Cell Cycle and Cell Division with NEET-focused notes. Mitosis, meiosis, phases, diagrams, and key differences for Class 11 Biology.

Biology is full of tiny miracles, and one of the most fascinating is how a single cell grows, replicates, and divides to give rise to trillions of cells in our body. This magical process is known as the cell cycle and cell division. Whether you're a Class 11 student or preparing for NEET, understanding this topic is essential.

What is the Cell Cycle?

The cell cycle is the life cycle of a cell — from the moment it's formed until it divides to form new cells. It includes phases of growth, DNA duplication, and actual cell division.

Quick Fact: Some cells like nerve cells exit the cycle and stop dividing. This stage is called G₀ phase

Phases of the Cell Cycle

The cell cycle is the series of events that a cell goes through from its formation to the time it divides into two daughter cells. It ensures proper growth, DNA replication, and division.

The cell cycle is mainly divided into two broad phases:

1. Interphase – The Preparation Phase

2. M Phase (Mitotic Phase) – The Division Phase

1. Interphase – The Preparation Phase

Interphase is the longest and most active phase of the cell cycle where the cell grows, performs normal functions, and prepares for cell division. Although no visible division happens, the cell is metabolically very active during this stage.

Also called the "resting phase", but this is misleading—the cell is not at rest.
DNA replication, RNA transcription, and protein synthesis occur.
Organelles and cytoplasmic contents increase.

Interphase is divided into 3 sub-phases:

1. G₁ Phase (Gap 1)

2. S Phase (Synthesis Phase)

3. G₂ Phase (Gap 2)

1. G₁ Phase (Gap 1)

The G₁ phase is the first sub-phase of Interphase, right after the cell has divided through mitosis or meiosis. It is a growth and preparation phase before the cell begins DNA replication in the S phase.

Cell growth begins — the daughter cell increases in size.
High level of metabolic activity – the cell performs its normal functions.
Protein synthesis and enzyme production needed for DNA replication starts.
RNA synthesis occurs actively.
Organelles (like mitochondria, ER, etc.) begin to duplicate.

Important Notes for NEET:

G₁ is the most variable phase in duration.
Cells like nerve and heart cells may enter G₀ phase permanently and never divide again.
No DNA replication occurs in G₁ — only growth and preparation.

2. S Phase (Synthesis Phase) –

The S phase is the second sub-phase of Interphase, where the cell synthesizes (replicates) its DNA in preparation for cell division. DNA replication occurs — each chromosome duplicates to form two identical sister chromatids.

Amount of DNA doubles, but the chromosome number remains unchanged.
For example, in humans:
Before S phase: 46 chromosomes, 2N DNA
After S phase: 46 chromosomes, 4N DNA
Histone proteins are also synthesized to package the newly formed DNA.
Centrosome duplication occurs (important for spindle formation in mitosis).

Important for NEET:

Chromosome number does not double — only DNA content does.
Proper duplication is essential; errors may cause mutations.

3. G₂ Phase (Gap 2) –

The G₂ phase is the final sub-phase of Interphase, following the S phase. During this phase, the cell gets ready for mitosis by checking and repairing DNA and producing the proteins necessary for division.

DNA is already replicated (in S phase) – now the cell prepares for mitosis.
Protein synthesis continues, especially proteins needed for spindle formation (like tubulin).
The cell grows further, increasing cytoplasmic volume and organelles.
The cell performs a "DNA damage check" – if errors are found, they are repaired before entering mitosis.
Energy stores (like ATP) are increased to support cell division.

Important for NEET:

G₂ ensures genetic stability by checking for any DNA damage.
If errors are found and not repairable, the cell may undergo apoptosis (programmed cell death).
G₂ is shorter than G₁ but critical for accurate cell division.

M Phase (Mitotic Phase) – The Division Phase

The M phase, also known as the Mitotic Phase, is the stage of the cell cycle where the actual division of the cell occurs. It follows the interphase and results in the formation of two genetically identical daughter cells.

It is a short but critical phase of the cell cycle.

Includes two main events:

1. Karyokinesis – Division of the nucleus

2. Cytokinesis – Division of the cytoplasm

Important for NEET:

No growth or DNA replication occurs during M phase
Results in 2 diploid daughter cells (in mitosis)
Ensures genetic stability from one generation of cells to the next

Mitosis – For Growth and Repair:

Mitosis is a type of cell division in which a parent cell divides into two genetically identical daughter cells, each having the same number of chromosomes as the parent. It plays a vital role in the growth, repair, and maintenance of multicellular organisms.

Occurs in somatic (body) cells
Produces 2 diploid (2n) daughter cells
No variation – daughter cells are genetically identical
One cell division cycle (unlike meiosis, which has two)

Why Mitosis is Important:

1. Growth – Helps increase cell number during development of an organism.

2. Repair – Replaces damaged or dead cells (e.g., skin cells, blood cells).

3. Regeneration – Regrowth of lost parts in some organisms (e.g., tail in lizards).

4. Asexual reproduction – In organisms like amoeba, hydra, etc.

NEET Notes:

Mitosis maintains chromosome number (e.g., human cells: 46 → 46)
Essential for tissue maintenance and wound healing

Stages of Mitosis –

Mitosis is the process of nuclear division in which a single diploid cell divides to form two genetically identical daughter cells. The mitotic division ensures equal distribution of chromosomes.

