IMAT Biology · Cell Division and Reproduction

Chromosomes, the Cell Cycle and Mitosis

A body cell copies its DNA, then splits it equally by mitosis, so each of the two daughter cells gets the same chromosomes as the parent.

All Cell Division and Reproduction notes

Why this matters

This page and the meiosis page carry almost all of the chapter's questions. The papers from 2012 to 2022 asked for the order of the mitosis stages, what happens in anaphase, which stage a root-tip cell shows, the alleles a cell holds before and after DNA copying, and how many cells repeated divisions make.

Concept 1 of 4: Chromosome structure: chromatids, centromere and homologous pairs

A chromosome is one long DNA molecule wound round proteins. Before a cell divides it copies every chromosome, and the two copies stay joined until the split. Body cells carry two sets of chromosomes, one from each parent, so chromosomes come in matching pairs.

Definition

Words you need for every question in this chapter:

Chromatin: DNA wound round histone proteins. Loose in interphase, tightly coiled (visible) during division.
Sister chromatids: the two identical copies of one chromosome made in S phase, joined at the centromere.
Kinetochore: the protein patch on the centromere where spindle fibres (microtubules) attach.
Homologous chromosomes: a pair with the same genes in the same order (one from each parent), possibly with different alleles.
Diploid (2n): two sets of chromosomes. Haploid (n): one set. Humans: $2n = 46$ (22 pairs of autosomes plus XX or XY), $n = 23$ .

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.

Practice this conceptself-check · 4 quick reps

Spindle fibres attach at the centromere, not the centriole

The names are close, and IMAT uses that. The centromere is on the chromosome; its kinetochore is where the fibres grip. The centrioles are at the poles of an animal cell and help organise the spindle. A chromosome never attaches to a centriole.

Concept 2 of 4: Phases of the cell cycle and their checkpoints

Most of a cell's life is spent in interphase, growing and copying its DNA. Division itself is short. The cell checks itself at fixed points, like a pilot's checklist, and only goes on when each check passes. Cancer starts when those checks fail.

Definition

The cell cycle is interphase (G1, S, G2) followed by M phase (mitosis and cytokinesis).

Interphase takes about 90% of the cycle. DNA is copied only in S phase.
G0 is a resting state outside the cycle: mature nerve cells and most muscle cells stay there.
Checkpoints: G1 (is the cell big enough, is the DNA undamaged?), G2/M (is all DNA copied correctly?), and the spindle checkpoint in metaphase (is every chromosome attached?).
The cycle is driven by cyclins binding cyclin-dependent kinases (CDKs). The protein p53 stops the cycle when DNA is damaged and can trigger apoptosis (programmed cell death).
Cancer is uncontrolled division after mutations in proto-oncogenes (which then become oncogenes) and tumour suppressor genes such as p53.

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.

Practice this conceptself-check · 4 quick reps

Interphase is not a resting phase

Older books call interphase resting, but the cell is busy: it grows, copies its DNA and checks it. The true resting state is G0, which is outside the cycle.

Concept 3 of 4: The stages of mitosis in order: prophase, metaphase, anaphase, telophase

Mitosis is a sorting job. First pack the long DNA threads into short chromosomes, line them up in the middle, pull each one apart into its two copies, then wrap a new nucleus round each group. Remember the order as PMAT, then cytokinesis.

Definition

Mitosis divides one nucleus into two genetically identical nuclei with the same chromosome number as the parent.

It is used for growth, repair, and asexual reproduction in eukaryotes.
Cytokinesis (division of the cytoplasm) follows it, usually starting in late anaphase or telophase.
In animal cells a cleavage furrow pinches the cell in two; in plant cells a cell plate forms from Golgi vesicles and becomes the new wall.
Root tips and shoot tips (meristems) are where plant cells divide, so they are used to see mitosis under a microscope.

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.

Practice this conceptself-check · 4 quick reps

In mitosis, chromatids separate, not homologous pairs

Homologous chromosomes never pair up in mitosis. Each chromosome lines up on its own and is split into its two sister chromatids in anaphase. Pairs of homologues lining up and separating is meiosis I.

DNA is copied before mitosis, not during it

DNA replication happens in S phase of interphase. An option placing DNA copying in prophase or anaphase is wrong: by prophase each chromosome already has two chromatids.

Concept 4 of 4: Counting cells, chromosomes, chromatids and DNA through mitosis

Each round of mitosis doubles the number of cells, so the count grows as a power of 2. Inside each cell, the DNA doubles in S phase and halves again when the cell splits, but the chromosome number never changes. The alleles behave like the DNA: they are copied, so a cell that was Aa becomes AAaa until it divides.

Definition

Three counts to keep apart:

Chromosomes: count the centromeres. Through mitosis the number stays at the parent's (2n stays 2n).
Chromatids: one per chromosome in G1, two per chromosome from S phase to metaphase.
DNA content (c): 2c in G1, 4c after S, back to 2c in each daughter cell.
Alleles: a heterozygous cell is Aa in G1 and AAaa after S phase (each allele now on two sister chromatids). Each daughter is Aa again.
Cell number: starting with one cell, after $k$ rounds of division there are $2^k$ cells (if none die).

Cells after k rounds of mitosis

N = N_0 \times 2^{k}

$N_0$ number of cells at the start
$k$ number of rounds of division
$N$ number of cells at the end

Worked example

A plant cell has 12 chromosomes and 10 pg of DNA in G1. It divides by mitosis 6 times and every cell survives. Give the chromosomes and DNA of one cell in G2, and the number of cells at the end.

Practice this conceptself-check · 4 quick reps

Ten divisions do not make ten or twenty cells

Every cell divides each round, so the count doubles every time: 2, 4, 8, 16 and so on. The total after k rounds is

2^k

, not

k + 1

2k

Copying DNA does not double the chromosome number

After S phase a human cell still has 46 chromosomes, but 92 chromatids and twice the DNA. Counting each chromatid as a chromosome is the classic wrong row in IMAT tables.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

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