Final answer:
During prophase, DNA condenses into X-shaped structures called chromosomes, consisting of sister chromatids joined at the centromere.
Step-by-step explanation:
During prophase, DNA condenses into X-shaped structures called chromosomes. These chromosomes are visible under a microscope due to the tightly wound chromatin. Each chromosome at this stage consists of two identical copies known as sister chromatids, which are joined together at a specific region named the centromere. The centromere contains proteins, such as kinetochores, where spindles attach during mitosis to assist in the movement of the chromosomes.