Maarg Manthan · Topic 1.6

Cell Division: Mitosis and Meiosis

Science & Technology › Cell Biology and Genetics · Topic 1.6

Cell division is the process by which a parent cell divides to form daughter cells. It is of two main kinds. Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent, and it is used for growth, repair and asexual reproduction. Meiosis is a reduction division in which one diploid cell divides twice to form four haploid cells that are genetically different, and it forms gametes in sexually reproducing organisms. Humans have 46 chromosomes in body cells and 23 in gametes. Mitosis was named by Walther Flemming in the 1880s. Both processes are part of the cell cycle, and failures in control of division lead to cancer.

5 min read · UPSC Prelims · Science & Technology · Cell Biology and Genetics

At a Glance

Mitosis One diploid cell gives two identical diploid cells
Meiosis One diploid cell gives four haploid cells
Human chromosomes 46 in body cells; 23 in gametes
Crossing over Prophase I of meiosis
Cell cycle G1, S, G2 and M phases
Exam link UPSC Prelims, Science & Technology: Cell Biology and Genetics

Where Does It Fit?

This is the sixth topic of the macro on cell biology and genetics. Division of cells links growth and repair to sexual reproduction, which the next topic covers. Previous topic: Cellular Respiration and Energy Metabolism (ATP Production). Next topic: Sexual Reproduction at the Cellular Level.

What Is the Cell Cycle and How Does Mitosis Work?

Interphase: the long preparation period, with G1 for growth, S phase when DNA is copied, and G2 for further growth. A cell spends most of its life here.
Prophase: chromosomes coil and become visible, the nuclear envelope breaks down and spindle fibres form.
Metaphase: chromosomes line up at the equator of the cell.
Anaphase: sister chromatids separate and move to opposite poles.
Telophase and cytokinesis: nuclear envelopes form again and the cytoplasm divides, giving two daughter cells. Plant cells form a cell plate, while animal cells pinch inward.
Checkpoints: control points in the cycle check for DNA damage, and proteins such as p53 can stop division or trigger cell death if damage cannot be repaired.

How Do Mitosis and Meiosis Differ?

Feature	Mitosis	Meiosis
Where	Body cells	Reproductive organs, forming gametes
Divisions	One	Two, meiosis I and meiosis II
Daughter cells	Two, identical	Four, genetically different
Chromosome number	Same as parent (diploid)	Halved (haploid)
Crossing over	Does not occur normally	Occurs in prophase I
Purpose	Growth, repair, asexual reproduction	Gamete formation and variation

What Else Should You Remember?

Crossing over: exchange of segments between homologous chromosomes in prophase I of meiosis, which creates new combinations of genes and is a source of variation.
Independent assortment: homologous pairs line up and separate randomly in meiosis I, so gametes carry different mixes of maternal and paternal chromosomes.
Chromatid and chromosome: after DNA is copied, each chromosome has two sister chromatids joined at the centromere.
Cancer: arises when mutations break the control of division, so cells divide without limit and may spread. Many drugs and radiation work by targeting dividing cells.
Errors in meiosis: failure of chromosomes to separate, called non-disjunction, causes conditions such as Down syndrome, as in Genetic Mutations: Types and Causes.
Amitosis: a simple direct division seen in some bacteria and yeast, and binary fission in bacteria is not the same as mitosis.
Chromosome counts in other species: the fruit fly has 8 chromosomes, the pea plant 14, maize 20, the dog 78 and some ferns have hundreds, so the number does not show the complexity of an organism.
Telomeres: protective ends of chromosomes that shorten with each division, and the enzyme telomerase can rebuild them, which is active in stem cells and many cancer cells.
Apoptosis: programmed cell death that removes damaged or unneeded cells and shapes tissues, such as the webbing between embryonic fingers.
Cytokinesis difference: in animal cells a ring of protein fibres pinches the cell in two, while in plant cells a new cell wall grows from the centre outward.
Binary fission: bacteria divide by this simple method after copying their circular DNA, which does not involve a spindle.
Cell cycle length: the cycle can last about 24 hours in cultured human cells, and the mitotic phase is only a small part of it.
Stem cells and tissue: cells that divide often include skin and gut lining cells, while nerve cells in adults rarely divide.

