Every multicellular organism begins as a single cell, yet an adult organism contains an enormous number of cells. Those cells do not simply appear. Existing cells divide to produce new ones, so cell division lies behind growth, replacement of worn-out cells and repair of damaged tissues.
Reproduction creates another challenge. When your skin produces replacement cells, the new cells should receive essentially the same genetic information as the cells they replace. When gametes are formed, however, the chromosome number must first be reduced so that fertilisation does not cause the chromosome number to double in every generation.
Organisms therefore use two different processes. Mitosis preserves the chromosome number for growth and repair, while meiosis reduces the chromosome number for sexual reproduction.
Try these five questions before reading. They test chromosomes, diploid and haploid cells, mitosis, meiosis and fertilisation.
- 5 diagnostic questions
- Core chromosome and cell-division ideas
- Instant explanation for every answer option
Chromosomes carry the genetic information that must be distributed
The nucleus contains DNA. Much of this DNA is organised into structures called chromosomes. Each chromosome carries many genes, and each gene contains genetic information that can influence a characteristic or control part of a cell's activity.
In most human body cells, chromosomes occur in homologous pairs. The two chromosomes in a homologous pair are similar in size and carry the same types of genes at corresponding positions. One chromosome of each pair was inherited from the mother and the other from the father.
Homologous chromosomes do not necessarily carry exactly the same genetic information. They carry the same genes at corresponding positions, but they may carry different alleles of those genes.
| Term | Meaning | Human example |
|---|---|---|
| Chromosome | A structure containing DNA and genes | Most human body cells contain 46 chromosomes |
| Homologous pair | A pair of corresponding chromosomes carrying the same types of genes, with one inherited from each parent | Most human body cells contain 23 homologous pairs |
| Diploid, 2n | Having two sets of chromosomes | Most human body cells: 2n = 46 |
| Haploid, n | Having one set of chromosomes | Human sperm and egg cells: n = 23 |
| Sister chromatids | Identical copies of a chromosome produced during DNA replication | They separate during mitosis and during the second meiotic division |
Diploid and haploid describe the number of chromosome sets
The symbols 2n and n describe chromosome sets rather than one fixed number that applies to every species.
A diploid cell contains two chromosome sets, while a haploid cell contains one chromosome set.
In humans:
- Diploid body cell: 2n = 46 chromosomes = 23 homologous pairs.
- Haploid gamete: n = 23 chromosomes = one chromosome from each homologous pair.
Other species have different chromosome numbers. What matters is the relationship between the diploid and haploid numbers.
For example, if a species has a diploid chromosome number of 12, its normal gametes contain 6 chromosomes. If another species has a diploid chromosome number of 20, its gametes contain 10.
Mitosis produces cells with the same chromosome number
Mitosis is nuclear division in which one nucleus produces two nuclei containing the same chromosome number as the original nucleus. When cell division is completed, this normally produces two genetically identical daughter cells.
Before mitosis begins, the cell replicates its DNA. Each chromosome is copied, producing two identical sister chromatids joined at a centromere.
DNA replication is essential because each daughter nucleus needs a complete copy of the genetic information. The replicated chromosomes can then be separated so that each new nucleus receives the correct chromosome set.
The basic sequence of mitosis
- DNA is replicated before mitosis. Each chromosome now consists of two sister chromatids.
- The chromosomes become organised for separation.
- Sister chromatids separate and move to opposite sides of the cell.
- Two nuclei form, each receiving a complete chromosome set.
- Division of the cytoplasm produces two daughter cells.
The key idea is:
One parent cell → mitosis → two genetically identical daughter cells with chromosome number maintained.
Why mitosis is important
Mitosis allows organisms to produce new cells without changing their chromosome number. This makes it essential for several processes.
- Growth: multicellular organisms increase their number of cells.
- Repair: damaged tissues can replace lost cells.
- Replacement: worn-out or dead cells can be replaced.
- Asexual reproduction in many eukaryotes: genetically similar offspring can be produced from one parent.
For example, when skin is damaged, cells near the wound can divide by mitosis. The replacement cells need the same chromosome number and essentially the same genetic instructions as the cells they replace.
