Class 11 Cell Cycle And Cell Division Notes for NEET by PW

Cell cycle and cell division explain how organisms grow, repair tissues, replace cells, and maintain chromosome number across generations. During the cell cycle, a cell grows, duplicates its DNA and cellular components, and divides. Mitosis produces daughter cells with the same chromosome number as the parent cell, whereas meiosis produces haploid cells and contributes to genetic variation.

For NEET, understanding the sequence of cell-cycle phases, DNA content, chromosome number, and the stages of mitosis and meiosis is essential. PW provides online preparation for NEET through live and recorded classes, study material, practice questions, and mock tests to help you revise concepts and strengthen your preparation.

Cell Cycle And Cell Division Overview 

Cell cycle and cell division explain how organisms grow, repair tissues, reproduce, and maintain chromosome number. A cell grows, duplicates its genetic material and organelles, and then divides. Mitosis forms genetically similar cells, while meiosis produces haploid cells and promotes variation through recombination.

Meaning of Cell Division And The Cell Cycle

Cell division is the process in which one cell divides into two daughter cells. In sexually reproducing organisms, life begins with a single zygote. Repeated divisions follow the pattern:

1 to 2 to 4 to 8 to 16 to 32 and so on

These repeated cell cycles form millions, billions, or trillions of cells in the body.

The cell cycle includes all events from the formation of a daughter cell until it divides again. During this cycle, the cell:

• Grows.

• Replicates its DNA.

• Duplicates organelles and other cellular components.

• Divides into two daughter cells.

Cell division is a part of the cell cycle. The cell cycle is broader than cell division because it also includes growth, DNA replication, and preparation.

Why Does Cell Division Occur?

A cell grows mainly by increasing its cytoplasmic volume. However, the nucleus does not increase in size at the same rate. When the cell becomes too large, one nucleus cannot efficiently control all its activities.

This causes a disturbance in the nucleocytoplasmic ratio:

• The cytoplasm-to-nucleus ratio increases.

• The nucleus-to-cytoplasm ratio decreases.

• The nucleus becomes less effective in controlling the enlarged cell.

• Division begins and produces smaller daughter cells.

For example, if the nucleus is 1 and the cytoplasm increases from 1 to 2, the C/N ratio changes from 1 to 2, while the N/C ratio decreases from 1 to 0.5.

Cell division restores the nucleocytoplasmic ratio by producing two smaller daughter cells, each controlled by a nucleus. Cell growth is a continuous process, so this imbalance may develop repeatedly.

Zygote Formation And Body Development

A sperm and an ovum are haploid and each contains 23 chromosomes. Their fusion forms a diploid zygote with 46 chromosomes:

• 23 chromosomes come from the mother.

• 23 chromosomes come from the father.

The zygote contains the genetic information needed to form the complete body. It undergoes repeated cell cycles and divisions, producing the large number of cells required for development.

Growth And Reproduction

At the cellular level, growth occurs before reproduction:

1. The cell increases in size.

2. Cytoplasmic volume increases.

3. The nucleocytoplasmic ratio becomes disturbed.

4. The cell divides into two daughter cells.

In unicellular organisms, growth is directly followed by reproduction. In multicellular organisms, growth increases body size and mass, while reproduction may be asexual or sexual.

Cell Division In Eukaryotes And Prokaryotes

In eukaryotic cells, DNA replication, chromosome segregation, and cytokinesis must be coordinated. Prokaryotes generally divide through binary fission, a process distinct from mitosis.

All cell-cycle events must occur in the correct order. DNA must replicate before division, and organelles must be duplicated before they are distributed. All events of the cell cycle must be coordinated.

Duration Of The Cell Cycle

The duration differs among organisms and cell types.

Adult heart cells and neurons generally remain in G0, or the quiescent phase. They are metabolically active but usually do not divide. The liver, however, can regenerate after limited damage. Stomach-lining cells divide regularly because hydrochloric acid continuously damages the lining.

Phases Of The Cell Cycle

The cell cycle has two major phases:

1. Interphase

2. M Phase

Interphase

Interphase is often called the resting phase, but this description is misleading. Interphase is not a true resting phase; it is primarily a preparatory phase.

More than 95% of the cell cycle is spent in interphase. It includes:

• G1 Phase: Growth and preparation for DNA replication.

• S Phase: DNA replication.

• G2 Phase: Final preparation for division.

