1. During S phase, the replication of the cell's genetic material, known as DNA, occurs. In eukaryotic cells, DNA is found in structures called chromosomes. Before replication, these chromosomes exist as long, thin chromatin fibers. After replication, each chromosome is condensed into one of two identical halves of a replicated chromosome called a chromatid. Sister chromatids are two identical copies of DNA, the original strand of DNA, and the newly created copy. Sister chromatids separate during mitosis. A centromere, the point on a chromosome that attaches to the spindle fibers during cell division, attaches the sister chromatids. The centromere is aided in binding sister chromatids together by one of several proteins called a cohesin. Once the DNA has been replicated, the cell moves to the second gap phase.2. During the G2 phase, a structure in the cytoplasm of animal cells that coordinates the formation of microtubules, called a centrosome, allows cell division to proceed during reproduction. The centrosome will organize a complex structure of microtubules, the mitotic spindle, involved in mitosis. Other cellular structures are duplicated during G2, such that each replicated daughter cell produced during mitosis will have all necessary organelles (such as mitochondria, endoplasmic reticulum, etc.). For most cells this phase is relatively short; once complete, the cell is ready to divide. Interphase takes most of the time in the cell cycle, comprising more than 95% of the duration of the cell cycle in most eukaryotes.