An X-shaped structure is still one replicated chromosome
One common source of confusion is the familiar X-shaped chromosome. After DNA replication, an X-shaped chromosome contains two sister chromatids joined at a centromere.
Before the sister chromatids separate, the whole X-shaped structure is regarded as one replicated chromosome. Do not automatically count the two halves of an X as two separate chromosomes.
Once the sister chromatids separate during nuclear division, each chromatid becomes an individual chromosome in its daughter nucleus.
Meiosis produces haploid cells for sexual reproduction
Sexual reproduction creates a different chromosome-number problem. If both gametes carried the full diploid chromosome number, fertilisation would double the chromosome number.
Meiosis prevents this by reducing the chromosome number from diploid, 2n, to haploid, n.
In humans, meiosis occurs during the formation of gametes in the reproductive organs. Sperm cells are produced in the testes, while egg cells develop in the ovaries.
Meiosis contains two nuclear divisions
DNA is replicated once before meiosis begins, but the nucleus then undergoes two divisions.
- DNA is replicated once.
- Homologous chromosomes separate during the first meiotic division. This reduces the chromosome number.
- Sister chromatids separate during the second meiotic division.
- The resulting nuclei or cells are haploid.
In the general model of meiosis, one diploid cell gives rise to four haploid products. During sperm formation, these can develop into four sperm cells.
Human egg formation is different because division of the cytoplasm is unequal. Only one large functional ovum is produced, together with smaller polar bodies. For chromosome-number questions, however, the key idea remains the same: meiosis halves the chromosome number.
Why meiosis must halve the chromosome number
Imagine that human sperm and egg cells each contained 46 chromosomes. Fertilisation would then give:
46 + 46 = 92 chromosomes.
If the same thing happened in every generation, chromosome number would continue increasing. Instead, meiosis first produces haploid gametes.
In humans:
23 chromosomes in sperm + 23 chromosomes in egg → 46 chromosomes in the zygote.
In symbols:
2n → meiosis → n
n + n → fertilisation → 2n
Meiosis and fertilisation therefore work together to maintain the chromosome number of a species from one generation to the next.
Meiosis also contributes to genetic variation
Mitosis is designed mainly to preserve genetic information. Meiosis has a different role: it produces gametes that are genetically different from one another.
Homologous chromosomes can be distributed into different combinations during meiosis. As a result, different gametes can contain different combinations of chromosomes inherited from the organism's parents.
Sexual reproduction creates further variation because fertilisation combines genetic information from two gametes. One genetically different sperm can fuse with one genetically different egg, producing a new combination of genetic information.
This is one reason siblings produced by sexual reproduction are generally genetically different from one another, except in special cases such as genetically identical twins.
Mitosis and meiosis solve different biological problems
Both processes involve chromosome movement and nuclear division, but their purposes are almost opposite. Mitosis must preserve the chromosome number, whereas meiosis must reduce it before sexual reproduction.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Main purpose | Growth, repair, replacement and some forms of asexual reproduction | Production of haploid cells for sexual reproduction |
| Number of nuclear divisions | One | Two |
| General number of products | Two | Four haploid products in the standard model |
| Chromosome number | Maintained | Halved |
| Genetic similarity | Daughter cells are normally genetically identical | Products are genetically different |
| What separates? | Sister chromatids | Homologous chromosomes first, then sister chromatids |
| Human example | Production of new skin cells | Formation of sperm or egg cells |
Cell division connects with asexual and sexual reproduction
The differences between mitosis and meiosis become especially clear when reproduction is considered.
Asexual reproduction
Asexual reproduction involves one parent and does not involve the fusion of gamete nuclei. In many eukaryotic organisms, mitosis produces the cells involved in asexual reproduction.
Offspring produced asexually are normally genetically identical to the parent and to one another, apart from changes caused by mutation. Genetically identical individuals are described as clones.
Asexual reproduction can be rapid and does not require finding a mate. However, low genetic variation can be a disadvantage if environmental conditions change or if a disease affects organisms sharing the same genotype.
