A cell copies or replicates its chromosomes when getting ready to divide. Living cells go through stages called the cell cycle. During the cycle, the cells grow, copy or replicate their chromosomes and then divide to form new cells.
The cycle of a cell happens in four phases. The first phase, or G1 phase, involves the cell growing. The S phase is when the cell makes copies of its chromosomes, and each chromosome then consists of two sister chromatids.The G2 phase is when the cell verifies that the duplicated chromosomes are correct and are ready to divide. Lastly, the M phase is when the cell separates the chromosomes to form two full sets and then divides into two new cells.
Cells divide in two ways to make new cells, by mitosis or meiosis. When mitosis is used, the cells produce daughter cells that are genetically identical to their parent cells. During this type of division, the cell copies its chromosomes and splits the copied chromosomes equally between each daughter cell so that each cell has a full set.
Meiosis is the process of making special cells, which are sperm and egg cells, that have half the number of chromosomes as normal. Meiosis reduces the 23 pairs to 23 single chromosomes. The cell then copies its chromosomes and separates the 23 pairs so that each daughter cell has only one copy of each chromosome. A second divide occurs, which divides each daughter cell to create four daughter cells.