Human cells showing the stages of cell division

Dividing we stand

Ellie Patten explores the stages and importance of mitosis and meiosis

Cell division is one of the most crucial processes of any living organism. It is necessary for mitosis, which is responsible for the growth of a multicellular organism, and for reproduction of a eukaryotic single-celled organism. Mitosis is also required for the repair of damaged tissues, as new cells are made to replace any that are lost, old or damaged. Another type of cell division is meiosis, the process by which sex cells are made.

The cell cycle is an organised series of events that take place in a cell. The longest stage of the cell cycle is interphase, which can be split into G1, S phase and G2. In the G1 phase, proteins required for cell division are made. This is followed by the S phase, in which chromosomes are duplicated, and then the G2 phase, in which more proteins are synthesised. Interphase is followed by mitosis, and then the cycle begins again.

The cell cycle must be tightly regulated for cell division to take place, as events such as protein synthesis or chromosome replication must occur at the correct stage.

Mitosis

Mitosis is the process of making new eukaryotic cells. This is divided into stages called prophase, metaphase, anaphase and telophase and is followed by cytokinesis. During interphase, a cell’s chromosomes (organised, thread-like structures composed of a mixture of DNA and proteins) are duplicated so that the nucleus contains twice the original DNA. The chromosomes are now X-shaped structures, composed of two identical sister chromatids.

At the start of mitosis, in a stage called prophase, the chromosomes condense and can be viewed under a microscope, and spindles (protein structures that will later attach to sister chromatids to separate them) begin to form in the cytoplasm.

Stages of mitosis

  • In prophase, the nuclear envelope breaks down.
  • During metaphase, the pairs of sister chromatids then line up along the equator of the cell.
  • In anaphase, spindle fibres attach to chromosomes via a protein complex (several proteins that are associated with each other) called a kinetochore and separate the sister chromatids to opposite poles of the cell.
  • In the last stage, telophase, new nuclear envelopes form around each set of chromosomes.

Finally, in cytokinesis, the cell cytoplasm splits to form two identical daughter cells, each with exactly the same DNA as the original cell. For images of the process, see our image gallery.

Regulation of mitosis

This process is highly regulated by a series of biochemical reactions to ensure that mitosis only occurs after sufficient cell growth and DNA replication have occurred. CDKs (cyclin-dependent kinases) are enzymes that add phosphate groups to proteins to activate them. One of their roles is to activate proteins that initiate DNA replication before mitosis, to ensure this happens at the correct stage.

In cancer cells, growth is incorrectly regulated, and this is often due to mutations in genes that control cell division. A tumour-suppressant gene called p53 is mutated in more than 50 per cent of

all human cancers. When regulation mechanisms for mitosis cannot function, mitosis becomes unregulated and this causes a mass of cancer cells called a tumour.

If we can find out more about the mechanisms of regulating mitosis, it might provide us with more information about how to prevent or cure cancer. In particular, lots of research is going into finding ways to target only the cells that divide abnormally; the targets of anti-cancer drugs can often also be found in ordinary cells, so they cause unpleasant side-effects.

Meiosis

Meiosis is the process in which haploid sperm and egg cells are generated. In humans, instead of containing 46 (2n) chromosomes, haploid cells only contain 23 (n) chromosomes. Each sex cell contains half of the DNA required to create an embryo.

During meiosis, the cell undergoes two rounds of division, named ‘meiosis I’ and ‘meiosis II’. As in mitosis, the DNA is replicated before the first division, and spindles begin to form.

Stages of meiosis I

  • In prophase I, the nuclear envelope breaks down and chromosomes condense into X-shaped structures. A process called ‘crossing-over’, or recombination, occurs. This involves the exchange of DNA between paternal and maternal chromosomes, so the chromatids are no longer identical and are referred to as non-sister chromatids.
  • In metaphase I, chromosome pairs line up along the equator of the cell and spindle fibres attach to each chromosome.
  • In anaphase I, chromosomes do not separate in the same way as they do in mitosis. Non-sister chromatids remain attached to one another, but pairs of homologous chromosomes separate. Homologous chromosomes are chromosomes of a similar size and structure and contain the same genes. Each chromosome in the pair may contain a different allele of the gene, as one homologous chromosome is inherited from the mother and the other is inherited from the father.
  • In telophase I, a new nuclear membrane forms around each set of chromosomes

Cytokinesis then occurs, resulting in two non-identical haploid cells. Before meiosis II, DNA is not duplicated, so prophase II begins with 23 chromosomes in each cell.

Stages of meiosis II

  • In prophase II, the chromosomes condense and the nuclear membrane breaks down.
  • In metaphase II, the chromosomes line up along the equator and spindle fibres attach.
  • In anaphase II, the sister chromatids are pulled to opposite ends of the cell.
  • In telophase II, a new nuclear envelope forms around each group of chromosomes.

Cytokinesis is then repeated. At the end of this division, there are four non-identical haploid cells.

Genetic variation and errors in meiosis

Meiosis introduces genetic variation into populations, which is important because more diverse populations are more likely to withstand disease or illness. Variation is further increased when a sperm and egg cell combine during sexual reproduction, as the embryo inherits half of the DNA from each parent, creating a unique combination of DNA.

Down’s syndrome (trisomy 21) is caused when non-sister chromatids of chromosome 21 fail to separate in meiosis II. The result of this is a sex cell that contains an extra version of chromosome 21. After fertilisation, this extra chromosome will be copied into every cell in the embryo as it grows, meaning that each of its cells will contain an extra chromosome.

Meiosis is a very complex topic to study and can provide information on fertility, plant and animal breeding, and sources of genetic variation. In particular, research into meiosis can provide information about why age-related fertility issues arise.

Differences between mitosis and meiosis

  Mitosis Meiosis
Number of cell divisions One Two
number of daughter cells produced Two Four
Similarity to parent (dividing) cell Identical Genetically unique
Chromosome number of daughter cells Diploid (2n) Haploid (n)
Organisms in which it occurs

Only in eukaryotes – animals, plants and fungi

Only in eukaryotes – animals, plants and fungi
Type of cells created Somatic cells (all body cells excluding egg or sperm cells) Only germ cells (egg or sperm)
Does recombination or crossing over occur? No Yes, during prophase I
What happens during anaphase? Sister chromatids are separated to opposite poles During anaphase I sister chromatids move to same pole. In anaphase II they are separated to opposite poles.

Lead image:

Human cells showing the stages of cell division.

Matthew Daniels/Wellcome Images

References

Further reading

Downloadable resources

About this resource

This resource was first published in ‘The Cell’ in September 2015.

Topics:
Cell biology, Health, infection and disease
Issue:
The Cell
Education levels:
16–19, Continuing professional development