In a eukaryotic cell, chromosome replication occurs during DNA synthesis, or the S phase of the cell cycle. In its normal state, a chromosome is a long, thin chromatin fiber containing one DNA molecule. Once duplicated, a chromosome consists of two identical sister chromatids, each containing a copy of the DNA molecule. During mitosis, the sister chromatids are separated and distributed to two daughter cells.
In eukaryotes, mitotic cell division involves the distribution of identical genetic material to two daughter cells. According to Hartnell College, the cell cycle is divided into two primary phases, interphase and mitosis. Interphase is further divided into three subphases: the first gap, or G1 phase; DNA synthesis; and the second gap, or G2 phase. When a cell is not dividing, it carries out its normal cellular functions in the G0 phase. During this phase, each chromosome is in its normal form of a long, thin chromatin fiber. When a cell enters the cell cycle for division, it first begins to grow by producing proteins and cytoplasmic organelles. This growth continues throughout all subphases of interphase. Within the S phase, the chromosomes are duplicated when DNA polymerases and other enzymes act on the DNA strands. The double helix structure of DNA is unwound, and each strand acts as a template for synthesis of the new strands. When a cell is ready to divide, it enters the mitotic phase. During mitosis, the cell divides into two daughter cells, with each cell containing one of the sister chromatids.