Why Does It Matter for the Exam?

Compare and contrast: the difference between mitosis and meiosis is a standard question.
Number questions: the chromosome numbers in body cells, gametes and after each division.
Health link: cancer, radiotherapy and chemotherapy are linked to the control of division.
Genetics link: crossing over and independent assortment explain variation, as in Inheritance Mechanisms (Mendelian Genetics).
Technology link: tissue culture and stem cell therapy use controlled division of cells.

As an analytical point, mitosis keeps the genetic information the same from cell to cell, while meiosis shuffles it. Together they explain both stability and variety in living things.

Why Is This Topic Important?

Facts Stages and numbers asked
Contrast Mitosis versus meiosis
Link Cancer and variation

What Came Next?

The next topic covers sexual reproduction at the cellular level. Next topic: Sexual Reproduction at the Cellular Level.

Key People and Terms

Diploid Two sets of chromosomes
Haploid One set of chromosomes
Chromatid One copy of a replicated chromosome
Spindle Fibres that move chromosomes
Cytokinesis Division of the cytoplasm
Non-disjunction Failure of chromosomes to separate

Exam Corner

Points to Remember

Mitosis: 2 identical cells.
Meiosis: 4 haploid cells.
Crossing over: prophase I.
Human body cells 46; gametes 23.
DNA copied in S phase.

Do Not Confuse With

Mitosis and meiosis Identical diploid versus varied haploid cells
Chromosome and chromatid Whole structure versus one copy
Cytokinesis and mitosis Cytoplasm division versus nuclear division

Memory Hook

Mitosis makes two the same, meiosis four to vary the game; crossing over in prophase one, the chromosomes halve.

Mains Angle

Questions ask for the difference between the two divisions and their importance. These points add depth.

Process Stages
Contrast Outcome and purpose
Health Cancer

A Question You May Face

An original practice question, not a past paper question.

“Compare mitosis and meiosis and explain their significance for growth, reproduction and variation.”

How to Answer

Cycle Phases.
Mitosis Stages.
Meiosis Reduction.
Link Cancer.

GS Relevance

Prelims: stages, differences, chromosome numbers, cancer link. GS Paper 3: science and technology, health.

Frequently Asked Questions

What is the difference between mitosis and meiosis?

Mitosis is a single division that produces two genetically identical diploid cells for growth and repair, while meiosis is a double division that produces four genetically different haploid cells for sexual reproduction. Crossing over occurs only in meiosis, and meiosis halves the chromosome number.

What is crossing over?

Crossing over is the exchange of segments of DNA between homologous chromosomes during prophase I of meiosis. It creates new combinations of genes on the chromosomes and is a major source of genetic variation among the gametes and the offspring of sexually reproducing organisms.

How many chromosomes do human cells have?

Human body cells are diploid and have 46 chromosomes, made up of 23 pairs, while the gametes, sperm and egg, are haploid with 23 chromosomes. Fertilisation restores the number to 46 in the zygote, which keeps the chromosome number constant across generations.

What are the phases of the cell cycle?

The cell cycle has interphase and the mitotic phase. Interphase includes G1 for growth, S phase when DNA is copied and G2 for further preparation, and the M phase includes mitosis and cytokinesis. Checkpoints control progress, and failure of control can lead to cancer.

How is cancer related to cell division?

Cancer arises when mutations damage the genes that control the cell cycle, such as the tumour suppressor p53, so cells divide without limit and may invade other tissues. Many treatments such as radiation and chemotherapy target rapidly dividing cells, which is why they also affect healthy dividing cells.

PYQ Practice — Statement Analysis

1 Meiosis produces four haploid cells from one diploid cell.

True

In two divisions.

2 Crossing over occurs during mitosis.

False

It occurs in prophase I of meiosis.

3 DNA replication occurs in the S phase.