Cell Cycle, Meaning, Phases, Significance of Mitosis and Meiosis

The cell cycle is the process through which a cell grows, duplicates its DNA, and divides into two new cells. It consists of different phases that help the cell prepare for division and ensure that everything is ready for the creation of two identical cells. Understanding the cell cycle is essential for biology students, especially for the NEET exam , as it helps explain how organisms grow and repair tissues. This topic is an important part of the NEET syllabus .

What is a Cell Cycle?

Phases of Cell Cycle

• Interphase
• M Phase (Mitosis phase)

Interphase

• G1 phase (Gap 1) : The G1 phase is the interval between the end of mitosis and the start of DNA replication. During this phase, the cell is metabolically active, growing in size, and performing its regular functions, but it does not replicate its DNA.
• S phase (Synthesis): During the S phase, the process of DNA replication takes place. The amount of DNA in the cell doubles during this time. For example, if the initial DNA amount is 2C, it increases to 4C after the S phase. However, the number of chromosomes remains unchanged. If the cell had a diploid (2n) number of chromosomes before the S phase, it still has 2n chromosomes afterward. In animal cells, DNA replication begins in the nucleus, and the centrioles duplicate in the cytoplasm .
• G2 phase (Gap 2): During the G2 phase, the cell continues to grow and prepares for mitosis. Proteins essential for cell division are synthesized, ensuring the cell is ready for the mitotic phase.
• Quiescent Stage (G0): Some cells, such as heart cells in adult animals, do not divide and enter a quiescent (G0) stage. In this stage, cells remain metabolically active but stop proliferating unless required to divide, such as in the case of tissue repair.

Also Check:

• NEET Biology MCQs
• NEET Biology Notes
• NEET Biology  Chapter wise Weightage

M Phase

• Prophase
• Metaphase
• Anaphase
• Telophase

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Cytokinesis

• Animal Cells : In animal cells, a shallow groove forms in the cytoplasm due to the contraction of microfilaments, which leads to cell division.
• Plant Cells : In plant cells, a structure known as the cell plate appears at the cell's equator, facilitating the division of the cell.

Significance of Mitosis

• Production of Daughter Cells: Mitosis typically produces diploid daughter cells that have identical genetic material.
• Growth of Multicellular Organisms : Mitosis contributes to the growth of multicellular organisms. As cells grow, the ratio of the nucleus to the cytoplasm is disturbed, necessitating division to restore balance.
• Cell Repair: Mitosis plays a vital role in cell repair and regeneration. Cells in the upper epidermis, gut lining, and blood are continuously replaced through mitosis.
• Continuous Growth in Plants : In plants, mitotic divisions in meristematic tissues, such as the apical and lateral cambium, support ongoing growth throughout their lives.

Meiosis

• Two Division Cycles : Meiosis consists of two sequential cycles of nuclear and cell division, referred to as meiosis I and meiosis II. However, there is only one cycle of DNA replication, which occurs during meiosis I.
• Initiation of Meiosis I: Meiosis I begins after the parental chromosomes have replicated, producing identical sister chromatids during the S phase of interphase.
• Pairing and Recombination : During meiosis, homologous chromosomes pair up and undergo recombination, allowing genetic material to be exchanged between them.
• Formation of Haploid Cells : At the conclusion of meiosis II, four haploid cells are formed, each containing a single set of chromosomes.
• Genetic Variation : The four haploid cells produced at the end of meiosis II are genetically dissimilar, contributing to genetic diversity in sexually reproducing populations.

Meiosis I

• Prophase I : Prophase I of the first meiotic division is typically longer and more intricate than prophase in mitosis. It can be subdivided into five phases based on chromosomal behavior: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
  • Leptotene: During this initial stage, chromosomes gradually become visible under a light microscope as they continue to condense.
  • Zygotene : In this stage, chromosomes begin to pair up, a process known as synapsis. The paired chromosomes are called homologous chromosomes. Electron micrographs show that synapsis is accompanied by the formation of a complex structure known as the synaptonemal complex. The structure formed by a pair of synapsed homologous chromosomes is referred to as a bivalent or tetrad. The initial two stages of prophase I are relatively brief in duration compared to the subsequent stage.
  • Pachytene: This stage is characterized by the distinct appearance of the four chromatids in each bivalent, which are now clearly visible as tetrads. Recombination nodules appear at this stage, marking the sites where crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over involves the exchange of genetic material and is mediated by an enzyme called recombinase. By the end of the pachytene stage, recombination is complete, and the chromosomes remain linked at the crossover sites.
  • Diplotene : The start of diplotene is marked by the breakdown of the synaptonemal complex and the partial separation of the recombined homologous chromosomes in the bivalents, except at the crossover sites. These X-shaped structures are known as chiasmata. In some vertebrate oocytes, diplotene can last for months or even years.
  • Diakinesis : The final stage of meiotic prophase I, diakinesis, is characterized by the terminalization of chiasmata. At this point, the chromosomes are fully condensed, and the meiotic spindle is assembled to prepare for the separation of homologous chromosomes. By the end of diakinesis, the nucleolus disappears, and the nuclear envelope breaks down, signaling the transition to metaphase.
• Metaphase I: The bivalent chromosomes position themselves along the equatorial plane. Microtubules extending from opposite poles of the spindle connect to the kinetochores of the homologous chromosomes.
• Anaphase I: In this stage, homologous chromosomes separate while sister chromatids remain connected at their centromeres.
• Telophase I : The nuclear membrane and nucleolus reappear, followed by cytokinesis, resulting in two daughter cells referred to as dyads. Although the chromosomes may undergo some dispersion, they do not reach the fully extended state seen in interphase nuclei.

Interkinesis

• The period between the two meiotic divisions is called interkinesis, which is generally short-lived and does not involve DNA replication. Interkinesis is succeeded by prophase II, which is much simpler than prophase I.

Meiosis II

• Prophase II : Meiosis II begins right after cytokinesis, typically before the chromosomes have fully elongated. Unlike meiosis I, meiosis II is similar to standard mitosis. By the end of prophase II, the nuclear membrane disappears, and the chromosomes condense again.
• Metaphase II : In this stage, the chromosomes align along the equatorial plane, and microtubules from opposite poles of the spindle attach to the kinetochores of sister chromatids.
• Anaphase II: Anaphase II begins with the simultaneous division of the centromeres that connect the sister chromatids. This allows the sister chromatids to move toward opposite poles of the cell, facilitated by the shortening of the microtubules attached to the kinetochores.
• Telophase II : Meiosis concludes with telophase II, during which the two groups of chromosomes are enclosed by a new nuclear envelope. Cytokinesis then occurs, resulting in the formation of four haploid daughter cells.

Significance of Meiosis

• Conservation of Chromosome Number: Meiosis ensures that offspring inherit the correct number of chromosomes.
• Genetic Variability : Meiosis increases genetic diversity within populations, which is crucial for evolution and adaptation. Variations arising from meiosis play a significant role in the evolutionary process.

MCQs of Cell Cycle

Q1. Which of the following does not occur during cell division?

1. DNA replication
2. Cell growth
3. Increase in the cytoplasm of daughter cell
4. Division of cell organelles

Q2. Identify the correct statement with regard to the G1 phase (Gap 1) of interphase:-

1. The cell is metabolically active and grows but does not replicate its DNA.
2. DNA synthesis or replication takes place.
3. The nuclear division takes place.
4. Reorganization of all cell components takes place.

Q3. Cell in G0 Phase:-

1. Exit the cell cycle
2. Enter the cell cycle
3. Suspend the cell cycle
4. Terminate the cell cycle

Answers of MCQs of Cell Cycle