Chromatid

Chromosomes
Chromosomes. Credit: MedicalRF.com/MedicalRF.com/Getty Images

What is a Chromatid?

Definition: A chromatid is one-half of two identical copies of a replicated chromosome. During cell division, the identical copies are joined together at the region of the chromosome called the centromere. Joined chromatids are known as sister chromatids. Once the joined sister chromatids separate from one another in anaphase of mitosis, each is known as a daughter chromosome.

Chromatids are formed from chromatin fibers.

Chromatin is DNA that is wrapped around proteins and further coiled to form chromatin fibers. Chromatin allows DNA to be compacted in order to fit within the cell nucleus. Chromatin fibers condense to form chromosomes.

Prior to replication, a chromosome appears as a single-stranded chromatid. After replication, the chromosome has the familiar X-shape. Chromosomes must be replicated and sister chromatids separated during cell division to ensure that each daughter cell receives the appropriate number of chromosomes. Every human cell contains 23 chromosome pairs for a total of 46 chromosomes. The chromosome pairs are called homologous chromosomes. One chromosome in each pair is inherited from the mother and the from the father. Of the 23 homologous chromosome pairs, 22 are autosomes (non-sex chromosomes) and one pair consists of sex chromosomes (XX-female or XY-male).

Chromatids in Mitosis

When cell replication is necessary, a cell enters the cell cycle.

Prior to the mitosis phase of the cycle, the cell undergoes a period of growth where it replicates its DNA and organelles.

Prophase

In the first stage of mitosis called prophase, the replicated chromatin fibers form chromosomes. Each replicated chromosome consists of two chromatids (sister chromatids) connected at the centromere region.

Chromosome centromeres serve as a place of attachment for spindle fibers during cell division.

Metaphase

In metaphase, the chromatin becomes even more condensed and sister chromatids line up along the mid region of the cell or metaphase plate.

Anaphase

In anaphase, sister chromatids are separated and pulled toward opposite ends of the cell by spindle fibers.

Telophase

In telophase, each separated chromatid is known as a daughter chromosome. Each daughter chromosome is enveloped in its own nucleus. Following the division of the cytoplasm known as cytokinesis, two distinct daughter cells are produced. Both cells are identical and contain the same number of chromosomes.

Chromatids in Meiosis

Meiosis is a two-part cell division process undergone by sex cells. This process is similar to mitosis consisting of prophase, metaphase, anaphase and telophase stages. In meiosis however, cells go through these stages twice. In meiosis, sister chromatids do not separate until anaphase II. After cytokinesis, four daughter cells are produced with half the number of chromosomes as the original cell.

Chromatids and Nondisjunction

It is vital that chromosomes be separated correctly during cell division. Any failure of homologous chromosomes or chromatids to separate correctly results in what is known as nondisjunction.

Nondisjunction during mitosis or meiosis II happens when sister chromatids fail to separate properly during anaphase or anaphase II, respectively. Half of the resulting daughter cells will have too many chromosomes, while the other half will have no chromosomes. Nondisjunction can also occur in meiosis I when homologous chromosomes fail to separate. The consequences of having either too many or not enough chromosomes are often serious or even fatal.

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Bailey, Regina. "Chromatid." ThoughtCo, Apr. 17, 2017, thoughtco.com/chromatid-373540. Bailey, Regina. (2017, April 17). Chromatid. Retrieved from https://www.thoughtco.com/chromatid-373540 Bailey, Regina. "Chromatid." ThoughtCo. https://www.thoughtco.com/chromatid-373540 (accessed December 12, 2017).