Final answer:
Chromosomes are copied during the S phase of interphase, with sister chromatids held together by the centromere. The centrosome, critical for cell division apparatus, is also duplicated during this phase.
Step-by-step explanation:
Chromosomes make a copy of themselves during a specific stage of interphase known as the S phase (Synthesis phase). It is in this phase where DNA replication occurs, leading to the formation of two identical copies of each chromosome, termed sister chromatids.
These sister chromatids are tightly connected at a region called the centromere. Additionally, the cellular structure known as the centrosome is also duplicated at this time. The centrosomes, comprising a pair of centrioles in animal cells, are essential for organizing the mitotic spindle during cell division.