IMAT Biology · Free notes

Cell Division and Reproduction: Mitosis, Meiosis and Gametes

Cell Division and Reproduction has 19 past questions since 2011, and the ministry papers from 2023 on have asked 1 of them. Almost every question is about mitosis or meiosis: putting the stages in order, naming the stage a description shows, or counting chromosomes, chromatids and DNA at a given moment. The difficulty is bookkeeping, not memory. Most wrong options count chromatids as chromosomes, call a cell diploid because its DNA has doubled, or swap what separates in meiosis I with what separates in meiosis II. Learn the counts for one species and you can redo them for any species in the exam.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

3 formulas · 10 reference tables · 18 gotchas across all subtopics — the exam-eve cheat-sheet

Chromosomes, the Cell Cycle and Mitosis

Formulas (1)

Counting cells, chromosomes, chromatids and DNA through mitosis · Cells after k rounds of mitosis $N = N_0 \times 2^{k}$

Reference tables (3)

Chromosome structure: chromatids, centromere and homologous pairs5 rows

Term	What it is	Fact IMAT tests
Gene	A section of DNA on a chromosome that codes for a product	Each chromosome carries hundreds to thousands of genes
Centromere	The joining point of two sister chromatids	Spindle fibres attach here, at the kinetochore
Centriole	A small cylinder of microtubules; two make a centrosome	Organises the spindle in animal cells; plants have none
Sister chromatids	Two copies of one chromosome after S phase	Genetically identical (barring a copying error)
Homologous pair	Two chromosomes, one from each parent, same genes	Can carry different alleles; X and Y in a male pair only partly

A replicated chromosome is still counted as ONE chromosome. Count centromeres to count chromosomes.

Phases of the cell cycle and their checkpoints5 rows

Phase	Main events	Chromosomes and DNA
G1	Cell grows, makes proteins and organelles	2n chromosomes, each one chromatid; DNA content 2c
S	DNA replication; centrosome duplicates	Still 2n chromosomes, now two chromatids each; DNA rises to 4c
G2	More growth, checks the copied DNA, prepares for division	2n chromosomes, two chromatids each; 4c
M	Mitosis splits the nucleus, cytokinesis splits the cytoplasm	Each daughter: 2n, one chromatid each; 2c
G0	Cell has left the cycle and does its job	Stays at 2n and 2c

c is the DNA content of one unreplicated set of chromosomes.

The stages of mitosis in order: prophase, metaphase, anaphase, telophase5 rows

Stage	What happens	How to spot it
Prophase	Chromatin condenses into visible chromosomes; nucleolus disappears; spindle forms; nuclear envelope breaks down at the end	Thick threads inside or just leaving the nucleus
Metaphase	Chromosomes line up singly on the equator; fibres from both poles attach to each kinetochore	One straight line of chromosomes across the middle
Anaphase	Centromeres split; sister chromatids are pulled to opposite poles as the fibres shorten	Two V-shaped groups moving apart
Telophase	Chromosomes reach the poles and uncoil; nuclear envelopes and nucleoli reform; spindle breaks down	Two clusters at the ends, new nuclei forming
Cytokinesis	Cytoplasm divides: furrow in animals, cell plate in plants	Two separate daughter cells

Some books add prometaphase (envelope breaks, fibres attach) between prophase and metaphase.

Watch out for (6)

Spindle fibres attach at the centromere, not the centriole
→ Chromosome structure: chromatids, centromere and homologous pairs
Interphase is not a resting phase
→ Phases of the cell cycle and their checkpoints
In mitosis, chromatids separate, not homologous pairs
→ The stages of mitosis in order: prophase, metaphase, anaphase, telophase
DNA is copied before mitosis, not during it
→ The stages of mitosis in order: prophase, metaphase, anaphase, telophase
Ten divisions do not make ten or twenty cells
→ Counting cells, chromosomes, chromatids and DNA through mitosis
Copying DNA does not double the chromosome number
→ Counting cells, chromosomes, chromatids and DNA through mitosis

Meiosis, Chromosome Numbers and Variation

Formulas (2)

Chromosome number (n) and DNA content (c) at each stage of meiosis · Ploidy and DNA through meiosis $2n,\,4c \xrightarrow{\text{meiosis I}} n,\,2c \xrightarrow{\text{meiosis II}} n,\,c$
Sources of variation in meiosis: crossing over, independent assortment, random fertilisation · Gamete combinations from independent assortment $\text{combinations} = 2^{n}$

Reference tables (2)

The stages of meiosis I and meiosis II7 rows

Stage	What happens	Chromosomes in each cell
Prophase I	Chromosomes condense, homologues pair into bivalents, crossing over	2n, each with two chromatids
Metaphase I	Bivalents (pairs) line up on the equator, each pair facing a random pole	2n, as pairs
Anaphase I	Homologous chromosomes go to opposite poles; sister chromatids stay joined	2n in the cell, n moving to each pole
Telophase I	Two cells form	n, each still with two chromatids
Metaphase II	Chromosomes line up singly on the equator	n, each with two chromatids
Anaphase II	Centromeres split; sister chromatids go to opposite poles	n moving to each pole
Telophase II	Four haploid cells form	n, each with one chromatid

Pairs on the equator means meiosis I; single chromosomes on the equator means mitosis or meiosis II.