It occurs in four continuous stages under Karyokinesis (nuclear division), followed by Cytokinesis (cytoplasmic division).

cell cycle and cell division with NEET-focused notes. Covers mitosis, meiosis, phases, diagrams, and key differences for Class 11 Biology.

1. Prophase

Chromatin condenses into visible chromosomes.
Each chromosome has two sister chromatids joined at a centromere.
Nuclear membrane breaks down.
Spindle fibers begin to form from centrosomes.

2. Metaphase

Chromosomes align at the equatorial plane (center of the cell).
Spindle fibers attach to the centromeres of the chromosomes.

3. Anaphase

Sister chromatids separate and move toward opposite poles.
Chromatids are now considered individual chromosomes.

4. Telophase

Chromosomes begin to decondense into chromatin.
Nuclear membranes reappear around both sets of chromosomes.
Spindle fibers disappear.

Cytokinesis (Post-mitosis)

Cytoplasm divides, forming two separate daughter cells.
In animal cells: cleavage furrow forms
In plant cells: cell plate forms

Important for NEET:

Mitosis occurs in diploid somatic cells
Daughter cells are genetically identical
Ensures equal chromosome number (2n → 2n)

Meiosis – For Gamete Formation

Meiosis is a special type of cell division that reduces the chromosome number by half and results in the formation of haploid gametes — sperm in males and egg in females. It is essential for sexual reproduction and ensures genetic diversity in offspring.

Occurs only in germ cells (testes and ovaries).
Involves two successive divisions:
Meiosis I (Reductional Division)
Meiosis II (Equational Division)
Produces 4 non-identical haploid (n) cells from 1 diploid (2n) cell.
Leads to variation due to genetic recombination and independent assortment.

Why Meiosis is Important:

1. Gamete Formation – Essential for forming sperm and egg.

2. Genetic Variation – Through crossing over and independent assortment.

3. Maintains Chromosome Number – Ensures stability across generations.

E.g., Human: 46 chromosomes → gametes with 23 chromosomes → zygote (46 again)

Important for NEET:

Meiosis is crucial for sexual reproduction.
Genetic variation is introduced during Prophase I (crossing over).
Failure in meiosis can lead to chromosomal disorders (e.g., Down syndrome).

Meiosis I –

Meiosis I is the first division of meiosis, also called the reductional division, because it reduces the chromosome number by half. It starts with one diploid (2n) cell and produces two haploid (n) daughter cells, each with half the number of chromosomes.

This stage is crucial for genetic variation, as it includes crossing over and independent assortment.
Chromosome number is reduced (2n → n).
Homologous chromosomes (one from each parent) pair up and then separate.
Introduces genetic diversity through recombination.

Stages of Meiosis I:

1. Prophase I (Longest and most complex stage)

Homologous chromosomes pair up (synapsis)
Crossing over occurs (exchange of genetic material)
Divided into 5 sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis

2. Metaphase I

Homologous pairs align at the equatorial plate
Spindle fibers attach to centromeres

3. Anaphase I

Homologous chromosomes separate and move to opposite poles
Sister chromatids remain attached

4. Telophase I

Nuclear envelope may reappear
Cytokinesis follows, forming 2 haploid cells

NEET Key Points:

Reductional division – chromosome number is halved
Crossing over introduces genetic variation
Occurs only in germ cells

Meiosis II –

Meiosis II is the second division of meiosis, also known as the equational division because the chromosome number remains the same in the daughter cells. It is similar to mitosis in behavior but occurs in haploid cells formed after Meiosis I.

This phase separates sister chromatids and results in four genetically unique haploid cells.

Purpose of Meiosis II:

To separate sister chromatids of each chromosome
To produce four haploid (n) gametes from the two haploid cells formed in Meiosis I
To ensure each gamete gets one complete set of chromosomes
Begins with 2 haploid cells
Ends with 4 non-identical haploid cells
No DNA replication before this division
Helps maintain chromosome stability across generations

Stages of Meiosis II:

1. Prophase II

Chromosomes condense again
New spindle fibers form
Nuclear envelope (if re-formed) dissolves

2. Metaphase II

Chromosomes align at the equator
Spindle fibers attach to centromeres

3. Anaphase II

Sister chromatids separate and move to opposite poles

4. Telophase II

Chromatids reach poles
Nuclear envelopes reform

Cytokinesis follows → 4 haploid daughter cells

Important NEET Points:

Meiosis II is similar to mitosis, but starts with haploid cells
No crossing over happens in Meiosis II
Results in genetic variation + haploid gametes

Key Differences – Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Type of cell division	Equational division	Reduction division
Number of divisions	1 (single division)	2 (Meiosis I and Meiosis II)
Number of daughter cells	2 daughter cells	4 daughter cells
Genetic similarity	Genetically identical to parent cell	Genetically different from parent cell
Chromosome number	Same as parent (diploid → diploid)	Halved (diploid → haploid)
Occurs in	Somatic (body) cells	Germ cells (testes and ovaries)
Purpose	Growth, repair, asexual reproduction	Formation of gametes (sperm and egg)
Crossing over	❌ Does not occur	✅ Occurs during Prophase I
Tetrad formation	❌ No tetrad formation	✅ Homologous chromosomes form tetrads
Duration	Shorter	Longer
Phases	Prophase, Metaphase, Anaphase, Telophase	Meiosis I: Prophase I to Telophase I Meiosis II: Prophase II to Telophase II