Describe cytokinesis, mitosis and meiosis, explain how meiosis halves the chromosome number and creates variation, and (HL) how the cell cycle is controlled and how cancer arises.
Each is in the Biology glossary, with every other term in the subject.
The phase of mitosis in which sister chromatids separate at the centromere and are pulled to opposite poles of the cell.
A tumour that grows slowly, stays in one place and does not invade neighbouring tissue or spread; it is not cancerous.
A pair of homologous chromosomes held together during prophase I and metaphase I of meiosis; it consists of four chromatids.
A form of asexual reproduction, as in yeast, in which a small outgrowth of the parent cell receives a nucleus after mitosis and is separated by unequal cytokinesis.
The sequence of events between one cell division and the next, consisting of interphase (G1, S and G2), mitosis and cytokinesis.
The process by which a parent cell (mother cell) divides to form two daughter cells.
A membrane-bound structure formed at the equator of a dividing plant cell by the fusion of vesicles from the Golgi apparatus; it develops into the new plasma membranes and cell wall.
An increase in the number of cells by repeated cycles of cell growth and division by mitosis, used for growth, cell replacement and tissue repair.
Cell division is the process by which one cell becomes two. It allows organisms to grow from a single cell, to replace and repair their tissues, and to pass genetic information to the next generation.
Eukaryotic cells divide their nucleus in one of two ways. Mitosis produces genetically identical nuclei for growth and repair; meiosis produces genetically varied haploid nuclei for sexual reproduction. Both depend on the same machinery for copying, packing and moving chromosomes.
45 exam-style questions on this subtopic, written in the style of the IB papers. The first has its markscheme open. Counted on
Variation is the raw material on which evolution acts.
Describe how meiosis generates genetic variation.
4 marks
a. crossing over between non-sister chromatids of homologous chromosomes;
b. occurs in prophase I;
c. produces new combinations of alleles on a chromosome / recombinant chromatids;
d. random orientation of pairs of homologous chromosomes in metaphase I;
e. independent assortment of chromosomes into the daughter cells;
f. possible combinations of chromosomes in gametes (8 million in humans);
g. each gamete is genetically different;
[4 max]
Outline how the selective breeding of crop plants provides evidence for evolution.
One of 1 short multiple-choice questions on D2.1. Pick an answer to see why it's right or wrong.
In which stage of mitosis do sister chromatids separate?
The official deck for this part of the syllabus.
Define cell division.
State three things that cell division allows organisms to do.
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A point in the cell cycle, such as the end of G1 or G2, at which the cell continues only if conditions are right, for example when DNA replication is complete.
A point where non-sister chromatids of a bivalent have crossed and exchanged sections of DNA during crossing over in prophase I.
One of the two identical DNA molecules that make up a chromosome after DNA replication; sister chromatids are held together at the centromere until anaphase.
The groove that forms around the equator of a dividing animal cell as a contractile ring of actin and myosin pulls the plasma membrane inwards during cytokinesis.
The exchange of sections of DNA between non-sister chromatids of a pair of homologous chromosomes during prophase I of meiosis.
One of a family of regulatory proteins whose concentrations rise and fall during the cell cycle; a cyclin activates a cyclin-dependent kinase, allowing the cell to pass a checkpoint.
An enzyme that is active only when bound to a cyclin; it phosphorylates target proteins that trigger the next stage of the cell cycle.
The division of the cytoplasm of a parent cell between the two daughter cells, after nuclear division.
Describes a nucleus or cell that has two sets of chromosomes, so that each chromosome has a homologous partner (2n).
A condition caused by trisomy 21, three copies of chromosome 21, usually resulting from non-disjunction during meiosis.
The first growth phase of interphase, in which the cell grows, synthesises proteins and increases its number of organelles before DNA replication.
The second growth phase of interphase, after DNA replication, in which the cell grows further and prepares for mitosis.
Describes a nucleus or cell that has one set of chromosomes (n), with one chromosome of each type.
The part of the cell cycle between divisions, in which the cell grows, synthesises proteins, replicates its DNA and carries out its normal functions.
A protein structure at the centromere of each chromatid to which spindle microtubules attach during nuclear division.
A cancerous tumour whose cells divide rapidly, invade neighbouring tissue and can spread to other parts of the body by metastasis.
Division of a diploid nucleus into four haploid nuclei by two successive divisions, producing nuclei that are genetically different from one another.
A region of undifferentiated plant cells that divide repeatedly by mitosis, producing new cells for growth.
The phase of mitosis in which chromosomes, attached to spindle microtubules from both poles, line up on the equator of the cell.
The spread of cancer cells from a primary tumour to other parts of the body, through blood or lymph, where they form secondary tumours.
Division of the nucleus into two genetically identical daughter nuclei, each with the same number of chromosomes as the parent nucleus.
The ratio of the number of cells in mitosis to the total number of cells in a sample of tissue, used as a measure of how fast cells are dividing.
