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Chromosomes are made up of two identical chromatids and play a role in cell division. Mitosis produces genetically identical daughter cells, while meiosis produces sex cells with half the DNA. Chromatids can increase genetic diversity by interbreeding and modifying genes.
Inside the nucleus of each cell, that organism’s DNA is found in the form of chromosomes. Depending on the type of organism, the number of different chromosomes will be different, but not the structure of the chromosomes. Each chromosome is made up of two identical strands of DNA called chromatids.
Most often, chromosomes appear as very long, thin strands of DNA called chromatin. When a cell isn’t dividing, it replicates its DNA so that each chromosome consists of two sister chromatids that lie next to each other. While they appear to run parallel to each other, there is a section where each chromatid is joined to the other, called the centromere.
Chromatids play an important role during cell division, both mitosis and meiosis. Mitosis is the duplication of DNA and nucleus to produce two nuclei, resulting in the production of two genetically identical daughter cells from a single mother cell. Mitosis is used for the growth, repair and replacement of cells. Meiosis occurs only in reproductive cells to produce sex cells or gametes which have half the amount of DNA as the parent cell.
During the initial stage of mitosis, chromosomes condense or become shorter and thicker. At this time, each pair of chromatids can be seen when the nucleus is stained. The chromosomes line up along the “equator” of the cell and the chromatids are separated. Each chromatid is pulled to opposite ends, or poles, of the cell so that a new nuclear membrane can form around them and then the cell can divide.
Once the cell has finished dividing, mitosis is complete. Right now, the chromatids are now the chromosomes of the new nucleus they inhabit. Before cell division repeats, they replicate to ensure that there are two identical copies of each chromosome.
A similar process occurs during meiosis. The main difference between the two is that meiosis consists of two mitotic divisions. During the first division, pairs of chromosomes line up along the equator and separate. The resulting cells only have half of the DNA in them now, as they only have a single copy of each chromosome. All chromosomes are still fully intact and consist of two sister chromatids.
The second division of meiosis is identical to mitosis. Chromosomes line up along the equator and chromatids are pulled apart to separate the poles of the cell. A new nucleus forms and the cell divides. However, the resulting cells are not genetically identical, having only half the DNA of the initial parent cell.
Genetic diversity can be further increased by chromatids during meiosis. In the prophase phase of meiosis I, or first division, a chromatid of one chromosome can interbreed with a chromatid of a different chromosome. When pairs of identical chromosomes, one from each parent, line up along the cell’s equator, the chromatids can twist around each other. Fragments of each chromatid can pass to the other chromosome, thus modifying the genes found on the initial chromosome. By changing the genetic information on chromosomes, non-identical daughter cells are formed, which can increase genetic diversity.
