What is a chromatid?

Inside the cells is the information that determines the inheritance of the traits that the parents transmit to their descendants. This information is contained in the deoxyribonucleic acid molecule, DNA , which associates with proteins called histones and forms chromatin . In turn, chromatin is packaged through complexes called nucleosomes , structures made up of histones wrapped around DNA. When chromatin condenses, that is, is tightly packed together, it gives rise to a filament- or rod-shaped structure called a chromosome .

Before a cell divides, it goes through a process called interphase in which DNA is duplicated and histones are synthesized on chromosomes. As a result, each chromosome generates a copy, so that it is made up of two filaments, which are joined by an area called the centromere . Each strand of the duplicated chromosome is called a chromatid .

When a chromosome is made up of a single strand it is known as a simple chromosome , but when it is made up of two strands or chromatids it is called a duplicated chromosome . For example, a cell in the human body contains 46 single chromosomes. During interphase, each of those 46 chromosomes generates a copy, or chromatid. At the end of this process the cell will have 46 duplicated chromosomes.

Usually, a duplicated chromosome consists of four arms: two short and two long, although sometimes they are practically the same. The arms of a chromosome, being copies, encode the information for the same genes and form sister chromatids . Therefore, each duplicated chromosome has two copies for the same gene. A gene is a segment of DNA that expresses a trait, such as skin color.

chromatids in mitosis

Chromatids are formed so that when a cell divides in two, each resulting cell has the same number of single chromosomes. This division process, called mitosis , produces a pair of daughter cells with the same genetic information as the parent cell. Mitosis proceeds in successive stages: prophase, metaphase, anaphase, and telophase.

Chromatids in meiosis

Meiosis is the cell division that produces gametes , that is, eggs and sperm . These cells have different genetic information from that of the parent cell, because the chromosomes are not equally distributed but are divided among the daughter cells; Furthermore, chromosomes combine genetic information between them.

The behavior of chromatids during meiosis is explained as follows:

1. Single chromosomes are duplicated before meiosis.
2. Homologous chromosomes line up and form a structure with four chromatids called a tetrad .
3. In prophase I, the chromosomes that make up the tetrads exchange genetic information between their chromatids.
4. After metaphase, anaphase and telophase I, two daughter cells are produced between which the duplicated chromosomes of the parent cell are equally distributed.
5. After prophase and metaphase II, and during anaphase II, the chromatids of the duplicated chromosomes separate. In telophase II the resulting single chromosomes are equally distributed among the four daughter cells that will form eggs or sperm.

Sources

Garibay Garcia, J. (2013). Treatment-induced sister chromatid exchange in breast cancer patients . (Thesis). Autonomous Mexico State University. Faculty of Chemistry, Toluca, Mexico.

Rodríguez, R. Persistence of lesions caused to DNA by alkylating agents, involved in the production of sister chromatid exchanges . Slopes, 3(1/2): 3-13, 2000.