Chromosomes appear as X shapes when they are duplicated and the two identical copies, called sister chromatids, are joined together at a central region called the centromere. This X-shaped structure is typical during cell division, specifically in mitosis and meiosis. These X-shaped structures are still called chromosomes, but more precisely, each "arm" of the X is a chromatid. When a chromosome has this X shape, it means the DNA has been replicated in preparation for cell division, so there are two sister chromatids joined as one chromosome. In summary:
- The X shape occurs during cell division.
- Each half of the X is a chromatid (sister chromatids).
- The joined chromatids together form one chromosome.
- The central joining point is the centromere.
This shape is not unique to any one chromosome but is a general appearance during cell division for all chromosomes, including autosomes and sex chromosomes.