Sexual reproduction
Sexual reproduction involves the fusion of the nucleus of a male gamete with the nucleus of a female gamete. The resulting cell is called a zygote.
Meiosis first produces haploid gametes. Fertilisation then combines two haploid chromosome sets and restores the diploid chromosome number.
Because the gametes are genetically different and fertilisation combines genetic information from two parents, sexual reproduction generally produces genetically varied offspring.
Do bacteria divide by mitosis?
No. Bacteria are prokaryotes and do not possess a nucleus. They reproduce by binary fission, not mitosis.
It is therefore more accurate to say that mitosis is involved in asexual reproduction in many eukaryotic organisms rather than saying that all asexual reproduction uses mitosis.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying mitosis halves the chromosome number | Mitosis normally maintains chromosome number. | Associate chromosome-number reduction with meiosis. |
| Saying meiosis produces genetically identical cells | Meiosis produces genetically different haploid products. | Use “identical” for normal mitotic daughter cells and “different” for meiotic products. |
| Calling two sister chromatids a homologous pair | Sister chromatids are copies of one chromosome. Homologous chromosomes are corresponding chromosomes inherited from different parents. | Ask whether you are comparing replicated copies of one chromosome or the two members of a homologous pair. |
| Counting the two halves of an X-shaped chromosome as two chromosomes | Before separation, the two halves are sister chromatids belonging to one replicated chromosome. | Treat the X-shaped structure as one replicated chromosome until its sister chromatids separate. |
| Saying every human cell contains 46 chromosomes | Normal human gametes contain 23 chromosomes. | Distinguish diploid body cells from haploid gametes. |
| Saying meiosis takes place throughout the body | Meiosis is associated with gamete formation in reproductive organs. | Think “growth and repair → mitosis; gamete formation → meiosis”. |
| Saying sexual reproduction simply means having two parents | The defining event is the fusion of haploid gamete nuclei. | Define sexual reproduction in terms of gametes and fertilisation. |
| Saying bacteria reproduce by mitosis | Bacteria are prokaryotes and reproduce by binary fission. | Restrict mitosis to nuclear division in eukaryotic cells. |
| Saying chromosome number permanently doubles when DNA replicates | DNA is copied, but the sister chromatids are later separated between daughter nuclei. | Distinguish DNA replication from a permanent increase in chromosome number. |
What you should be able to do
Define chromosome terms
Explain chromosome, homologous pair, sister chromatid, diploid and haploid.
Use chromosome numbers
Relate the human diploid number of 46 to the haploid number of 23.
State the outcome of mitosis
Describe two genetically identical daughter cells with chromosome number maintained.
Explain why mitosis matters
Connect mitosis with growth, repair, replacement and appropriate forms of asexual reproduction.
State the outcome of meiosis
Explain that chromosome number is reduced from diploid to haploid.
Explain reduction division
Show why halving chromosome number before fertilisation prevents chromosome number doubling.
Compare mitosis and meiosis
Compare purpose, chromosome number, number of divisions, products and genetic similarity.
Connect meiosis with fertilisation
Use n + n → 2n to explain restoration of the diploid chromosome number.
Explain genetic variation
Relate meiosis and sexual reproduction to genetically different offspring.
Distinguish reproductive strategies
Compare asexual and sexual reproduction without confusing mitosis with binary fission.
These ten questions test chromosome counting, mitosis, meiosis, fertilisation, genetic variation and unfamiliar chromosome-number situations.
- 10 mastery questions
- Application and biological reasoning
- Instant explanation for every answer option
Frequently asked questions
Cell division: questions students often ask
Cell division becomes much easier once chromosome number, homologous chromosomes and sister chromatids are kept separate in your mind. These questions connect the ideas students most commonly confuse.
What exactly is a chromosome?
A chromosome is a structure containing a long DNA molecule together with associated proteins. The DNA carries genes. Before cell division, the DNA is replicated so that genetic information can be distributed to new nuclei.
What is the difference between a chromosome and a chromatid?
