what is a chromatid

A chromatid is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. Chromatids are condensed chromosomes distinguishable during cell division. The two "sister" chromatids are joined at a constricted region of the chromosome called the centromere. Sister chromatids are like two halves of a puzzle piece. During cell division, spindle fibers attach to the centromere and pull each of the sister chromatids to opposite sides of the cell. Soon after, the cell divides in two, resulting in daughter cells with identical DNA. Chromatid pairs are normally genetically identical, and said to be homozygous. However, if mutations occur, they will present slight differences, in which case they are heterozygous.