noun, plural: sister chromatids
The chromosomes in the nucleus of a cell appear as threadlike strands prior to cell division. They are specifically referred to as chromatin. In preparation to cell division, such as mitosis, the chromatins condense into thicker structures and the DNA molecule contained in each chromosome duplicates (by DNA replication). Following DNA replication, each chromatin would have two copies of DNA. During the first stage of mitosis (i.e. prophase), the chromatins condense and become more visible. At this point, the chromatins are referred to as chromosomes. Each chromosome would be comprised of two strands joined by a kinetochore. Either one of the two strands is called a chromatid. The chromatids may be sister chromatids or non-sister chromatids. Each of the two identical strands joined by a common kinetochore is called a sister chromatid. The chromosomes (each made up of sister chromatids) align at the equatorial region during metaphase. At anaphase, the sister chromatids separate and move toward the opposite poles of the cell. During telophase, the chromatids become threadlike structures again resulting in two daughter cells, each with their own set of now-called chromosomes.
Word origin: Greek khrōma, khrōmat- ("color")