Cell Cycle And Cell Division: Complete Chapter Notes For Class 11 NEET by PW

Keeping track of G1, S, G2, mitosis and meiosis becomes harder when the chapter is studied as a list of stages instead of one continuous process. The role of DNA replication, chromosome number, crossing over and cytokinesis also needs to be connected to the correct stage of cell division. 

The PW Cell Cycle And Cell Division: Complete Chapter One-Shot Revision Video For Class 11 NEET helps you revise these concepts together. From interphase and the M phase to mitosis, meiosis, crossing over, and cytokinesis, the revision covers the key processes needed for Class 11 NEET Biology.

Definition And Sequence Of The Cell Cycle

The cell cycle is the sequence of events through which a cell:

1. Increases in size and cytoplasmic material.

2. Duplicates its DNA.

3. Synthesises organelles, RNA, proteins, and ATP.

4. Divides into two genetically identical daughter cells.

 The division phase has two steps:

• Karyokinesis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

 The complete sequence is:

Interphase → Karyokinesis → Cytokinesis → Two Daughter Cells

Cytoplasmic growth continues throughout the cell cycle, but DNA replication occurs only during the S phase. A typical human cell cycle lasts about 24 hours. Interphase takes nearly 23 hours, or 95%, while M phase takes about 1 hour, or 5%.

Phases Of The Cell Cycle

The cell cycle has two major phases:

1. Interphase: Preparatory phase.

2. M Phase: Mitotic or division phase.

Interphase: Preparatory And “Resting” Phase

Interphase is also called the preparatory phase and the resting phase. The cell does not divide during this stage, but it is highly metabolically active. It performs cell growth, DNA synthesis, RNA synthesis, protein synthesis, ATP production, and organelle duplication.

Interphase is called the resting phase because the cell does not divide, not because it is metabolically inactive.

Interphase includes:

• G1 Phase

• S Phase

• G2 Phase

G1 Phase: Gap One Or Post-Mitotic Phase

G1 begins after M phase, when two daughter cells have formed. It is therefore called the post-mitotic phase.

Major events include:

• Rapid cell growth.

• Increase in cytoplasm.

• High metabolic activity.

• RNA and protein synthesis.

• ATP and nucleotide production.

• Increase or duplication of most organelles.

RNA synthesis is called transcription, while protein synthesis is called translation.

DNA replication does not occur during G1. G1 is the phase between the end of mitosis and the beginning of DNA replication.

S Phase: Synthesis Phase

The S phase is the stage of DNA replication. One DNA molecule produces two DNA molecules that remain attached at the centromere.

Before S phase, one chromosome has one DNA molecule and one chromatid. After S phase, it has two identical sister chromatids joined at the centromere.

The chromosome number remains unchanged, but DNA content doubles.

Histone proteins are synthesised during S phase. In animal cells, centriole duplication also occurs during S phase. Centrioles are present in animal cells but absent in plant cells.

G2 Phase: Gap Two Or Pre-Mitotic Phase

G2 occurs after S phase and immediately before M phase. It is also called the pre-mitotic phase.

Major events include:

• Continued cell growth.

• Tubulin protein synthesis for spindle-fibre formation.

• Continued transcription and replication-related activity.

• Duplication of Golgi bodies, mitochondria, and plastids.

Protein synthesis occurs throughout interphase:

• G1: Most proteins.

• S: Histone proteins.

• G2: Tubulin protein.

M Phase: Mitotic Or Division Phase

M phase includes:

1. Karyokinesis: Nuclear division.

2. Cytokinesis: Cytoplasmic division.

It produces two daughter cells.

G0 Phase: Quiescent Or Inactive State

A cell may leave G1 without entering S phase and enter G0, also called the quiescent or inactive state. G0 cells do not divide but may remain metabolically active.

• Permanent G0: Nerve cells and heart cells generally do not resume division.

• Temporary G0: Liver, kidney, pancreatic, and plant parenchyma cells may re-enter the cycle when required.

The overall cycle is:

G1 → S → G2 → M

Stages Of Mitosis

Mitosis includes karyokinesis and cytokinesis. Karyokinesis has four phases:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

During prophase:

• Chromosomes condense, shorten, and become thicker.

• Each chromosome has two sister chromatids joined by a centromere.

• The nuclear envelope and nucleolus disappear.

• Golgi bodies and endoplasmic reticulum also disappear.

• In animal cells, centrosomes move to opposite poles and form the mitotic apparatus.

• In plant cells, spindle fibres form from cytoplasmic microtubules because centrioles and centrosomes are absent.

Spindle fibres are made of tubulin protein.

Metaphase

During metaphase:

• The nuclear envelope completely disintegrates.

