Difficulty: Intermediate | Prerequisites: DNA structure, DNA replication (see companion notes)
Big Picture
Every living organism grows, repairs damage, and (in sexually reproducing species) produces offspring by dividing its cells. The cell cycle is the master programme that governs when a cell grows, copies its DNA, and splits in two. Mitosis produces genetically identical daughter cells for growth and repair, while meiosis halves the chromosome number to make gametes (sex cells) and introduces genetic variation through crossing over and independent assortment. You should already understand DNA structure and replication before working through this material.
Cells follow a repeating cycle of growth, DNA replication, and division. Mitosis splits one cell into two genetically identical diploid daughters (for growth and repair), while meiosis produces four genetically unique haploid cells (gametes) through two rounds of division. Errors in either process, such as non-disjunction or checkpoint failures, can lead to disorders or cancer.
Cell cycle
The ordered sequence of events in which a cell grows, replicates its DNA, and divides. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis + cytokinesis).
Think of it as the cell's life programme: grow, copy, divide, repeat.
Interphase
The longest phase of the cell cycle, during which the cell grows, carries out its normal functions, and replicates its DNA. Divided into G1 (first gap/growth), S (synthesis, where DNA is copied), and G2 (second gap/growth, preparation for division).
This is the "working" phase. The cell spends most of its life here, not in mitosis.
Mitosis
A type of nuclear division that produces two daughter nuclei, each genetically identical to the parent nucleus and to each other. Consists of prophase, metaphase, anaphase, and telophase (PMAT).
In simple terms, mitosis is copying and splitting the nucleus so both new cells get exactly the same DNA.
Cytokinesis
The physical division of the cytoplasm that follows mitosis, producing two separate daughter cells.
In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms down the middle because the rigid cell wall prevents pinching.
Meiosis
A type of cell division that reduces the chromosome number by half, producing four genetically unique haploid daughter cells (gametes). Involves two sequential divisions: meiosis I (separates homologous chromosomes) and meiosis II (separates sister chromatids).
This is how the body makes sperm and egg cells with half the usual chromosome count, so that fertilisation restores the full number.
Diploid (2n)
A cell containing two complete sets of chromosomes, one from each parent. In humans, 2n = 46.
Most body cells (somatic cells) are diploid.
Haploid (n)
A cell containing one complete set of chromosomes. In humans, n = 23.
Gametes (sperm and egg) are haploid. When two haploid gametes fuse at fertilisation, the resulting zygote is diploid.
Allele
A variant form of a gene. Diploid organisms carry two alleles for each gene, one on each homologous chromosome.
Think of alleles as different "versions" of the same instruction, for example, one allele for brown eyes and one for blue.
Chromatin
The loosely coiled complex of DNA and histone proteins found in the nucleus during interphase. This is the "unwound" form of genetic material, allowing genes to be read and copied.
Histone
A small protein that DNA wraps around for packaging and organisation. Eight histones form a nucleosome core, and DNA winds around it like thread on a spool.
Chromosome
A highly condensed, tightly coiled structure of chromatin that becomes visible under a light microscope during cell division. Each chromosome consists of one (unreplicated) or two (replicated) chromatids.
Chromatid
One copy of a duplicated chromosome, joined to the other copy at the centromere. After DNA replication, each chromosome has two sister chromatids. These are separated during anaphase of mitosis (or anaphase II of meiosis).
Before replication: one chromosome = one chromatid. After replication: one chromosome = two sister chromatids joined at the centromere.
Homologous chromosomes
A matched pair of chromosomes, one inherited from each parent, carrying the same genes (though potentially different alleles) at the same loci. They pair up during meiosis I.
Crossing over
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis. This produces recombinant chromosomes and increases genetic variation.
Non-disjunction
The failure of chromosomes (or chromatids) to separate properly during cell division. This results in daughter cells with abnormal chromosome numbers (aneuploidy). Can occur during meiosis I, meiosis II, or mitosis.
In simple terms, chromosomes that should have gone to opposite ends of the cell instead travel together, so one daughter gets too many and the other too few. Down syndrome (trisomy 21) is a well-known result.
G1 (Gap 1): the cell grows, carries out normal metabolic functions, and produces organelles. Cyclins and checkpoints decide whether the cell is ready to proceed.
S (Synthesis): the cell replicates all of its DNA. Each chromosome goes from one chromatid to two sister chromatids joined at the centromere.
G2 (Gap 2): the cell continues to grow and synthesises proteins needed for division (e.g. tubulin for spindle fibres). A second checkpoint verifies that DNA replication is complete and undamaged.
Growth and development: a single fertilised egg becomes a multicellular organism through repeated cell division.
Repair and replacement: damaged or worn-out cells (skin, blood, gut lining) are replaced.
Reproduction: in unicellular organisms, division is reproduction; in multicellular organisms, meiosis produces gametes.