The sequence is: G1 to S to G2

During G1, maximum growth, protein synthesis, organelle replication, ATP production, and formation of DNA-replication enzymes occur. During S phase, DNA and centrosomes replicate. During G2, remaining organelles are duplicated and tubulin is produced for spindle formation.

The relation between genetic information and protein synthesis is: DNA to RNA to Protein

DNA Content And Chromosome Number

During S phase, DNA content doubles, but chromosome number remains unchanged. Chromosomes are counted by their centromeres. A replicated chromosome has two sister chromatids but still counts as one chromosome.

For a diploid cell:

During S phase, DNA content doubles, but chromosome number remains unchanged.

G0 Phase

Some cells leave the cycle from G1 and enter G0 phase. These cells remain alive and metabolically active but do not divide. Some G0 cells can re-enter the cycle during healing or repair. Adult heart cells and neurons generally remain permanently in G0.

M Phase: Mitosis And Cytokinesis

M phase is the actual division phase and occupies less than 5% of the cell cycle.

It includes:

• Karyokinesis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

Karyokinesis follows the sequence:

Prophase to Metaphase to Anaphase to Telophase

This is remembered as PMAT. Memory Tip: Remember the cell-cycle sequence as G1, S, G2, PMAT, and Cytokinesis.

Mitosis

Mitosis is an equational division because daughter cells retain the same chromosome number and ploidy as the parent cell.

• Prophase: Chromosomes condense, centrosomes move toward opposite poles, and nuclear structures disappear.

• Metaphase: Chromosomes align at the equator and form the metaphase plate. This is the best stage for observing chromosome morphology and counting chromosomes. Memory Tip: M for Metaphase and M for Morphology.

• Anaphase: The centromere splits, and sister chromatids move toward opposite poles.

• Telophase: Chromosomes decondense, and nuclear envelopes, nucleoli, endoplasmic reticulum, and Golgi complexes reappear.

Spindle fibres attach to the kinetochores, which are disc-like structures associated with centromeres.

In animal cells, centrosomes and asters form the mitotic apparatus. Such division is called amphiastral division. Plant cells lack centrosomes and asters and show anastral division.

Cytokinesis In Animal And Plant Cells

Animal cells form an inward furrow because their membrane is flexible. Plant cells form a cell plate because their rigid cell wall cannot constrict inward.

Importance Of Mitosis

Mitosis is important for:

• Growth from the zygote.

• Repair and replacement of damaged cells.

• Renewal of skin and alimentary-canal lining.

• Continuous formation of red blood cells, which live for about 120 days.

• Primary growth in plants through root and shoot apical meristems.

• Secondary growth, which increases stem thickness.

• Restoration of the nucleocytoplasmic ratio.

Colchicine is a mitotic poison. If karyokinesis occurs but cytokinesis fails, chromosome sets remain in one cell, producing polyploidy.

Meiosis: Reductional Division

Meiosis produces four genetically different haploid cells from one diploid cell. DNA replicates once, but the cell divides twice:

1. Meiosis I

2. Meiosis II

Meiosis I is reductional because homologous chromosomes separate and chromosome number becomes half. Meiosis II is equational because sister chromatids separate without another reduction.

In humans:

• Meiosis produces sperm and ova containing 23 chromosomes.

• Fertilisation restores 46 chromosomes in the zygote.

• The zygote develops through mitosis.

DNA replicates once, but the cell divides twice.

Homologous Chromosomes And Sister Chromatids

Homologous chromosomes are maternal and paternal chromosomes of the same type. Humans have 23 pairs of homologous chromosomes.

• Sister chromatids: Two chromatids of the same replicated chromosome.

• Non-sister chromatids: Chromatids belonging to different homologous chromosomes.

Crossing over occurs between non-sister chromatids during pachytene of Prophase I.

Prophase I Of Meiosis

Prophase I has five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis

Interkinesis occurs between Meiosis I and Meiosis II, but DNA replication does not occur during interkinesis.

Cell cycle and cell division are fundamental topics in biology because they explain growth, tissue repair, chromosome continuity, and genetic variation. For NEET 2026, focus on the G1, S, and G2 phases, the PMAT sequence of mitosis, the difference between DNA content and chromosome number, and the stages of Prophase I. PW supports NEET preparation through classes, study material, practice questions, and mock tests that help you revise these concepts.