The failure of homologous chromosomes (in anaphase I) or sister chromatids (in anaphase II) to separate during meiosis, producing gametes with an extra or a missing chromosome.
A mutated form of a proto-oncogene whose product is permanently active or overproduced, stimulating uncontrolled cell division.
The production of egg cells in the ovary; its divisions are unequal, so one cell keeps most of the cytoplasm and the others form small polar bodies.
The tumour that forms first, at the site in the body where a cancer began.
The first phase of mitosis, in which chromosomes condense, the spindle forms and the nuclear membrane breaks down.
A normal gene whose product stimulates cell division, such as a growth factor receptor; a mutation can convert it into an oncogene.
The independent and random alignment of each bivalent on the equator in metaphase I, which gives 2ⁿ possible combinations of chromosomes in the gametes.
The synthesis phase of interphase, in which DNA is replicated so that each chromosome comes to consist of two sister chromatids.
A tumour formed in another part of the body by cancer cells that have spread from a primary tumour.
An array of microtubules that forms during nuclear division, attaches to chromosomes at their centromeres and moves chromosomes or chromatids to opposite poles.
The repeated coiling of DNA and its histones into more and more compact structures, which condenses chromosomes before nuclear division.
The last phase of mitosis, in which chromosomes reach the poles and uncoil and a nuclear membrane forms around each group.
An abnormal mass of tissue formed by uncontrolled cell division.
A gene whose product restrains the cell cycle, repairs DNA or triggers the death of damaged cells; mutations that inactivate it allow uncontrolled division.
Every living organism grows, repairs itself and reproduces by making new cells. New cells are never assembled from non-living material: they are produced only when an existing cell divides. This is one of the three principles of cell theory, which states that all living things are made of cells, that the cell is the smallest unit of life, and that cells arise only from pre-existing cells.
In cell division the original cell is called the parent cell or mother cell, and the two cells it forms are the daughter cells. Before a cell divides it copies its genetic material, so that each daughter cell receives a full set of instructions. It then shares its cytoplasm and organelles between the two new cells.
Cell division happens in all three domains of life, but the mechanism differs. A prokaryote such as Escherichia coli has a single circular chromosome and no nucleus; it copies the chromosome and splits in two by binary fission. A eukaryote keeps most of its DNA in a nucleus on several linear chromosomes, so it must first divide the nucleus (nuclear division) and then divide the cytoplasm (cytokinesis). The account that follows concerns division in eukaryotic cells.
Organism or tissue | What cell division achieves |
|---|---|
Unicellular organisms (yeast, Amoeba, bacteria) | Asexual reproduction: each division makes a new individual. |
Multicellular organisms during development | Growth: a single-celled zygote becomes an organism with many millions of cells. |
Adult tissues (skin, gut lining, bone marrow) | Replacement of cells that are worn out or lost, and repair of damaged tissue. |
Reproductive organs (testes, ovaries, anthers, ovules) | Production of cells that develop into gametes for sexual reproduction. |
These roles connect the levels of organisation. Division of single cells explains how whole organisms grow, how populations of unicellular organisms increase, and how genetic information passes from one generation to the next, which is the theme of continuity and change.
Cytokinesis usually begins near the end of nuclear division, once two daughter nuclei have formed at opposite ends of the cell. It separates the cytoplasm, the organelles and the plasma membrane (also called the cell membrane) into two cells. Animal cells and plant cells achieve this in different ways because plant cells are surrounded by a rigid cell wall.
In an animal cell, a ring of protein filaments forms just inside the plasma membrane around the equator of the cell. The ring is made of actin filaments and myosin, the same two proteins that cause muscle contraction. Myosin uses energy from ATP to pull the actin filaments past each other, so the ring tightens like a drawstring.
As the contractile ring tightens, it pulls the plasma membrane inwards and makes a groove around the cell called the cleavage furrow. The furrow deepens until the membrane on opposite sides meets and fuses. The cell is pinched into two daughter cells, each enclosed by its own plasma membrane.
A plant cell cannot be pinched in two, because the cellulose cell wall does not bend inwards. Instead, a new wall is built across the middle of the cell. Vesicles from the Golgi apparatus, carrying cell-wall materials such as pectins and other polysaccharides, move along microtubules to the equator. There they fuse with one another to form a flat, membrane-bound structure called the cell plate.
The cell plate grows outwards from the centre until it joins the existing wall at the sides. The membranes of the fused vesicles become the new plasma membranes of the two daughter cells, and their contents become the middle lamella. Cellulose is then laid down on each side to complete the new cell walls.
The two methods are a clear example of form and function. Both produce two daughter cells from one, but the presence or absence of a cell wall determines which mechanism can work.
Feature | Animal cell | Plant cell |
|---|---|---|
Structure that divides the cell | Cleavage furrow | Cell plate |
Direction of division | From the outside inwards | From the centre outwards |
Main components | Contractile ring of actin and myosin | Vesicles from the Golgi apparatus carrying wall materials |
What separates the daughter cells | Plasma membrane pinched and fused | New plasma membranes and a new cell wall |
In most dividing cells the cytoplasm is shared roughly equally, so the two daughter cells are similar in size. This is the usual result in body cells such as skin cells or the cells of a root tip. Equal division is not universal, however, and in some cells one daughter receives much more cytoplasm than the other.
