Final answer:
The increase in chromosome count from 2n to 4n occurs due to DNA replication, which takes place during the S phase of the cell cycle, resulting in duplicated chromosomes with sister chromatids.
Step-by-step explanation:
The chromosome count goes from 2n to 4n due to DNA replication. This process occurs during the S phase of the cell cycle, where each chromosome replicates and forms two identical sister chromatids that are attached at the centromere, creating what looks like an 'X' shaped molecule. This is essential for cell division, as it ensures that when cells divide, by processes such as mitosis, each new cell will have a complete set of chromosomes.
During the S phase, chromatin condenses into chromosomes, and each one duplicates, resulting in a matched pair of sister chromatids firmly attached at the centromere. Prior to mitosis, each chromosome is made of two sister chromatids and is considered a duplicated chromosome. The centrosomes, structures that help organize the mitotic spindle for chromosome movement during cell division, are also replicated during this phase.