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).

1. The cell cycle is a series of steps by which a cell grows, duplicates its DNA, and divides to form two daughter cells.
2. It has two main phases: Interphase (growth and preparation) and M-phase (mitosis or actual division).
3. This cycle ensures orderly cell growth, DNA replication, and equal distribution of genetic material.

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 the cytoplasm that follows nuclear division (karyokinesis), resulting in the formation of two separate daughter cells.
• In animal cells, it occurs by the formation of a cleavage furrow, while in plant cells, a cell plate forms at the centre to divide the cytoplasm.

• Meiosis is a reduction division that results in haploid gametes (sex cells) with half the chromosome number.
• In humans, it occurs in the testes (to form sperm) and ovaries (to form ova).
• In flowering plants, it occurs in the anthers (to form pollen grains) and ovaries (to form ovules).
• Fertilisation restores the diploid (2n) number, maintaining chromosome count across generations.

• Meiosis is a two-stage process: Meiosis I and Meiosis II, responsible for forming haploid gametes from diploid germ cells.
• Prophase I is a complex phase subdivided into five stages where homologous chromosomes pair up and undergo crossing over, enabling genetic recombination.
• In Metaphase I, homologous pairs (not sister chromatids) align along the equatorial plate, attached to spindle fibers from opposite poles.
• During Anaphase I, homologous chromosomes are separated and pulled to opposite poles, reducing the chromosome number by half.
• Telophase I results in the formation of two haploid daughter cells, each with half the chromosome number, setting the stage for Meiosis II.

• Meiosis II is similar to mitosis and occurs in both haploid cells formed after Meiosis I, dividing the recombined sister chromatids.
• It includes four stages: Prophase II, Metaphase II, Anaphase II, and Telophase II, resulting in four haploid daughter cells.
• The process ensures genetic variation, as each resulting cell is genetically different from the parent and from each other.
• Meiosis II plays a vital role in sexual reproduction, producing gametes (in animals) or spores (in plants) with half the chromosome number.