Cell Cycle and Cell Division: Complete Guide for Class 11 NCERT Biology

Overview of the Cell Cycle and Its Phases

The cell cycle is the sequence of events that a cell undergoes to grow and divide. It consists of two major phases:

• Interphase: The cell prepares for division by growing and replicating its DNA. It has three sub-phases:
• G1 phase (Gap 1): Cell grows and synthesizes proteins.
• S phase (Synthesis): DNA replication occurs, doubling the chromosome content.
• G2 phase (Gap 2): Cell prepares for mitosis by producing organelles and molecules.

• Mitotic (M) phase: The cell divides its nucleus and cytoplasm to form two daughter cells.

Most cells spend about 90% of their time in interphase. For example, a typical mammalian cell completes the entire cell cycle in about 24 hours.

Some cells exit the cycle into a resting phase called G0 phase, where they do not divide but perform normal functions, such as nerve and muscle cells.

Understanding these phases is crucial for grasping how cells maintain life and reproduce.

Detailed Stages of Mitosis in Cell Division

Mitosis is an equational division process where one parent cell divides to form two genetically identical diploid daughter cells. It ensures the chromosome number remains constant.

Mitosis has five stages:

1. Prophase: Chromosomes condense and become visible. The nuclear envelope starts to break down. 2. Metaphase: Chromosomes align at the cell's equatorial plate, attached to spindle fibers via kinetochores. 3. Anaphase: Sister chromatids separate as centromeres divide and move toward opposite poles. 4. Telophase: Chromosomes reach poles, decondense, and nuclear envelopes reform. 5. Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Worked example:

If a diploid cell with 46 chromosomes undergoes mitosis, each daughter cell will have 46 chromosomes, maintaining the chromosome number.

Understanding Meiosis: Reductional Division Explained

Meiosis is a special type of cell division that reduces the chromosome number by half, producing haploid gametes essential for sexual reproduction.

Meiosis consists of two divisions: Meiosis I and Meiosis II.

Meiosis I (Reductional Division)

• Prophase I is the longest and most complex stage, subdivided into five sub-stages:
• Leptotene: Chromosomes condense and become visible.
• Zygotene: Homologous chromosomes pair up (synapsis) forming bivalents or tetrads.
• Pachytene: Crossing over occurs between non-sister chromatids, promoting genetic recombination.
• Diplotene: Synaptonemal complex dissolves; chiasmata become visible.
• Diakinesis: Chromosomes fully condense; nuclear envelope breaks down.

• Metaphase I: Bivalents align at the equatorial plate.
• Anaphase I: Homologous chromosomes separate and move to opposite poles.
• Telophase I: Nuclear membranes reform; cytokinesis produces two haploid cells.

Meiosis II

Similar to mitosis, sister chromatids separate, resulting in four haploid daughter cells.

This process ensures genetic diversity and maintains chromosome number across generations.

Key Differences Between Mitosis and Meiosis

Understanding the differences between mitosis and meiosis is essential for Class 11 students. Here is a comparison:

This table helps clarify how each process contributes differently to organismal development and reproduction.

Interphase: The Preparation Phase for Cell Division

Interphase is the longest phase of the cell cycle where the cell prepares for division.

It includes three sub-phases:

• G1 phase: Cell grows in size and synthesizes proteins and organelles.
• S phase: DNA replication occurs, doubling the genetic material.
• G2 phase: Cell prepares for mitosis by producing necessary molecules and checking for DNA errors.

During interphase, the cell is metabolically active but does not divide. The DNA is in the form of loosely packed chromatin.

Example:

If a cell has 2 picograms of DNA at the start of G1, after S phase it will have 4 picograms, ensuring each daughter cell receives an identical copy.

Cells that do not divide enter the G0 phase, a resting state where they perform specialized functions.

Cytokinesis vs Karyokinesis: Completing Cell Division

Cell division involves two main processes:

• Karyokinesis: Division of the nucleus, where chromosomes are separated into daughter nuclei.
• Cytokinesis: Division of the cytoplasm, producing two separate daughter cells.

In mitosis and meiosis, karyokinesis is followed by cytokinesis.

Differences:

For example, in animal cells, cytokinesis occurs by the formation of a cleavage furrow, while in plant cells, a cell plate forms.