Final answer:
After DNA replication, two identical copies of DNA, known as sister chromatids, remain attached at a region called the centromere. These sister chromatids form part of the X-shaped chromosome observable during cell division.
Step-by-step explanation:
DNA Replication and Chromatids
After each DNA molecule replicates, the two identical copies remain closely attached to each other. These copies are referred to as sister chromatids. They are connected at a specific region known as the centromere. During the S phase of interphase, DNA replication occurs, resulting in each chromosome being composed of two linked sister chromatids. When the DNA achieves its highly compact form, the sister chromatids are visible under a light microscope as part of the X-shaped chromosome. The centromeric region, where the sister chromatids are closely held together by cohesin proteins, is highly condensed and appears as a constricted area.
In summary, when the question asks what forms when two identical copies of DNA remain closely attached together after replication, the answer is: B. Chromatids. These chromatids are also known as sister chromatids, an essential concept in cell division and genetics.