Before S phase, a chromosome is a single DNA molecule (one chromatid).
After S phase, each chromosome consists of two identical sister chromatids joined at the centromere.
Mitosis separates sister chromatids, so each daughter cell receives one chromatid from each chromosome.
The word "chromosome" refers to the unit regardless of whether it has been replicated. A chromatid is one arm of that replicated unit.
During interphase, DNA exists as chromatin (loosely coiled around histones), allowing access for transcription and replication.
As the cell enters mitosis, chromatin condenses into visible chromosomes.
Histones are the protein spools that DNA wraps around. They help package roughly 2 metres of DNA into a nucleus about 6 micrometres across.
Prophase: chromatin condenses into visible chromosomes (each is two sister chromatids). The nuclear envelope begins to break down. Centrioles (in animal cells) move to opposite poles and spindle fibres begin to form.
Metaphase: chromosomes line up along the metaphase plate (the cell's equator). Spindle fibres attach to the centromere of each chromosome.
Anaphase: sister chromatids are pulled apart to opposite poles of the cell by the shortening spindle fibres. Once separated, each chromatid is now called a chromosome.
Telophase: chromosomes arrive at the poles, decondense back into chromatin, and nuclear envelopes re-form around each set. The spindle disassembles.
One division produces two daughter cells, each diploid (2n) and genetically identical to the parent cell.
The correct answer on the review sheet is B: 2 daughter cells, diploid.
Animal cells: a contractile ring of actin filaments pinches the membrane inward, forming a cleavage furrow that deepens until the cell splits in two.
Plant cells: vesicles from the Golgi apparatus fuse at the centre of the cell to form a cell plate, which grows outward until it divides the cell. A new cell wall is then deposited on either side.
The mitotic index is the fraction of cells in a sample that are undergoing mitosis: (number of cells in mitosis) / (total number of cells).
In normal tissue, the mitotic index is low (roughly 1 to 5 percent of cells dividing at any given time).
An elevated mitotic index may indicate rapid growth (e.g. embryonic tissue, wound healing) or uncontrolled division (cancer).
Meiosis I (the reduction division):
Interphase (before meiosis I): DNA is replicated, so each chromosome has two sister chromatids.
Prophase I: homologous chromosomes pair up (synapsis), forming tetrads. Crossing over occurs here, swapping segments between non-sister chromatids of homologous pairs. This is a major source of genetic variation. The nuclear envelope breaks down and spindle fibres form.
Metaphase I: homologous pairs line up along the metaphase plate (as pairs, not individual chromosomes). Which homologue faces which pole is random (independent assortment).
Anaphase I: homologous chromosomes are pulled to opposite poles. Sister chromatids stay together.
Telophase I and cytokinesis: two haploid cells form, each with one chromosome from each homologous pair (still as sister chromatids).
Meiosis II (similar to mitosis):
No DNA replication occurs between meiosis I and meiosis II.
Prophase II: chromosomes condense (if they decondensed), nuclear envelope breaks down, spindle forms.
Metaphase II: chromosomes line up individually at the metaphase plate.
Anaphase II: sister chromatids are pulled apart to opposite poles.
Telophase II and cytokinesis: four haploid daughter cells are produced, each genetically unique.
Non-disjunction is the failure of chromosomes or chromatids to separate properly.
In meiosis I, homologous chromosomes fail to separate, so both go to one daughter cell.
In meiosis II, sister chromatids fail to separate.
Non-disjunction can also occur during mitosis.
The result is aneuploidy: cells with too many or too few chromosomes. A well-known example is trisomy 21 (Down syndrome), where an individual has three copies of chromosome 21.
Mitosis | Meiosis | |
|---|---|---|
Number of divisions | 1 | 2 |
Number of daughter cells | 2 | 4 |
Chromosome number in daughters | Diploid (2n) | Haploid (n) |
Genetically identical to parent? | Yes | No (genetic variation from crossing over and independent assortment) |
Function | Growth, repair, asexual reproduction | Production of gametes (sex cells) |
Understanding mitosis is central to cancer research: tumour cells are cells that have lost normal cell-cycle control and divide without restraint. Anti-cancer drugs like taxol work by disrupting spindle fibres during mitosis, preventing cell division.
Meiosis and crossing over are the basis of genetic diversity in sexually reproducing organisms, which is why offspring are not clones of their parents. Agricultural breeding programmes rely on the variation meiosis creates to develop crops with desirable traits.
Students often confuse mitosis with the entire cell cycle. Mitosis is just the division phase. The cell spends most of its time in interphase.
A frequent error is saying meiosis produces diploid cells. Meiosis produces haploid cells. Mitosis produces diploid cells (in a diploid organism).
Students sometimes think crossing over happens in mitosis. It does not. Crossing over occurs in prophase I of meiosis only.