Whatever the split, each daughter cell must receive at least one mitochondrion. Mitochondria cannot be made from scratch: new ones form only by the growth and division of existing ones, because they contain their own DNA and ribosomes. The same is true of chloroplasts in plant cells. A daughter cell that received no mitochondria could never make any, and could not carry out aerobic respiration. Other organelles, such as the endoplasmic reticulum and Golgi apparatus, can be rebuilt from membranes in the cell.
Oogenesis is the production of egg cells in the ovary. Both divisions of meiosis that lead to a mature ovum are unequal. One daughter cell keeps almost all of the cytoplasm and becomes the egg cell; the other is a tiny cell called a polar body, which contains a nucleus but very little cytoplasm and later breaks down. The large egg cell keeps the food reserves, mitochondria and other organelles that the early embryo needs before it can obtain nutrients from the mother.
Yeast (Saccharomyces cerevisiae) is a unicellular fungus that reproduces asexually by budding. A small outgrowth, the bud, forms on the surface of the parent cell. After mitosis, one nucleus moves into the bud along with a share of the organelles, and the bud is then cut off by cytokinesis. The bud is much smaller than the parent cell when it separates, and it grows to full size afterwards. Each budding event leaves a bud scar on the wall of the parent cell.
A eukaryotic cell must divide its nucleus before it divides its cytoplasm. If the cytoplasm split first, one daughter cell would have no nucleus at all. Such an anucleate cell has no genes to direct the synthesis of new proteins, so it cannot repair itself or divide and soon dies. Mature human red blood cells are a rare exception: they lose their nucleus deliberately as they develop, and so they live only about 120 days and cannot divide.
Eukaryotes use two kinds of nuclear division, mitosis and meiosis, for different purposes.
Mitosis maintains the chromosome number and the genome. Each daughter nucleus receives an exact copy of every chromosome in the parent nucleus, so the daughter cells are genetically identical to the parent and to each other. Mitosis is used for growth, tissue repair, the replacement of cells and asexual reproduction.
Meiosis, by contrast, halves the chromosome number and produces nuclei that are genetically different from one another and from the parent nucleus. It is used to make the cells that develop into gametes, so it is part of every sexual life cycle.
Feature | Mitosis | Meiosis |
|---|---|---|
Number of divisions | One | Two (meiosis I and meiosis II) |
Number of daughter nuclei | Two | Four |
Chromosome number | Maintained (2n → 2n) | Halved (2n → n) |
Genetic identity of daughter nuclei | Identical to each other and to the parent | Different from each other and from the parent |
Main role | Growth, repair, replacement, asexual reproduction | Production of gametes; genetic variation |
Both mitosis and meiosis can happen only after the DNA has been copied. DNA replication takes place during interphase, the period between divisions, and produces two identical DNA molecules from each original molecule. Without this step, a dividing nucleus would have only one copy of each chromosome to share between two daughter nuclei.
Before replication, each chromosome is a single, very long DNA molecule wrapped around proteins. After replication the chromosome consists of two identical DNA molecules, called sister chromatids. The two chromatids remain held together, mainly at the centromere, by a ring-shaped protein complex called cohesin. They stay attached until anaphase, when they finally separate and move to opposite ends of the cell.
Counting chromosomes therefore needs care. A human body cell has 46 chromosomes. After replication it still has 46 chromosomes, but each one consists of two chromatids, so there are 92 chromatids and 92 DNA molecules. At anaphase of mitosis the sister chromatids separate, and each separated chromatid is then counted as a chromosome in its own right; for a short time the cell contains 92 chromosomes, 46 moving to each pole.
Two physical problems must be solved in every nuclear division, in mitosis and meiosis alike. The chromosomes are far too long to be moved without tangling, so they must first be packed into a compact form. Then they must be pulled accurately to opposite ends of the cell. The cell solves the first problem with histone proteins and the second with microtubules.
The DNA in one human cell is about 2 m long if laid end to end, yet it fits inside a nucleus only a few micrometres across. During interphase the DNA is wound around histones, small positively charged proteins that attract the negatively charged phosphate groups of DNA. Eight histones form a core around which the DNA wraps about twice, making a nucleosome; the chain of nucleosomes looks like beads on a string.
At the start of nuclear division the chain of nucleosomes is coiled again and again into loops and coils of increasing size. This repeated coiling of an already coiled structure is called supercoiling. It shortens each chromosome many thousands of times, so that chromosomes become short, thick and visible with a light microscope. Condensed chromosomes can be moved without breaking or tangling, but their genes cannot be transcribed, so very little protein synthesis happens during division.
