Final answer:
The formation of two identical sister chromatids within a chromosome occurs during the S phase of interphase due to DNA replication. These sister chromatids are held together by the centromere, separated later during mitosis or meiosis.
Step-by-step explanation:
The process leading to the formation of two chromatids in a chromosome is known as DNA replication, which occurs during the S phase of interphase, preceding both mitosis and meiosis. During this phase, each chromosome is replicated to produce two identical copies, referred to as sister chromatids. These chromatids are held together at a specific region called the centromere by cohesin proteins. This connection is maintained until the sister chromatids are separated into individual chromosomes during subsequent phases of cell division.
The formation of sister chromatids ensures that each daughter cell receives an identical set of chromosomes following cell division, whether it's by the process of mitosis, leading to the formation of two identical diploid cells, or by meiosis, resulting in four haploid cells. During meiosis I, each chromosome pairs with its homologous partner to form a tetrad, where crossing over can occur, contributing to genetic diversity.