Cell and nuclear division
🎯What you need to be able to do
- Explain cell division and cytokinesis in animal and plant cells, including unequal cytokinesis.
- Distinguish the roles of mitosis and meiosis, and explain why DNA replication must come first.
- Explain condensation of chromosomes and their movement by microtubules and motor proteins.
- Describe the phases of mitosis and identify them in diagrams and micrographs.
- Explain meiosis as a reduction division, its two rounds of segregation, and its role in a sexual life cycle.
- Explain non-disjunction using Down syndrome, and how meiosis generates variation.
- AHL Explain cell proliferation, the phases of the cell cycle, growth in interphase and control by cyclins.
- AHL Explain the effects of mutations in proto-oncogenes and tumour suppressor genes, distinguish types of tumour, and calculate the mitotic index.
📚The biology
Cell division
In all living organisms, new cells are made by cell division: a parent cell (often called the mother cell) divides to produce two daughter cells. Cells are never assembled from scratch.
Cytokinesis
Cytokinesis is the splitting of the cytoplasm of the parent cell between the daughter cells. It happens differently in animals and plants:
A ring of contractile actin and myosin proteins forms just inside the plasma membrane around the middle of the cell. It contracts, pinching the membrane inwards (a cleavage furrow) until the cell is split in two.
The rigid cell wall cannot be pinched. Instead, vesicles collect at the middle of the cell and fuse, assembling sections of membrane and cell wall (a cell plate) that grow outwards until they join the existing walls, dividing the cell.
Equal and unequal cytokinesis
Division of cytoplasm is usually equal, but not always. Whatever the proportions, both daughter cells must receive at least one mitochondrion (and, in plants, a chloroplast) and any other organelle that can only be made by dividing a pre-existing one.
- Oogenesis in humans: the divisions of meiosis give almost all the cytoplasm to one cell, the egg, while the others become tiny polar bodies that degenerate. The egg keeps a large store of cytoplasm for the early embryo (D3.1).
- Budding in yeast: a small bud grows out from the parent cell, receives a nucleus, and pinches off as a smaller daughter cell.
Mitosis and meiosis
Nuclear division must happen before cell division, otherwise one daughter cell would have no nucleus (be anucleate). Eukaryotes have two types:
Produces two nuclei with the same chromosome number and the same genome as the parent — genetically identical. Used for growth, tissue repair, replacement of cells and asexual reproduction.
Halves the chromosome number and generates genetic diversity, producing four genetically different nuclei. Used to produce gametes (in animals) or spores (in plants and fungi) for sexual reproduction.
DNA replication first
Both mitosis and meiosis are preceded by DNA replication during interphase. After replication, each chromosome consists of two identical DNA molecules, called sister chromatids, held together at the centromere until anaphase, when they are separated.
Condensation and movement of chromosomes
During interphase, DNA is spread out so that genes can be transcribed. For division, chromosomes must condense into compact structures that can be moved without tangling or breaking. DNA is wound around histones into nucleosomes (A1.2), and this string is coiled again and again — supercoiling — making each chromosome thousands of times shorter.
Chromosomes are moved by a spindle of microtubules. Microtubules attach to chromosomes at their centromeres. Microtubule motor proteins use ATP to move chromosomes along microtubules, and microtubules shorten by losing subunits, pulling chromosomes towards the poles.
The phases of mitosis
Chromosomes condense and become visible, each as two sister chromatids. The spindle forms. The nuclear membrane breaks down at the end.
Spindle microtubules attach to the centromeres. Chromosomes line up individually on the equator (metaphase plate) of the cell.
Centromeres split and sister chromatids are pulled apart to opposite poles. Each chromatid is now a chromosome. Chromosomes appear V-shaped as they are dragged by their centromeres.
Chromosomes arrive at the poles and uncoil. Nuclear membranes re-form around each set. Cytokinesis usually follows.
Because the sister chromatids were identical copies, each pole receives a complete, identical set of chromosomes. The two daughter cells are therefore genetically identical to each other and to the parent.
Identifying the phases
In a stained root tip squash or a micrograph, look for:
- Interphase: a visible nucleus with a nucleolus and no distinct chromosomes.
- Prophase: thread-like chromosomes visible inside a still-rounded nuclear region.
- Metaphase: chromosomes in a line across the middle of the cell.
- Anaphase: two groups of V-shaped chromosomes moving apart.
- Telophase: two separate clusters of chromosomes at opposite ends, often with a new cell wall or furrow forming between them.
Meiosis: a reduction division
A diploid cell (2n) has two sets of chromosomes, one from each parent, in homologous pairs. A haploid cell (n) has one set. Meiosis consists of two divisions, following a single round of DNA replication, and produces four haploid nuclei from one diploid nucleus.
Homologous chromosomes pair up to form bivalents, and crossing over may occur. Bivalents line up on the equator. The homologous chromosomes of each pair are pulled to opposite poles. Each of the two nuclei is haploid, but each chromosome still has two chromatids.
In each of the two cells, chromosomes line up on the equator, and sister chromatids are pulled to opposite poles, as in mitosis. Four haploid nuclei result.
These are the two rounds of segregation: homologous chromosomes separate in meiosis I, and sister chromatids separate in meiosis II.
Why meiosis is needed: in a sexual life cycle, two gametes fuse at fertilization. If gametes were diploid, the chromosome number would double every generation. Meiosis halves the number, so that fertilization restores the diploid number (D3.1).
Non-disjunction and Down syndrome
Non-disjunction is the failure of chromosomes to separate correctly in meiosis — either a homologous pair in meiosis I, or sister chromatids in meiosis II. One gamete receives an extra copy of the chromosome and another receives none.