Another common mix-up is treating "chromosome" and "chromatid" as interchangeable. After replication, one chromosome is made of two sister chromatids. Once those chromatids separate (at anaphase), each one is called a chromosome.
⚠️ Know the stages of mitosis (PMAT) and what happens at each. Expect to label diagrams and describe chromosome movements.
⚠️ Be able to outline all stages of meiosis I and meiosis II and explain what distinguishes each from the corresponding stage of mitosis.
⚠️ The mitosis vs meiosis comparison table is a standard exam question. Memorise the number of divisions, daughter cells, ploidy, and function for each.
⚠️ Crossing over and independent assortment are the two main sources of genetic variation in meiosis. Know when each occurs and why it matters.
⚠️ Non-disjunction questions will ask you to identify where it can occur (meiosis I, meiosis II, or mitosis) and what disorders it causes (e.g. trisomy 21).
⚠️ Distinguish cytokinesis in animal vs plant cells (cleavage furrow vs cell plate). This is a favourite short-answer question.
⚠️ Understand the mitotic index: how to calculate it, what a normal value looks like, and what an elevated index suggests.
True or false: Mitosis produces four haploid daughter cells. (False. Mitosis produces two diploid daughter cells.)
Fill in the blank: Crossing over occurs during ______ of meiosis. (Prophase I.)
True or false: After DNA replication, each chromosome consists of two sister chromatids joined at the centromere. (True.)
Fill in the blank: In animal cells, cytokinesis occurs by formation of a ______. (Cleavage furrow.)
True or false: Non-disjunction can only occur during meiosis. (False. It can also occur during mitosis.)
Q: Outline the events of interphase, naming all three sub-phases.
A: G1 (cell growth and normal function), S phase (DNA replication, each chromosome becomes two sister chromatids), G2 (further growth and preparation for mitosis, including synthesis of proteins for the spindle apparatus).
Q: Why do cells divide?
A: For growth and development of the organism, repair and replacement of damaged or worn-out cells, and reproduction (meiosis produces gametes in multicellular organisms; binary fission or mitosis in unicellular organisms).
Q: Describe the events of prophase in mitosis.
A: Chromatin condenses into visible chromosomes (each consisting of two sister chromatids). The nuclear envelope begins to break down. In animal cells, centrioles migrate to opposite poles and spindle fibres start to form.
Q: Identify the outcome of one division by mitosis.
A: Two daughter cells, each diploid (2n) and genetically identical to the parent cell.
Q: Distinguish between cytokinesis in animal and plant cells.
A: In animal cells, a cleavage furrow forms as a ring of actin filaments contracts, pinching the cell in two. In plant cells, a cell plate forms from vesicles at the centre and grows outward to divide the cell, because the rigid cell wall prevents pinching.
Q: State the function of meiosis.
A: To produce haploid gametes (sex cells) with half the chromosome number of the parent cell, introducing genetic variation through crossing over and independent assortment.
Q: During which stage of meiosis does crossing over occur, and what is its effect?
A: Crossing over occurs during prophase I. Homologous chromosomes exchange segments of DNA, producing recombinant chromosomes that carry new combinations of alleles. This increases genetic diversity among offspring.
Q: What is non-disjunction, and what disorder can it cause?
A: Non-disjunction is the failure of chromosomes or chromatids to separate properly during cell division. It results in daughter cells with abnormal chromosome numbers. A well-known example is trisomy 21 (Down syndrome), caused by three copies of chromosome 21.
Q: Compare mitosis and meiosis in terms of number of divisions, daughter cells produced, and ploidy of daughters.
A: Mitosis involves one division, produces two diploid (2n) daughter cells. Meiosis involves two divisions, produces four haploid (n) daughter cells.
Q: What is the mitotic index, and what might an elevated mitotic index indicate?
A: The mitotic index is the proportion of cells in a tissue sample that are undergoing mitosis. An elevated mitotic index may indicate rapid cell growth (e.g. embryonic tissue) or uncontrolled cell division, which can be a sign of cancer.
DNA replication (covered in the companion study notes) must happen during S phase before either mitosis or meiosis can begin. Errors in replication feed directly into the mutation and cancer topics covered in the third set of notes.
Meiosis connects to the genetics unit: independent assortment and crossing over during meiosis are the basis of Mendel's laws and explain why siblings look different from one another. Non-disjunction links to the human genetic disorders section of the course.
Cell cycle, interphase, G1 phase, S phase, G2 phase, mitosis, prophase, metaphase, anaphase, telophase, PMAT, cytokinesis, cleavage furrow, cell plate, meiosis, meiosis I, meiosis II, homologous chromosomes, crossing over, recombination, independent assortment, non-disjunction, aneuploidy, trisomy 21, Down syndrome, diploid, haploid, 2n, n, allele, chromatin, histone, chromosome, chromatid, sister chromatids, centromere, spindle fibres, mitotic index, gametes, sex cells, genetic variation, BIO 101, cell division review