What Is a Chromatid?

3D diagram describing the different parts of homologous chromosomes.

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A chromatid is one half of a replicated chromosome. Prior to cell division, chromosomes are copied and identical chromosome copies join together at their centromeres. Each strand of one of these chromosomes is a chromatid. Joined chromatids are known as sister chromatids. Once linked sister chromatids separate from one another during anaphase of mitosis, each is known as a daughter chromosome.

Chromatids

  • A chromatid is one of two strands of a copied chromosome.
  • Chromatids that are joined together at their centromeres are called sister chromatids. These chromatids are genetically identical.
  • Chromatids are formed in both the cellular division processes of mitosis and meiosis.

Chromatid Formation

Chromatids are produced from chromatin fibers during both meiosis and mitosis. Chromatin is composed of DNA and skeletal proteins and is called a nucleosome when wrapped around these proteins in sequence. Even more tightly wound nucleosomes are called chromatin fibers. Chromatin condenses DNA enough to fit within a cell's nucleus. Condensed chromatin fibers form chromosomes.

Before replication, a chromosome appears as a single-stranded chromatid. After replication, a chromosome appears in an X-shape. Chromosomes are first replicated and their sister chromatids are then separated during cell division to ensure that each daughter cell receives the appropriate number of chromosomes.

Chromatids in Mitosis

When it is time for a cell to replicate, the cell cycle begins. Before the mitosis phase of the cycle, the cell undergoes a period of growth called interphase where it replicates its DNA and organelles to prepare for division. The stages that follow interphase are listed chronologically below.

  • Prophase: Replicated chromatin fibers form chromosomes. Each replicated chromosome consists of two sister chromatids. Chromosome centromeres serve as a place of attachment for spindle fibers during cell division.
  • Metaphase: Chromatin becomes even more condensed and sister chromatids line up along the mid-region of the cell or the metaphase plate.
  • Anaphase: Sister chromatids are separated and pulled toward opposite ends of the cell by spindle fibers.
  • Telophase: Each separated chromatid is known as a daughter chromosome and each daughter chromosome is enveloped in its own nucleus. Two distinct but identical daughter cells are produced from these nuclei following the division of the cytoplasm known as cytokinesis.

Chromatids in Meiosis

Meiosis is a two-part cell division process carried out by sex cells. This process is similar to mitosis in that it consists of prophase, metaphase, anaphase and telophase stages. During meiosis, however, cells go through the stages twice. Because of this, sister chromatids do not separate until anaphase II of meiosis.

After cytokinesis at the end of meiosis II, four haploid daughter cells, containing half the number of chromosomes of the original cell, are produced.

Illustration of sex cells produced during meiosis, showing Interphase, Prophase, Metaphase.
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Nondisjunction

It is vital that chromosomes separate correctly during cell division. Any failure of homologous chromosomes or chromatids to separate correctly is known as nondisjunction. Nondisjunction occurs during anaphase of mitosis or either stage of meiosis. Half of the resulting daughter cells from nondisjunction have too many chromosomes and the other half have none at all.

The consequences of having either too many or not enough chromosomes are often serious or even fatal. Down syndrome is an example of nondisjunction resulting from an extra chromosome and Turner syndrome is an example of nondisjunction resulting from a missing whole or partial sex chromosome.

Sister Chromatid Exchange

When sister chromatids are in close proximity to one another during cell division, the exchange of genetic material can occur. This process is known as sister-chromatid exchange or SCE. During SCE, DNA material is swapped as portions of chromatids are broken and rebuilt. A low level of material exchange is typically considered safe, but when the exchange reaches excessive levels, it can be hazardous to the individual.