Chromosomes are moved by the spindle, an array of protein fibres made of microtubules. In animal cells the microtubules grow out from two centrosomes, which move to opposite ends (poles) of the cell at the start of division. Plant cells have no centrosomes but still build a spindle from microtubules.
Some spindle microtubules attach to each chromosome at its centromere through a protein structure called the kinetochore. Each chromatid has its own kinetochore, so in mitosis the two sister chromatids become attached to microtubules from opposite poles. Motor proteins at the kinetochores and the poles then move the chromosomes. They use energy from ATP to "walk" along the microtubules, while the microtubules themselves shorten by losing tubulin subunits at their ends. Together these actions pull chromatids towards the poles.
Condensation by histones and movement by microtubules are shared features of mitosis and meiosis. Both divisions use the same machinery and differ only in which structures it separates: homologous chromosomes in meiosis I, and sister chromatids in mitosis and meiosis II.
Mitosis is a continuous process, but biologists divide it into four phases so that each stage can be described: prophase, metaphase, anaphase and telophase. Before mitosis the cell is in interphase, during which it grows and replicates its DNA; interphase is not itself a phase of mitosis.
Chromosomes condense by supercoiling and become visible as two sister chromatids joined at the centromere.
The nucleolus disappears.
In animal cells the two centrosomes move to opposite poles, and spindle microtubules grow between them.
At the end of prophase the nuclear membrane breaks down into small vesicles, so the spindle microtubules can reach the chromosomes.
Spindle microtubules attach to the kinetochore of each chromatid; sister chromatids are attached to opposite poles.
The microtubules pull with equal tension from both sides, so the chromosomes line up in a single row on the equator (the metaphase plate) of the cell.
The cell checks that every chromosome is attached correctly before anaphase begins.
The cohesin holding the sister chromatids together is broken down, so the centromeres separate.
The spindle microtubules shorten and pull the chromatids, centromere first, to opposite poles. Each separated chromatid is now a chromosome in its own right.
Each pole receives one copy of every chromosome, so the two groups are genetically identical.
The chromosomes reach the poles and begin to uncoil (decondense).
A nuclear membrane forms around each group of chromosomes, and nucleoli reappear.
The spindle breaks down. Cytokinesis usually begins during telophase and completes the division into two daughter cells.
Mitosis produces two genetically identical nuclei because of three features working together. DNA replication makes two identical copies of each chromosome; the sister chromatids are attached to opposite poles in metaphase; and in anaphase each pole receives exactly one chromatid of every chromosome. Any error in this sequence would give daughter nuclei with missing or extra chromosomes.
YouTubeThe phases of mitosis can be seen with a light microscope in tissues where cells divide often. The root tip of a plant contains a region of rapidly dividing cells a few millimetres behind the tip. Onion (Allium cepa) and garlic (Allium sativum) root tips are used because their chromosomes are large and easy to stain.
Grow roots by suspending an onion bulb or garlic clove over water for several days.
Cut off the terminal 5 mm of a root and place it in warm dilute hydrochloric acid (about 1 mol dm⁻³ at 60 °C) for a few minutes. The acid breaks down the middle lamella that holds the cells together, and it stops division, fixing the cells in their phase.
Rinse the root tip and place it on a slide with a stain that binds to DNA, such as toluidine blue, acetic orcein or Feulgen stain.
Cut away all but the very tip, add a cover slip, and press down firmly through folded filter paper to squash the tissue into a single layer of cells.
Examine at low power to find the dividing region, then at high power (×400) to identify the phases.
In a micrograph like the onion root tip squash, each phase has features that identify it. The decision rests on where the chromosomes are and what they look like.
Phase | How to recognise it |
|---|---|
Interphase | Intact nucleus with a clear outline and a nucleolus; chromosomes not visible as separate threads. |
Prophase | Chromosomes visible as distinct threads inside the nuclear area; nuclear membrane may be fading. |
Metaphase | Chromosomes in a line across the middle of the cell; no nuclear membrane. |
Anaphase | Two groups of chromosomes moving apart, often V-shaped as their centromeres lead. |
Telophase | Two groups of chromosomes at opposite ends; nuclear membranes re-forming; a cell plate may be visible between them. |
Most cells taken from the human body and grown in a laboratory divide only a limited number of times before they stop. For decades this made it hard to study human cell division outside the body. The first human cells shown to divide without limit in culture are known as HeLa cells.
On 8 February 1951, a sample of tissue was taken from a cervical tumour of Henrietta Lacks, a 31-year-old African American woman treated at Johns Hopkins Hospital in Baltimore, Maryland. The sample was taken without her knowledge or consent, which was common practice at the time. Unlike earlier samples, her tumour cells kept dividing by mitosis when grown in culture. The cell line was named HeLa from the first two letters of her first and last names. Henrietta Lacks died of cervical cancer in October 1951.
Because HeLa cells divide continuously, a small sample can be grown into very large numbers of genetically identical cells. Laboratories around the world have used them to study the phases of mitosis, the effects of drugs on dividing cells, and the behaviour of viruses inside human cells. In the 1950s HeLa cells were grown in large quantities to test the polio vaccine, and they have since been used in research on cancer, radiation and gene mapping.
