What are sister chromatids?

Sister chromatids are indeed identical copies of a chromosome that are formed during DNA replication. Prior to cell division, each chromosome is duplicated, resulting in two copies, known as sister chromatids, that are connected at a region called the centromere. These chromatids are genetically identical because they are produced from the same DNA molecule, ensuring that when the cell divides, each daughter cell receives an exact copy of the genetic material.

The other options refer to different concepts in genetics and molecular biology. Different alleles of a gene describe variations of a gene that can lead to different traits but do not pertain to the identical nature of sister chromatids. Chromosomes from different parents relate to the concept of homologous chromosomes, which contain different genetic information inherited from each parent. Segments of RNA are quite distinct from chromosomes entirely, as they typically serve roles in the expression of genes rather than being structural components of genetic organization.