Sister Chromatids: Definition and Example

Definition: Sister chromatids are two identical copies of a single replicated chromosome that are connected by a centromere. Chromosome replication takes place during interphase of the cell cycle. DNA is synthesized during the S phase or synthesis phase of interphase to ensure that each cell ends up with the correct number of chromosomes after cell division. The paired chromatids are held together at the centromere region by a special protein ring and remain joined until a later stage in the cell cycle. Sister chromatids are considered to be a single duplicated chromosome. Genetic recombination or crossing over can occur between sister chromatids or non-sister chromatids (chromatids of homologous chromosomes) during meiosis I. In crossing over, chromosome segments are exchanged between sister chromatids on homologous chromosomes.

Chromosomes

Chromosomes are located in the cell nucleus. They exist most of the time as single-stranded structures that are formed from condensed chromatin. Chromatin consists of complexes of small proteins known as histones and DNA. Prior to cell division, single-stranded chromosomes replicate forming double-stranded, X-shaped structures known as sister chromatids. In preparation for cell division, chromatin decondenses forming the less compact euchromatin. This less compact form allows the DNA to unwind so that DNA replication can occur. As the cell progresses through the cell cycle from interphase to either mitosis or meiosis, the chromatin once again becomes tightly packed heterochromatin. The replicated heterochromatin fibers condense further to form sister chromatids. Sister chromatids remain attached until anaphase of mitosis or anaphase II of meiosis. Sister chromatid separation ensures that each daughter cell gets the appropriate number of chromosomes after division. In humans, each mitotic daughter cell would be a diploid cell containing 46 chromosomes. Each meiotic daughter cell would be haploid containing 23 chromosomes.

Sister Chromatids In Mitosis

In prophase of mitosis, sister chromatids begin to move toward the cell center.

In metaphase, sister chromatids align along the metaphase plate at right angles to the cell poles.

In anaphase, sister chromatids separate and begin moving toward opposite ends of the cell. Once the paired sister chromatids separate from one another, each chromatid is considered a single-stranded, full chromosome.

In telophase and cytokinesis, separated sister chromatids are divided into two separate daughter cells. Each separated chromatid is referred to as a daughter chromosome.

Sister Chromatids In Meiosis

Meiosis is a two-part cell division process that is similar to mitosis. In prophase I and metaphase I of meiosis, events are similar with regard to sister chromatid movement as in mitosis. In anaphase I of meiosis, however, sister chromatids remain attached after homologous chromosomes move to opposite poles. Sister chromatids do not separate until anaphase II. Meiosis results in the production of four daughter cells, each with one half the number of chromosomes as the original cell. Sex cells are produced by meiosis.

Related Terms

• Chromatid - one-half of two identical copies of a replicated chromosome.
• Chromatin - DNA and protein complex that forms chromosomes.
• Chromosomes - DNA strands containing genes that code for the production of proteins.
• Daughter Chromosome - single-stranded chromosome resulting from the separation of sister chromatids.