.
• Reformation of Nucleus: A new nuclear envelope develops around each chromosome cluster, forming two distinct daughter nuclei.
• Reappearance of Organelles: The nucleolus, Golgi complex, and ER reform.

Cytokinesis

Following nuclear division (karyokinesis), the cell itself divides its cytoplasm in a process called cytokinesis to form two separate daughter cells. The mechanism differs between animal and plant cells.

• In Animal Cells: Cytokinesis occurs by the formation of a furrow in the plasma membrane. This furrow gradually deepens and eventually joins in the center, pinching the cytoplasm into two.
• In Plant Cells: Due to their rigid cell wall, plant cells divide differently. Wall formation begins in the center of the cell with a precursor called the cell-plate. This plate grows outward until it meets the existing side walls, dividing the cell in two. The cell-plate represents the middle lamella between the new cells.

Significance of Mitosis

Mitosis is a crucial process for life, with several key functions:

• Growth: The growth of multicellular organisms from a single zygote to a full-sized adult is the result of countless mitotic divisions.
• Genetic Identity: Mitosis produces daughter cells that are genetically identical to the parent cell, ensuring that all somatic (body) cells have the same genetic makeup.
• Cell Repair and Replacement: Mitosis constantly replaces old or damaged cells. For example, cells in the upper layer of your skin, the lining of your gut, and your blood cells are continuously replaced.
• Restoring Cell Size: As a cell grows, the ratio between its nucleus and cytoplasm is disturbed. Division restores this balance, known as the nucleo-cytoplasmic ratio.
• Plant Growth: In plants, mitotic divisions in meristematic tissues (like the apical and lateral cambium) allow for continuous growth throughout their life.

Meiosis

Sexual reproduction involves the fusion of two specialized cells called gametes (like sperm and egg), each carrying a complete haploid (single) set of chromosomes. These gametes are formed from diploid cells through a special type of cell division called meiosis.

Meiosis is a reduction division because it reduces the chromosome number by half. This process ensures that when two gametes fuse during fertilization, the resulting zygote has the correct diploid number of chromosomes, restoring the species' characteristic chromosome count.

Key features of meiosis include:

• It involves two sequential cycles of division, Meiosis I and Meiosis II, but only a single round of DNA replication.
• It involves the pairing of homologous chromosomes (one from each parent).
• Recombination (crossing over) occurs between these homologous chromosomes, creating new genetic combinations.
• It results in the formation of four haploid daughter cells.

Meiosis I

This is the first meiotic division, where homologous chromosomes are separated.

Prophase I

This is a very long and complex phase, divided into five sub-stages:

1. Leptotene: Chromosomes gradually become visible and start to compact.
2. Zygotene: Homologous chromosomes begin to pair up. This process of association is called synapsis. The paired chromosomes are called a bivalent or a tetrad. A structure called the synaptonemal complex forms between them.
3. Pachytene: The four chromatids of each bivalent become clearly visible. Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs at sites called recombination nodules. This process is mediated by an enzyme called recombinase and leads to genetic recombination.
4. Diplotene: The synaptonemal complex dissolves. The homologous chromosomes start to separate but remain attached at the sites of crossing over. These X-shaped points of attachment are called chiasmata.
5. Diakinesis: This is the final stage. The chiasmata move towards the ends of the chromosomes (terminalisation). The chromosomes are fully condensed, the nucleolus disappears, and the nuclear envelope breaks down.

Metaphase I

The bivalent chromosomes align on the equatorial plate. Microtubules from opposite poles attach to the kinetochores of the homologous chromosomes.

Anaphase I

The homologous chromosomes separate and move to opposite poles. Importantly, the sister chromatids remain attached at their centromeres. This is a key difference from mitosis.

Telophase I

The chromosomes arrive at the poles. The nuclear membrane and nucleolus reappear, and cytokinesis follows. This results in two haploid cells, often called a dyad. The stage between Meiosis I and Meiosis II is called interkinesis. No DNA replication occurs during this short phase.

Meiosis II

The second meiotic division is much simpler and resembles a normal mitotic division.

Prophase II

The nuclear membrane disappears, and the chromosomes become compact again.

Metaphase II

The chromosomes align at the equator. Microtubules from opposite poles attach to the kinetochores of the sister chromatids.

Anaphase II

The centromere of each chromosome finally splits, allowing the sister chromatids to separate and move to opposite poles.

Telophase II

Meiosis ends with Telophase II. A nuclear envelope forms around the two groups of chromosomes at each pole. Cytokinesis follows, resulting in the formation of a tetrad of cells—four haploid daughter cells.

Significance of Meiosis

Meiosis is profoundly important for sexually reproducing organisms for two main reasons:

1. Conservation of Chromosome Number: By halving the chromosome number to create haploid gametes, meiosis ensures that the species-specific chromosome number is maintained from one generation to the next after fertilization.
2. Genetic Variation: The crossing over that occurs during Prophase I creates new combinations of genes on the chromosomes. This genetic recombination increases the genetic variability within a population. These variations are the raw material for natural selection and are essential for the process of evolution.