Date | Event |
|---|---|
8 February 1951 | Tumour cells taken from Henrietta Lacks without her consent; the HeLa line is established. |
1950s | HeLa cells grown in large quantities and used in testing the polio vaccine. |
1953–2018 | More than 110,000 scientific publications cite research using HeLa cells. |
2013 | The HeLa genome is sequenced and published; in August the NIH and the Lacks family agree on controlled access to HeLa genome data, with family members on the committee that reviews requests. |
Source: NIH Office of Science Policy, HeLa cells resource; NIH press release, 7 August 2013.
HeLa cells also raise questions about the ethics of science. Henrietta Lacks and her family were not asked for consent, were not told for many years that her cells were in use, and did not benefit from the research. The genome of HeLa cells contains information about her living relatives, which is why the 2013 agreement gave the family a role in deciding who may use it. Modern research ethics require informed consent for the use of human tissue, partly because of cases like this one.
Most cells in the human body contain two sets of chromosomes, one set inherited from the mother and one from the father. The two chromosomes that carry the same genes in the same sequence, one from each parent, form a pair of homologous chromosomes. Human body cells have 23 pairs, 46 chromosomes in total.
A diploid nucleus is described as 2n, where n is the number of chromosomes in one set. A haploid nucleus contains only one set (n), so it has one chromosome of each type and no homologous pairs. In humans 2n = 46 and n = 23; in the onion, 2n = 16.
In sexual reproduction, two gametes fuse at fertilisation to form a zygote. If gametes were diploid, the zygote would have four sets of chromosomes, and the number would double in every generation. Meiosis prevents this by halving the chromosome number when gametes are made. A haploid sperm (n = 23) fuses with a haploid egg (n = 23), restoring the diploid number (2n = 46) in the zygote.
Every sexual life cycle therefore has two events that balance each other: meiosis halves the chromosome number, and fertilisation doubles it again. In animals, meiosis happens in the testes and ovaries; in flowering plants, it happens in the anthers and ovules.
Meiosis begins after DNA replication, so each chromosome already consists of two sister chromatids. It then involves two divisions, with no DNA replication between them. The first division separates homologous chromosomes; the second separates sister chromatids. This sequence of two rounds of separation is how one diploid nucleus becomes four haploid nuclei.
Prophase I. Chromosomes condense. Homologous chromosomes pair up side by side to form a bivalent (a pair of homologous chromosomes, four chromatids in total). Crossing over can occur between non-sister chromatids. The nuclear membrane breaks down and the spindle forms.
Metaphase I. Bivalents line up on the equator. Spindle microtubules from one pole attach to one chromosome of each pair, and from the other pole to its homologue.
Anaphase I. Homologous chromosomes are pulled to opposite poles. The sister chromatids stay together, because the cohesin at their centromeres is not yet broken down.
Telophase I. Each pole has one chromosome from each homologous pair, so each new nucleus is haploid. Cytokinesis usually follows, giving two cells.
Prophase II. In each of the two cells the chromosomes condense again (if they uncoiled) and a new spindle forms.
Metaphase II. The chromosomes, each still made of two chromatids, line up on the equator of each cell.
Anaphase II. The centromeres separate and the sister chromatids are pulled to opposite poles.
Telophase II. Nuclear membranes form around four groups of chromosomes. After cytokinesis there are four haploid cells.
YouTubeFeature | Mitosis | Meiosis |
|---|---|---|
DNA replication beforehand | Yes | Yes |
Pairing of homologous chromosomes | No | Yes, in prophase I (bivalents) |
Crossing over | No | Yes, in prophase I |
What lines up on the equator | Individual chromosomes (metaphase) | Bivalents (metaphase I); chromosomes (metaphase II) |
What separates | Sister chromatids (anaphase) | Homologues (anaphase I); sister chromatids (anaphase II) |
Result | Two diploid, genetically identical nuclei | Four haploid, genetically different nuclei |
Meiosis usually distributes chromosomes with great accuracy, but errors can happen. Non-disjunction is the failure of chromosomes to separate correctly during anaphase. It can happen in anaphase I, when both chromosomes of a homologous pair move to the same pole, or in anaphase II, when both sister chromatids move to the same pole.
The gametes produced by non-disjunction have an abnormal number of chromosomes. A gamete with an extra chromosome has n + 1 chromosomes (24 in humans), and a gamete lacking one has n − 1 (22). If an n + 1 gamete fuses with a normal gamete, the zygote has 2n + 1 chromosomes (47 in humans), with three copies of one chromosome. This condition is called trisomy. Most trisomies are not compatible with survival, but a few, involving small chromosomes, are.
