What is mitosis?

Mitosis is one of the two types of cell division. It is the way cells in the body reproduce and is what is responsible for all your cells. It is used to grow and to replace old cells. During the process of mitosis, all the cell’s contents are copied and the cell splits into two cells, with each taking one copy of everything. The two cells will be identical to each other, as they both contain the same DNA.

The process is split into five stages, called phases: interphase, prophase, metaphase, anaphase, and telophase. Each stage is responsible for one important action, and only occurs after the previous stage is completed. The process differs slightly between different organisms, for example, some processes occur in different parts of the cell in fungi compared to animals.

Interphase

This is the phase where the chromosomes are copied to produce two identical copies of each. It is divided into three phases, called first gap, synthesis, and second gap. In the first gap phase, the cell starts to grow. In the synthesis phase, the chromosomes are copied, while the cell continues to grow. In the second gap phase, after chromosome copying is complete, the cell grows further.

Prophase

In prophase, the chromosomes condense into the familiar chromosome shape, similar to the letter X. At this stage, each chromosome is composed of two parts, called chromatids, which are identical to each other, with one part being destined for each cell. Every cell contains two different copies of each chromosome, one inherited from the mother and one from the father. After the chromosomes have condensed, the two copies of each chromosome pair up, and two structures called centrioles move to opposite sides of the cell.

Metaphase

In metaphase, the chromosomes line up across the center of the cell. A type of fiber called meiotic spindle fibers starts to extend out from each centriole and attach to the chromosomes. One spindle fiber from each centriole attaches to each chromosome, one to each chromatid.

Anaphase

Anaphase is composed of two phases, called anaphase A and anaphase B. During anaphase A, the two chromatids that make up each chromosome split apart. They are now known as daughter chromosomes. In anaphase B, the meiotic spindle fibers contract and pull one daughter chromosome from each pair towards the centrioles, at opposite ends of the cell. In some species, anaphase B occurs before anaphase A.

Telophase

At the start of telophase, there is a complete copy of all the chromosomes at each end of the cell. A nucleus, which is the part of the cell that contains most of the genes, and is responsible for the regulation of many processes that occur in the cell, forms around each set of chromosomes. The chromosomes then decondense and return to the state they were in before prophase. The cell then elongates and then splits apart, forming two cells, called daughter cells, each with one nucleus. This process is known as cytokinesis.

When does mitosis occur?

Mitosis occurs in different situations and is used for different purposes. It is used for the growth of the body. The larger a body is, the more cells it is composed of. Therefore, the production of more cells is required for a body to grow. Mitosis is entirely responsible for turning you from a newborn baby into the size you are now. It is also responsible for the growth of hair and nails. Another purpose it is used for is cell replacement.

Cells need replacing for two reasons. One is that they become too old to keep functioning properly, which happens to all cells. However, the rate at which this occurs varies depending on the type of cell. Some cell types, such as red blood cells, need replacing much more frequently than other types. The other reason that cells need replacing is that some types of cells are constantly being lost. These include skin cells and cells in the digestive system. When cells are lost they need replacing, or else the body would shrink as more and more cells were lost.

Cells need replacing for two reasons. One is that they become too old to keep functioning properly, which happens to all cells. However, the rate at which this occurs varies depending on the type of cell. Some cell types, such as red blood cells, need replacing much more frequently than other types. The other reason that cells need replacing is that some types of cells are constantly being lost. These include skin cells and cells in the digestive system. When cells are lost they need replacing, or else the body would shrink as more and more cells were lost.

There are also some more unusual situations where mitosis occurs. These situations are confined to a much smaller group of organisms, whereas growth and cell replacement occur in every multicellular organism. One of these more unusual situations is regeneration. This is a form of growth, but instead of it being new growth it is the regrowing of a lost body part. There are only a limited number of species where this occurs. The obvious example is lizards, which can regenerate their tails. However, there are some less well-known examples, including tadpoles, axolotls, and starfish.

Another of the more unusual situations is during asexual reproduction. This is where an organism reproduces entirely by itself. Because there is only one parent, the offspring have the same genes as their parent – there is no combining of genes like there is in sexual reproduction. This means that mitosis is sufficient for producing offspring in species that engage in asexual reproduction, whereas in sexual reproduction meiosis is required, which results in cells with only one copy of each chromosome.

Errors during mitosis

Occasionally, mitosis will occur incorrectly in a cell. Nondisjunction is when chromosomes don’t separate into daughter chromosomes, causing one cell to end up with only one chromosome and the other to have three, conditions known as monosomy and trisomy respectively.

Anaphase lag is when one daughter chromosome is prevented from moving across the cell properly during anaphase. This results in it not getting to the end of the cell before the nucleus forms. Because of this, one of the daughter cells will have only one copy of that chromosome. Endoreplication is when the chromosomes are copied but the rest of mitosis fails to occur, resulting in a cell with double the normal number of chromosomes, which is known as a polyploid cell.

References

1. http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html
2. https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/v/mitosis