• Chromosomes become maximally condensed.

• Spindle fibres attach to kinetochores.

• Chromosomes align at the centre on the equatorial plate or metaphase plate.

A kinetochore is a protein disc present on each side of the centromere. Metaphase is the best stage for studying chromosome structure and morphology.

Anaphase

The defining event is centromere splitting.

• Sister chromatids separate.

• Each separated chromatid becomes a daughter chromosome.

• Daughter chromosomes move toward opposite poles.

• The centromere leads the movement and chromosome arms trail behind.

Telophase

During telophase:

• Daughter chromosomes reach opposite poles.

• Nuclear envelopes and nucleoli reappear.

• Golgi bodies and endoplasmic reticulum reappear.

• Chromosomes decondense and lose their distinct identity.

• Two nuclei form within the cell.

Telophase is broadly the reverse of prophase.

Cytokinesis

In animal cells, the plasma membrane forms an inward cleavage furrow. This furrow grows from outside to inside, called centripetal movement.

In plant cells, a phragmoplast forms at the centre. It produces a cell plate that grows from inside to outside, called centrifugal movement. The cell plate later forms the middle lamella.

If cytokinesis does not immediately follow nuclear division, several nuclei may remain in one cell. This is called free nuclear division and can produce a multinucleated syncytial condition, such as liquid coconut endosperm.

Significance Of Mitosis

Mitosis is an equational division. It produces two genetically identical daughter cells and maintains the parent cell’s chromosome number.

Its functions include:

• Growth in multicellular organisms.

• Repair of damaged tissues.

• Replacement of worn-out skin, intestinal, and blood cells.

• Asexual reproduction through binary fission in organisms such as amoeba and Paramecium.

• Continuous plant growth through meristematic cells.

Mitosis can occur in haploid, diploid, triploid, and polyploid cells. In animals, it usually occurs in diploid somatic cells. The haploid male honeybee, or drone, is an important exception.

Meiosis: Reductional Division

Meiosis consists of two successive divisions but only one round of DNA replication. It produces four genetically different haploid cells from one diploid parent cell.

• Meiosis I: Reductional division.

• Meiosis II: Equational division and similar to mitosis.

DNA replication occurs only during the S phase before Meiosis I. No replication occurs during interkinesis.

Meiosis I

Meiosis I includes Prophase I, Metaphase I, Anaphase I, Telophase I, and cytokinesis. It produces two haploid cells called a dyad.

Prophase I

Prophase I is divided into five stages:

In zygotene, homologous chromosomes pair through synapsis and form a synaptonemal complex. The paired chromosomes form a bivalent, also called a tetrad because they contain four chromatids.

During pachytene, crossing over occurs between non-sister chromatids of homologous chromosomes. It produces genetic variation. During diplotene, homologous chromosomes separate but remain joined at chiasmata. During diakinesis, chiasmata move toward chromosome ends through terminalization.

Metaphase I

Bivalents align on two metaphase plates. Each chromosome is attached to one spindle fibre, and homologous chromosomes remain connected at chiasmata.

Anaphase I

Homologous chromosomes separate and move to opposite poles. Centromeres do not split, so sister chromatids remain attached. This separation reduces the chromosome number by half.

Telophase I And Cytokinesis I

Nuclear envelopes reform, and cytokinesis produces two haploid daughter cells. Each contains one chromosome from every homologous pair.

Meiosis II

After a short interkinesis, Meiosis II begins without DNA replication.

• Prophase II: Chromosomes condense and spindle fibres form.

• Metaphase II: Chromosomes align individually at one metaphase plate.

• Anaphase II: Centromeres split and sister chromatids separate.

• Telophase II: Nuclear envelopes reform and chromosomes decondense.

• Cytokinesis II: Four haploid daughter cells are formed.

Numerical Rules For Meiosis

Track chromosome number, chromatids, and DNA content separately:

1. DNA content doubles during S phase.

2. Homologous chromosomes separate during Meiosis I.

3. Sister chromatids separate during Meiosis II.

4. No DNA replication occurs during interkinesis.

5. The final four cells are haploid.

For a diploid cell with 8 chromosomes and 4 pg DNA:

 Mitosis And Meiosis Comparison

Mitosis maintains chromosome number and supports growth and tissue maintenance, while meiosis reduces chromosome number and introduces genetic variation during gamete formation. Comparing their divisions, daughter cells, and genetic outcomes helps distinguish their roles in the body. 

 The Cell Cycle and Cell Division chapter explains how cells grow, replicate their genetic material, and divide while maintaining genetic continuity. The PW Cell Cycle And Cell Division: Complete Chapter One-Shot Revision Video For Class 11 NEET can help you revise the important stages and differences before practising NEET questions. 

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