Final answer:
A duplex chromosome consists of two sister chromatids held together by one centromere after DNA replication during the S phase.
Step-by-step explanation:
A duplex chromosome consists of two chromatids and one centromere. After DNA replicates during the S phase of interphase, each chromosome is replicated to produce two identical copies known as sister chromatids. These sister chromatids are held together at a specific region called the centromere. The centromere serves as the attachment point for kinetochore proteins, where spindle fibers attach during mitosis to separate the chromatids. It's crucial not to confuse this with a homologous pair of chromosomes, which are two separate chromosomes inherited from each parent.