What is formed during the S phase regarding chromosomes?

During the S phase of the cell cycle, DNA synthesis takes place. This phase is crucial for ensuring that when the cell divides, each daughter cell receives an identical set of chromosomes.

Specifically, during the S phase, each chromosome is replicated, resulting in two identical copies known as sister chromatids. These sister chromatids are joined together at a region called the centromere. The formation of sister chromatids is essential for the accurate distribution of chromosomes during cell division, specifically in mitosis and meiosis.

The other options do not accurately represent what occurs during the S phase. Chromosomes are not broken into smaller fragments, homologous chromosomes align during a different stage of cell division (metaphase), and cellular respiration processes are separate from the events of the S phase. Thus, the formation of sister chromatids is a key defining feature of this phase, making it the correct answer.