Truly, the typical X shaped chromosomes is the result of DNA replication. When the DNA has replicated, it then condenses and coil into the X-shaped of the chromosome. This then implies that replicated chromosomes take the X-shaped structures seen under a microscope.
When this replication occurs, the chromosome is made of two structures that are identical. They are known as the sister chromatids. These chromatids are actually joined at the centromere.