Down syndrome is caused by trisomy 21: a person has three copies of chromosome 21 instead of two. It is usually the result of non-disjunction during meiosis in the mother, most often in meiosis I. The extra copies of the genes on chromosome 21 cause a range of effects, including learning difficulties, characteristic facial features, reduced muscle tone and a higher risk of heart defects. The condition can be detected before birth from a karyogram of fetal cells, which shows 47 chromosomes with three chromosome 21s.
The chance of non-disjunction rises with the age of the mother. Human oocytes begin meiosis I before the mother is born and then stop in prophase I, sometimes for more than 40 years, until ovulation. Over this long pause the cohesin that holds each bivalent together is thought to weaken, so chromosomes are more likely to separate incorrectly.
Age of mother at birth | Approximate chance of a baby with Down syndrome |
|---|---|
20 | 1 in 1,500 |
30 | 1 in 800 |
35 | 1 in 270 |
40 | 1 in 100 |
45 and over | about 1 in 50 |
Source: UK maternal age risk table, GPnotebook (based on UK screening data), 2024.
The four nuclei produced by meiosis are genetically different from one another and from the parent nucleus. Meiosis is therefore a source of genetic variation, which is the raw material for natural selection and evolution. Two processes in meiosis I produce this variation: random orientation of bivalents and crossing over.
In metaphase I each bivalent lines up on the equator. Which homologue faces which pole is a matter of chance, and the orientation of each bivalent is independent of every other bivalent. This is called random orientation. As a result, each gamete receives a random mixture of maternal and paternal chromosomes.
The number of possible combinations is , where n is the haploid number. In a cell with two pairs of homologous chromosomes there are possible combinations. In humans, with 23 pairs, random orientation alone can produce different combinations of chromosomes in the gametes of one person.
In prophase I, while homologous chromosomes are paired in a bivalent, non-sister chromatids can break at the same point and rejoin with each other. Sections of DNA are exchanged between the two homologues, a process called crossing over. The points where chromatids have crossed are visible with a microscope as X-shaped structures called chiasmata (singular chiasma).
Because homologous chromosomes carry the same genes but often different alleles, exchanging sections produces chromatids with new combinations of alleles. These are called recombinant chromatids. The position of crossing over varies from one cell to the next, and usually one to three chiasmata form per bivalent in humans, so the number of possible recombinant chromosomes is effectively unlimited.
Random orientation, crossing over and the random fusion of gametes at fertilisation together make every offspring of sexual reproduction genetically unique, apart from identical twins. This variation is why siblings with the same parents differ, and it gives natural selection a wide range of phenotypes to act on. It connects the behaviour of chromosomes inside a single cell with the evolution of whole populations, a link between levels of organisation.
Cell proliferation is used in three main ways in multicellular organisms: for growth, for replacing cells that are lost, and for repairing damaged tissue. In each case new cells are produced by mitosis followed by cytokinesis, so the new cells are genetically identical to the cells they came from.
Plants grow throughout their lives, but only in particular regions. A meristem is a tissue of small, unspecialised cells that keep dividing by mitosis. Apical meristems at the tips of roots and shoots add new cells that make the plant longer; lateral meristems (the cambium) in woody plants add cells that make stems and roots wider. Cells produced by a meristem later enlarge and differentiate into the specialised tissues of the plant. The onion root tip used for a squash preparation is an apical meristem, which is why it contains so many dividing cells.
In animals, rapid proliferation happens in the early-stage embryo. After fertilisation the zygote divides by mitosis again and again, first into 2 cells, then 4, 8, 16 and so on. In the earliest divisions (cleavage) the cells do not grow between divisions, so the embryo stays about the same size while its cells become smaller and more numerous. Later in development, growth and division continue together, and cells begin to differentiate into tissues.
Many adult tissues lose cells all the time and must replace them. The outer layer of the skin (the epidermis) is a clear example: cells at the surface are worn away continuously, and stem cells in the deepest layer of the epidermis divide to replace them. The new cells are pushed towards the surface, where they die and form a protective layer before they are shed. Bone marrow is another site of continuous proliferation; it produces about 2 million red blood cells every second in an adult human.
When tissue is damaged, proliferation repairs it. After a cut to the skin, cells at the edges of the wound are stimulated to divide by mitosis and to migrate into the gap. Cells in the dermis below lay down new connective tissue, and new epidermis grows over the wound until it is closed. Once the gap is filled, contact between cells and falling levels of growth signals slow the rate of division again.
Cells that proliferate pass repeatedly through a sequence of events called the cell cycle. Each turn of the cycle produces two daughter cells from one parent cell. The cycle has two main parts: interphase, during which the cell grows and copies its DNA, and the mitotic phase, which consists of mitosis and cytokinesis.
