Revision: Class 11 >> Cell Cycle and Cell Division NEET (UG) Cell Cycle and Cell Division

The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

• The cell cycle is the sequence of events in which a cell grows, duplicates its DNA, and divides into two daughter cells.
• Cell division is essential for growth and reproduction in all living organisms, starting from a single cell.
• A single cell forms a large organism through repeated cycles of growth and division, producing many cells.
• DNA replication and cell growth must be coordinated to ensure that daughter cells receive a complete and correct genome.
• Cell growth is continuous, but DNA synthesis occurs only at a specific stage of the cell cycle, followed by the distribution of chromosomes during division.

• The cell cycle has two main phases: Interphase (growth and DNA replication) and M phase (cell division).
• Interphase is the longest phase, covering about 95% of the cell cycle.
• Interphase includes G₁ (growth), S (DNA replication), and G₂ (preparation for mitosis).
• In S phase, DNA content doubles (2C → 4C) but chromosome number remains the same (2N).
• M phase involves karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division).
• Some cells enter G₀ phase, where they become inactive and do not divide (e.g., heart cells).
• Duration of cell cycle varies: human cells ~24 hours, yeast ~90 minutes.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

• Cytokinesis is the division of cytoplasm into two daughter cells after karyokinesis (nuclear division).
• In animal cells, a furrow appears in the plasma membrane, deepens, and joins in the centre to divide the cytoplasm.
• In plant cells, a cell plate forms in the centre and grows outward to meet the lateral walls, forming a new cell wall. The cell plate represents the middle lamella between two adjacent cells.
• During cytokinesis, organelles like mitochondria and plastids are distributed between the two daughter cells.
• In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).

• Mitosis is an equational division that produces genetically identical daughter cells, maintaining chromosome number.
• It is responsible for growth in multicellular organisms by increasing the number of cells.
• Mitosis helps in repair, regeneration, and wound healing by replacing damaged or lost cells.
• It maintains the nucleo-cytoplasmic (surface/volume) ratio, which gets disturbed during cell growth.
• Mitosis also enables asexual reproduction in unicellular organisms and continuous cell replacement (e.g., skin, blood, plant meristems).

• Meiosis is a specialised reduction division that produces four haploid daughter cells with half the chromosome number.
• It involves two sequential divisions (Meiosis I and II) but only one round of DNA replication during interphase.
• Occurs during gametogenesis — in humans (testes and ovaries) and in plants (anthers and ovaries).
• It includes pairing of homologous chromosomes and crossing over, leading to genetic recombination.
• Maintains chromosome number across generations by alternating haploid and diploid phases; fertilisation restores the diploid (2n) condition.

• Meiosis I is a reductional division, where the chromosome number reduces from diploid to haploid.
• Prophase I is the longest and most complex stage, divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• In Leptotene, chromosomes become visible and start condensing.
• In Zygotene, homologous chromosomes pair (synapsis), forming bivalents/tetrads with the help of the synaptonemal complex.
• In Pachytene, crossing over occurs between non-sister chromatids, causing genetic recombination.
• In Diplotene and Diakinesis, homologous chromosomes begin to separate; chiasmata appear, chromosomes fully condense, and the nuclear membrane breaks down.
• During Metaphase I, Anaphase I, and Telophase I:
  • Chromosomes align at the equator (Metaphase I)
  • Homologous chromosomes separate (Anaphase I)
  • Two haploid cells (dyads) are formed after Telophase I and cytokinesis

• Formation of Gametes - Meiosis is responsible for the formation of gametes in sexually reproducing organisms.
• Reduces Chromosome Number - Meiosis reduces the chromosome number by half in gametes, preventing doubling of chromosomes in each generation.
• Restores Chromosome Number - The original chromosome number is restored when gametes fuse during fertilisation.
• Conserves Species-Specific Number - Meiosis ensures that each species maintains its specific chromosome number across generations.
• Genetic Variability - Meiosis increases genetic variability in the population from one generation to the next, which is crucial for the process of evolution.