What are sister chromatids?

Sister chromatids are defined as two identical copies of a single chromosome that are joined together at a region called the centromere. This formation occurs during the S phase of the cell cycle, when DNA replication takes place, resulting in two identical copies of each chromosome. These sister chromatids remain attached until they are separated during mitosis or meiosis, ensuring that each daughter cell receives an identical set of chromosomes.

The other options refer to different concepts. Different alleles of the same gene pertain to variations of a gene located at the same position on homologous chromosomes, not to sister chromatids. Individual chromosomes before mitosis refer to chromosomes that have not undergone replication, which means they are not yet in the sister chromatid state. Unreplicated parts of a chromosome describe segments that have not been duplicated, which does not correlate to the concept of sister chromatids, as they specifically involve duplicated chromosomes.