Phase | Main events |
|---|---|
G1 (first gap) | The cell grows. Proteins are synthesised, including the enzymes needed for DNA replication. Organelles increase in number. |
S (synthesis) | DNA is replicated, so each chromosome comes to consist of two sister chromatids. |
G2 (second gap) | The cell continues to grow and makes the proteins needed for division, such as tubulin for spindle microtubules. Replicated DNA is checked for errors. |
M (mitosis) | Prophase, metaphase, anaphase and telophase: the nucleus divides into two genetically identical nuclei. |
Cytokinesis | The cytoplasm divides, forming two daughter cells. |
G1 phase, S phase and G2 phase together make up interphase. In a human cell dividing in culture, one turn of the cycle takes about 24 hours, of which mitosis takes only about one hour. Some cells leave the cycle after G1 and enter a non-dividing state called G0. Most nerve cells and muscle cells remain in G0 for life; liver cells stay in G0 but can return to the cycle when the liver is damaged.
Interphase is the most metabolically active part of the cell cycle, because the cell must roughly double its contents before it divides, or each daughter cell would be smaller than the parent. During interphase the cell:
synthesises proteins by transcription and translation, including enzymes, structural proteins and the histones needed to package new DNA
replicates its DNA in S phase
carries out cell respiration to supply ATP, and many other metabolic reactions
increases the number of mitochondria and, in plant cells, chloroplasts; both grow and divide by a process similar to binary fission, so that each daughter cell can receive some
carries out its normal functions in the tissue, such as secretion or absorption
A cell must not divide before its DNA is fully replicated, or before every chromosome is attached to the spindle. The cell cycle is therefore controlled at several checkpoints, points where the cell will continue only if conditions are right. The main checkpoints are at the end of G1 (before DNA replication), at the end of G2 (before mitosis) and in metaphase (before anaphase).
Progress through the checkpoints is controlled by cyclins. A cyclin has no enzyme activity of its own. It binds to an enzyme called a cyclin-dependent kinase (CDK), and the cyclin–CDK complex adds phosphate groups to target proteins. Phosphorylation activates or inactivates these targets, which then trigger the events of the next phase, such as the start of DNA replication or the condensation of chromosomes.
CDKs are present throughout the cycle, but they are active only when bound to the right cyclin. Each type of cyclin is synthesised at a particular stage, and its concentration builds up until it passes a threshold. At the threshold enough CDK is activated for the cell to pass the next checkpoint. After the checkpoint the cyclin is broken down rapidly, so the cell cannot go back or repeat the step.
The graph of cyclin concentrations shows the pattern described. One cyclin peaks at the end of G1, allowing entry to S phase; another rises during S and stays high through G2; a mitotic cyclin rises through G2 and peaks at the start of mitosis. Its sudden destruction in anaphase allows the cell to complete mitosis and return to G1.
Cyclins were discovered in the early 1980s by Tim Hunt, working with fertilised sea urchin eggs. He found a protein whose concentration rose before each cleavage division and then disappeared abruptly, a pattern that matched the timing of division exactly. Hunt shared the 2001 Nobel Prize in Physiology or Medicine with Leland Hartwell and Paul Nurse, who had identified the genes controlling the cell cycle in yeast. The same cyclin–CDK system is found in yeast, sea urchins, plants and humans, an example of unity at the level of molecules across very different eukaryotes.
Two groups of genes are central to the control of cell division. Their normal products act like the accelerator and the brake of a car: one group stimulates division and the other restrains it. Mutations in either group can lead to uncontrolled division.
Proto-oncogenes code for proteins that promote progress through the cell cycle when the cell receives a signal to divide, such as growth factor receptors and cyclins. A gene mutation can turn a proto-oncogene into an oncogene, a gene whose product is permanently active or produced in excess. For example, mutations in the RAS genes produce a Ras protein that keeps signalling "divide" even without a growth factor. Only one of the two copies of a proto-oncogene needs to mutate for the effect to appear.
Tumour suppressor genes code for proteins that slow or stop the cell cycle, repair DNA, or trigger the death of damaged cells. The best known is TP53, which codes for the protein p53. When DNA is damaged, p53 stops the cycle at the G1 checkpoint while the damage is repaired; if the damage is too great, p53 triggers programmed cell death (apoptosis). Mutations that stop tumour suppressor genes working remove this brake. TP53 is mutated in about half of all human cancers.
A cell becomes cancerous only after several mutations have built up in the same cell lineage, typically in both proto-oncogenes and tumour suppressor genes. Each mutation gives the cell a small advantage in growth, and its descendants divide faster than neighbouring cells. Mutations are caused by errors in DNA replication and by mutagens such as tobacco smoke, ultraviolet radiation and some viruses. Because mutations accumulate over time, most cancers become more common with age.
HeLa cells, described in the case study on cell division, came from a cervical tumour caused by human papillomavirus 18 (HPV-18), whose proteins inactivate the products of tumour suppressor genes, including p53. This is one reason HeLa cells divide without limit.
A tumour is an abnormal mass of tissue formed by uncontrolled cell division. Tumours differ in how fast their cells divide and whether they spread, and these differences determine whether they are cancerous.
