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Mitosis

The cell cycle is the process of cell replication.

Cells spend most of their life in interphase . Interphase can be sub-divided into the following stages:

  • $$\ce{G0}$$: if a cell is not dividing, it stays in this phase.
  • $$\ce{G1}$$ (gap phase 1) : Cell growth while performing normal function.
  • S phase (synthesis) : DNA is replicated. Chromosomes move from being single DNA strands to two strands (sister chromatids) joined at a centromere.
  • $$\ce{G2}$$ (gap phase 2) : Further cell growth and microtubule synthesis. Preparations for cell division now complete.

After G2, cells are ready for mitosis . During mitosis, the genetic material divides. This stage only takes up 10% of the total cell cycle!

Remember, a cell entering mitosis already has two copies of diploid genetic material.

Mitosis is the process in which the genetic material in the nucleus of a cell is divided. It occurs only in eukaryotic organisms.

This image shows a cell just after mitosis.
This image shows a cell just after mitosis.

In order for an organism to grow, its cells need to divide. Mitosis leads to the formation of two identical daughter cells.

Important! Mitosis describes the formation of two nuclei from one parent nucleus, not the replication of DNA. DNA replication happens before mitosis.

The replication of genetic material is very accurate. Errors in replicating the genetic code can lead to the cell not functioning properly.

Mitosis is used by multicellular organisms for:

  • Growth: As children grow, their cells divide by mitosis.
  • Repair: Skin cells are constantly dividing to replace old or damaged skin.
  • Asexual reproduction: Single-celled eukaryotes like yeast replicate by mitosis.

Mitosis is the division of one nucleus with two copies of 'x-shaped' chromosomes to form two nuclei, each with one copy of single-stranded chromosomes.

The table below summarises the four stages of mitosis:

Stage Diagram
Prophase

The spindle network forms.

Inside the nucleus, the genetic material condenses into visible chromosomes.

Metaphase

The nucleus breaks down.

Chromosomes line up along the cell equator.

Anaphase

Spindle network contract.

Chromosomes are split, each chromatid goes to the opposite side of the cell.

Telophase

Two new nuclei form.

Cell membrane pinches in, forming two new cells.

At the end of mitosis, both nuclei are diploid; they contain two versions of each chromosome.

There are four stages in mitosis defined by the behaviour of chromosomes: prophase, metaphase, anaphase and telophase.

Prophase
Prophase

In prophase, the first stage of mitosis:

  • The nucleolus disappears.
  • Chromosomes condense to become visible and thread-like.
  • In the cytoplasm, the centrosomes (microtubule organizing centres) begin to move towards the opposite poles of the cell.
  • Microtubule components come together, elongate from the centrosomes and begin to form the mitotic spindle.

Mitotic spindles will eventually attach to the centromeres of chromosomes and separate the sister chromatids.

Metaphase
Metaphase

In metaphase, the second stage of mitosis:

  • The nuclear envelope disappears between late prophase and metaphase.
  • The centrosomes which were migrating during prophase are now at opposite poles.
  • The mitotic spindle is fully formed.
  • The mitotic spindle attaches itself to the centromeres of the chromosome.
  • The mitotic spindle retracts and arranges the chromosomes at the equator of the cell, midway between the two centrosomes.

The mitotic spindle will now be able to separate the sister chromatids during anaphase.

Since each sister chromatid is attached to just one set of spindle fibres, retraction of the fibres pulls only one sister chromatid on each chromosome to each pole.

This ensures the daughter cells have the same number of chromosomes.

Anaphase
Anaphase

Anaphase is the third stage of mitosis:

  • The centromeres divide, causing the two sister chromatids to separate.
  • They are now classed as daughter chromosomes.
  • The microtubules forming the spindle fibres begin to retract, causing the daughter chromatids to migrate towards the poles.
Telophase
Telophase

Telophase is the final stage of mitosis:

  • Marks the end of mitosis as nuclear division is complete
  • New nuclear envelopes form around each set of daughter chromosomes at the two poles.
  • Two new nuclei form within the cell.

Cytokinesis is the final step in cell replication, but lies outside the mitosis process. This is when the cell fully divides , giving two complete and genetically identical daughter cells.

Mitosis refers to the division of chromosomes, not the formation of a new cell. Although these processes are strongly linked, mitosis is considered to have ended before cytokinesis.

In an animal cell, a ring forms at the centre of the cytoplasm and contracts, dividing it in two. Two daughter cells are created with a newly formed nucleus in each.

The separation of plant cells occurs slightly differently.

Rather than contractile elements (made from cytoskeleton) forming, a cell plate (made from both cell membrane and cell wall) forms and spreads across the centre of the cell, dividing it in two.