If a gamete with an extra chromosome 21 (24 chromosomes) fuses with a normal gamete (23), the zygote has 47 chromosomes, including three copies of chromosome 21 (trisomy 21). This causes Down syndrome, with characteristic features including learning difficulties and an increased risk of heart defects. The risk of non-disjunction increases with the age of the mother.
Meiosis as a source of variation
Meiosis produces genetically different gametes in two ways:
- Random orientation of bivalents. At metaphase I, each bivalent lines up independently of the others: which homologue (from the mother or father) faces which pole is random. With 23 pairs in humans, there are 223 ≈ 8.4 million possible combinations of chromosomes in a gamete, even without crossing over.
- Crossing over. In prophase I, non-sister chromatids of homologous chromosomes exchange sections of DNA. This produces chromatids with new combinations of alleles not present in either parent’s chromosomes.
Cell proliferation AHL
Cell proliferation is an increase in the number of cells by repeated division. It is needed for:
In plant meristems (at shoot and root tips) and in early-stage animal embryos, cells divide rapidly to increase size.
In skin, cells in the lower layer divide constantly to replace those lost from the surface.
After a wound in skin, cells proliferate faster to close the gap and heal.
The cell cycle AHL
Proliferation is achieved through the cell cycle, the sequence of events between one division and the next:
- G1 (first gap phase) — the cell grows and carries out its normal functions.
- S (synthesis phase) — DNA is replicated.
- G2 (second gap phase) — the cell continues to grow and prepares for division.
- Mitosis — nuclear division.
- Cytokinesis — division of the cytoplasm.
G1, S and G2 together make up interphase.
Growth during interphase AHL
Interphase is not a resting stage: it is a metabolically very active period, usually the longest part of the cycle. The cell grows by biosynthesis of its components: it synthesizes proteins (including the enzymes and histones it will need), replicates its DNA, makes membranes, and increases its stores. The numbers of mitochondria and chloroplasts are increased by growth and division of these organelles, so that both daughter cells will receive enough.
Cyclins AHL
Progress through the cell cycle is controlled at checkpoints. Proteins called cyclins regulate this. The concentrations of different cyclins rise and fall at different stages of the cycle. To pass each checkpoint, a threshold concentration of a specific cyclin must be reached: cyclins bind to and activate enzymes (cyclin-dependent kinases) that trigger the events of the next phase. Once that phase is under way, the cyclin is broken down. This ensures that each stage happens in the right order, and only when the previous one is complete. (You do not need to know the roles of specific cyclins.)
Mutations and uncontrolled division AHL
Proto-oncogenes are normal genes that stimulate cell division when needed. A mutation can turn one into an oncogene that is permanently switched on, stimulating division all the time — like a stuck accelerator.
These genes normally inhibit cell division, or trigger repair or cell death if DNA is damaged (p53 is an example). A mutation that inactivates both copies removes this brake.
Usually several mutations in different genes must accumulate in the same cell before division becomes uncontrolled, which is why most cancers become more common with age. Cells dividing without control form a tumour.
Tumours AHL
Grow relatively slowly, stay in one place, often within a capsule, and do not invade neighbouring tissue or spread. They do not cause cancer, though they can cause harm by pressing on organs.
Cells divide rapidly, invade neighbouring tissue, and can undergo metastasis: cells break away and travel in blood or lymph to other parts of the body. Malignant tumours are cancer.
The tumour where a cancer first develops is the primary tumour; a tumour formed elsewhere by cells that have spread from it by metastasis is a secondary tumour.
The mitotic index AHL
The mitotic index is the proportion of cells in a sample that are in mitosis (any of the four phases):
It indicates how rapidly a population of cells is dividing. A high mitotic index is found in growing regions such as root tips, and in tumours, where it helps to judge how aggressive a cancer is.
✏️Worked example
(a) Calculate the mitotic index.
(b) The cell cycle in these cells takes about 20 hours. Estimate the duration of mitosis, stating the assumption made.
(c) Suggest why prophase had the most cells of the four phases.
(d) State the chromosome number of an onion (2n = 16) cell in prophase of mitosis, and of a nucleus at the end of meiosis II.
(a) Cells in mitosis = 14 + 9 + 6 + 7 = 36.
(b) Assuming that the proportion of cells in a stage is proportional to the time spent in that stage (cells are dividing asynchronously at a steady rate):
(c) The number of cells in a phase reflects how long that phase lasts. Prophase is the longest phase of mitosis — chromosomes must condense extensively and the spindle must form — so at any moment more cells are caught in prophase. Anaphase is brief, so few cells are seen in it.
(d) In prophase, the cell still has 16 chromosomes (each made of two chromatids, so 32 chromatids). A nucleus at the end of meiosis II is haploid: 8 chromosomes, each a single chromatid.
📝Practise
Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.
1. Compare cytokinesis in animal cells and plant cells.
2. Describe the events of mitosis.
3. Explain why meiosis is described as a reduction division, and why it is needed in a sexual life cycle.
4. Explain two ways in which meiosis generates genetic variation.
5. AHL Explain how cyclins control the cell cycle.
6. AHL Distinguish between benign and malignant tumours, and between primary and secondary tumours.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- The Biology Project (University of Arizona) — an online onion root tip activity for identifying phases and estimating their duration.
- HHMI BioInteractive — animations of mitosis, meiosis and the cell cycle checkpoints.
- Cancer Research UK — accessible explanations of oncogenes, tumour suppressor genes and how cancers spread.