After DNA replication, a chromosome consists of two identical sister chromatids joined at a centromere. When the sister chromatids separate during nuclear division, each becomes an individual chromosome in its new nucleus.
What is a homologous pair of chromosomes?
A homologous pair contains two corresponding chromosomes, one inherited from the mother and one from the father. They carry the same types of genes at corresponding positions, although they may carry different alleles.
What do 2n and n mean?
The symbol 2n means that a cell has two chromosome sets and is diploid. The symbol n means that a cell has one chromosome set and is haploid. In humans, 2n = 46 and n = 23.
Why do human gametes contain 23 chromosomes instead of 46?
Gametes are produced through meiosis, which halves the chromosome number. A sperm containing 23 chromosomes can therefore fuse with an egg containing 23 chromosomes to form a diploid zygote containing 46 chromosomes.
Does mitosis always produce diploid cells?
Not necessarily. The important feature of mitosis is that chromosome number is maintained. A diploid cell undergoing mitosis normally produces diploid daughter cells, while a haploid cell undergoing mitosis can produce haploid daughter cells. In human examples at O-Level, mitosis usually involves diploid body cells.
Why is mitosis important for growth?
Growth of a multicellular organism requires an increase in cell number. Mitosis produces new cells while maintaining the chromosome number and genetic instructions required by the tissue.
Why is meiosis called reduction division?
Meiosis reduces the chromosome number from diploid to haploid. Homologous chromosomes are separated so that each resulting haploid cell receives one chromosome from each homologous pair.
Why does meiosis involve two divisions if DNA is copied only once?
The first meiotic division separates homologous chromosomes. The second meiotic division separates sister chromatids. Because DNA is not replicated again between these divisions, meiosis can produce haploid cells.
Does meiosis always produce four functional gametes?
The standard model of meiosis produces four haploid nuclear products. During sperm formation, these can develop into four sperm cells. During human egg formation, division of the cytoplasm is unequal, so one large functional ovum forms together with smaller polar bodies.
Why can cells produced by meiosis be genetically different?
Homologous chromosomes can be distributed into different combinations during meiosis. The resulting gametes therefore do not all receive exactly the same combination of genetic information. Fertilisation then creates further variation by combining genetic material from two gametes.
Are cells produced by mitosis always perfectly genetically identical?
Mitosis normally produces cells with essentially the same genetic information because replicated DNA is distributed between the daughter cells. Mutations can occasionally occur, so “genetically identical” describes the normal outcome rather than meaning that DNA can never change.
Where does mitosis occur compared with meiosis?
Mitosis occurs in many tissues where new cells are required for growth, repair or replacement. Meiosis is associated with the production of gametes in reproductive organs such as the testes and ovaries.
What is the connection between meiosis and fertilisation?
Meiosis reduces chromosome number from 2n to n. Fertilisation combines two haploid gametes, so n + n produces a diploid zygote, 2n. The two processes therefore work together to maintain chromosome number across generations.
What is the main difference between sexual and asexual reproduction?
Asexual reproduction involves one parent and does not involve fusion of gamete nuclei, so offspring are normally genetically identical to the parent apart from mutation. Sexual reproduction involves fusion of haploid gamete nuclei and normally produces genetically varied offspring.
Do bacteria use mitosis when they reproduce asexually?
No. Bacteria are prokaryotes and do not possess a nucleus, so they do not undergo mitosis. They reproduce by binary fission.
What should I remember first when comparing mitosis and meiosis?
Start with the biological purpose. Mitosis needs to preserve chromosome number for growth and repair, so it normally produces two genetically identical cells. Meiosis needs to produce haploid cells for sexual reproduction, so chromosome number is halved. Once you understand those purposes, most of the differences become logical rather than facts that must be memorised separately.
Mitosis and meiosis are easiest to understand when you focus on the biological problem each process solves. Mitosis allows an organism to make new cells while maintaining chromosome number, making growth, repair and replacement possible. Meiosis does a different job: it halves chromosome number before sexual reproduction so that fertilisation can restore the diploid number without doubling it. Once chromosome behaviour is connected to these purposes, cell division becomes a logical sequence rather than a list of facts to memorise.