Feature | Benign tumour | Malignant tumour |
|---|---|---|
Rate of cell division and growth | Usually slow | Often rapid |
Boundary | Often enclosed in a capsule; clear edge | No capsule; irregular edge |
Invasion of neighbouring tissue | No; it pushes tissue aside | Yes; it grows into surrounding tissue |
Metastasis | Does not spread | Cells spread in blood or lymph and form secondary tumours |
Cancerous? | No, though it can cause harm by pressing on organs | Yes; this is cancer |
A benign tumour is not cancer, but it can still be harmful if it presses on a blood vessel or nerve, or grows inside the skull. A malignant tumour is a cancer. The tumour that forms first, at the site where the cancer began, is the primary tumour. Metastasis is the spread of cancer cells from the primary tumour, through the blood or lymph, to other parts of the body. A tumour that grows from these cells in another organ is a secondary tumour; it is made of the same type of cell as the primary tumour, so a secondary tumour in the liver from a primary bowel cancer is still bowel cancer.
The rate of division in a tissue can be estimated from a single micrograph by counting how many cells are in mitosis. The proportion of cells in mitosis is called the mitotic index. Tissues that divide rapidly, such as a root tip meristem or a malignant tumour, have a high mitotic index; tissues that divide slowly have a low one.
Cells in prophase, metaphase, anaphase and telophase all count as "in mitosis". Cells in interphase do not, even though they may be about to divide. The index is a ratio between 0 and 1; it can also be given as a percentage by multiplying by 100. Doctors use the mitotic index of a tumour biopsy to help judge how aggressive a cancer is.
Questions on cell division appear in every part of the assessment. Paper 1A often asks for the identification of a phase from a diagram or for the number of chromosomes or chromatids at a given stage. Paper 1B uses data such as cell counts from a root tip squash or the effect of a drug on the mitotic index. Paper 2 asks for drawings of cells in a phase of mitosis, comparisons of mitosis and meiosis, and extended explanations of how meiosis produces variation. The command terms most often used are identify, draw, outline, explain, compare and contrast and, at HL, calculate (the mitotic index).
Cell division links many other subtopics: DNA replication (D1.1) happens before it, mutations (D1.3) can disrupt its control, and gametes made by meiosis are the starting point for reproduction (D3.1) and inheritance (D3.2).
I can explain why new cells arise only by division of existing cells.
I can describe cytokinesis in animal and plant cells and give examples of unequal cytokinesis.
I can state the roles of mitosis and meiosis and explain why nuclear division comes before cell division.
I can explain why DNA replication is needed before division and count chromosomes and chromatids correctly.
I can describe the roles of histones in condensation and of microtubules in moving chromosomes.
I can describe the phases of mitosis, draw cells in each phase and identify them in micrographs.
I can explain how meiosis halves the chromosome number and outline its two divisions.
I can explain how non-disjunction causes Down syndrome.
I can explain how random orientation and crossing over generate genetic variation.
I can describe the cell cycle, interphase growth and the role of cyclins at checkpoints. (HL)
I can explain how mutations in proto-oncogenes and tumour suppressor genes lead to tumours, and distinguish benign and malignant tumours. (HL)
I can calculate a mitotic index from cell counts. (HL)
a. humans choose plants with desired traits as parents in each generation;
b. the traits are heritable / variation is genetic;
c. over generations the characteristics of the crop population change;
d. crop varieties differ greatly from their wild ancestors;
e. example: maize bred from teosinte / cabbage, broccoli, kale and cauliflower from wild cabbage;
f. different varieties produced from one wild species / one ancestor;
g. changes happened over a short time / a few thousand years or less;
h. shows that selection can change heritable characteristics, so natural selection could do the same over longer periods;
[4 max]
Explain how natural selection can lead to evolutionary change in a population.
7 marks
a. there is variation between individuals in a population;
b. variation arises from mutation and from sexual reproduction / meiosis and fertilization;
c. populations produce more offspring than the environment can support;
d. (so) there is competition for resources / a struggle for survival;
e. selection pressures such as predation / disease / abiotic factors act on the population;
f. individuals with traits better suited / adapted to the environment are more likely to survive;
g. (and) reproduce and pass on their alleles;
h. only heritable traits are passed on / acquired characteristics are not;
i. the frequency of favourable alleles / traits increases over generations;
j. evolution is this change in the heritable characteristics of the population;
k. example of natural selection, such as antibiotic resistance in bacteria / beak size in finches;
[7 max]
One additional mark is available for the construction of the answer.
Species are units of biodiversity, but defining them is not straightforward.
Outline how geographic isolation can lead to the formation of new species.
4 marks
Discuss the difficulties of deciding whether two populations belong to the same species.
7 marks
Describe how meiosis halves the chromosome number, and explain why diploid cells have an even number of chromosomes.
4 marks
Chromosome number is a characteristic of each species.
Describe how a karyogram can be used to diagnose Down syndrome, and outline how non-disjunction causes this condition.
4 marks
Explain why hybrids between closely related species with different chromosome numbers are usually infertile.
7 marks
Outline the difficulties of applying the biological species concept to bacteria and to other organisms that reproduce asexually.
4 marks