Mitosis compared with meiosis7 rows

Feature	Mitosis	Meiosis
Divisions after one S phase	One	Two
Cells made from one cell	Two	Four
Chromosome number	Same as parent (2n gives 2n)	Halved (2n gives n)
Genetic make-up	Identical to parent and to each other	Different from parent and from each other
Homologues pair, crossing over	Never	In prophase I
Where in humans	Somatic (body) cells everywhere	Germ cells in the testes and ovaries
Role	Growth, repair, asexual reproduction	Making gametes (animals) or spores (plants)

Watch out for (6)

Homologous pairs moving apart means anaphase I
→ The stages of meiosis I and meiosis II
A cell after meiosis I is haploid, even though its DNA is 2c
→ Chromosome number (n) and DNA content (c) at each stage of meiosis
Daughter cells of mitosis are not haploid
→ Chromosome number (n) and DNA content (c) at each stage of meiosis
Use n, not 2n, in 2 to the power n
→ Sources of variation in meiosis: crossing over, independent assortment, random fertilisation
Crossing over is between non-sister chromatids
→ Sources of variation in meiosis: crossing over, independent assortment, random fertilisation
Meiosis is not limited to multicellular organisms or to special machinery
→ Mitosis compared with meiosis

Making Sperm and Eggs, Fertilisation and the Early Embryo

Reference tables (3)

Spermatogenesis: the stages of sperm production in the testis5 rows

Cell	Ploidy	Made by
Spermatogonium	Diploid (2n)	Mitosis of germ cells
Primary spermatocyte	Diploid (2n), DNA copied	Growth of a spermatogonium
Secondary spermatocyte	Haploid (n), two chromatids per chromosome	Meiosis I (2 per primary)
Spermatid	Haploid (n)	Meiosis II (4 per primary)
Spermatozoon	Haploid (n)	Differentiation of a spermatid, no division

Oogenesis: egg production and its pauses5 rows

Cell	Ploidy	When
Oogonium	Diploid (2n)	Multiplies by mitosis before birth
Primary oocyte	Diploid (2n), DNA copied	Formed before birth; paused in prophase I until puberty
Secondary oocyte	Haploid (n), two chromatids per chromosome	Made by meiosis I; paused in metaphase II; ovulated
Polar bodies	Haploid (n)	Small cells from each unequal division; they degenerate
Ovum	Haploid (n)	Completed only after a sperm enters

Fertilisation and early development of the human embryo3 rows

Germ layer	Position	Gives rise to
Ectoderm	Outer	Epidermis of the skin, hair, nervous system (brain, spinal cord, nerves)
Mesoderm	Middle	Muscle, bone, heart, blood and vessels, kidneys, gonads
Endoderm	Inner	Lining of the gut and airways, liver, pancreas

Watch out for (4)

Sperm are made by meiosis, not mitosis
→ Spermatogenesis: the stages of sperm production in the testis
Spermatogenesis gives four gametes, oogenesis gives one
→ Oogenesis: egg production and its pauses
The secondary oocyte is already haploid
→ Oogenesis: egg production and its pauses
Identical twins share one zygote
→ Fertilisation and early development of the human embryo

Asexual and Sexual Reproduction and Life Cycles

Reference tables (2)

Asexual and sexual reproduction: methods, advantages and costs5 rows

Method	How it works	Example
Binary fission	One cell copies its DNA and splits in two	Bacteria, Amoeba
Budding	An outgrowth grows and breaks off	Yeast, Hydra
Fragmentation	A piece of the body regrows into a whole organism	Flatworms (planarians), starfish
Vegetative propagation	A new plant grows from a stem, root or leaf	Potato tubers, strawberry runners, onion bulbs
Parthenogenesis	An egg develops without being fertilised	Aphids, male honeybees, some lizards

Sexual reproduction is favoured when the environment changes; asexual reproduction when it is stable.

Life cycles: where meiosis, mitosis and fertilisation fall4 rows

Life cycle	Main body	Meiosis makes	Example
Diplontic	Diploid	Gametes	Humans and other animals
Haplontic	Haploid	Haploid cells from the zygote	Most fungi, Chlamydomonas
Alternation, gametophyte dominant	Haploid gametophyte	Spores	Mosses
Alternation, sporophyte dominant	Diploid sporophyte	Spores	Ferns, flowering plants

Watch out for (2)

Variation is not only for sexual reproducers
→ Asexual and sexual reproduction: methods, advantages and costs
Gametes are not always made by meiosis
→ Life cycles: where meiosis, mitosis and fertilisation fall

PYQ weightage by concept

13 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Chromosomes, the Cell Cycle and Mitosis0 PYQs · 0%

Concept	PYQs	Share
Chromosome structure: chromatids, centromere and homologous pairsfoundation	—	—
Phases of the cell cycle and their checkpointsfoundation	—	—
The stages of mitosis in order: prophase, metaphase, anaphase, telophasefoundation	—	—
Counting cells, chromosomes, chromatids and DNA through mitosisfoundation	—	—

Meiosis, Chromosome Numbers and Variation0 PYQs · 0%

Concept	PYQs	Share
The stages of meiosis I and meiosis IIfoundation	—	—
Chromosome number (n) and DNA content (c) at each stage of meiosisfoundation	—	—
Sources of variation in meiosis: crossing over, independent assortment, random fertilisationfoundation	—	—
Mitosis compared with meiosisfoundation	—	—

Making Sperm and Eggs, Fertilisation and the Early Embryo0 PYQs · 0%

Concept	PYQs	Share
Spermatogenesis: the stages of sperm production in the testisfoundation	—	—
Oogenesis: egg production and its pausesfoundation	—	—
Fertilisation and early development of the human embryofoundation	—	—

Asexual and Sexual Reproduction and Life Cycles0 PYQs · 0%

Concept	PYQs	Share
Asexual and sexual reproduction: methods, advantages and costsfoundation	—	—
Life cycles: where meiosis, mitosis and fertilisation fallfoundation	—	—