During the S phase of the cell cycle, which occurs before cell division, DNA replication takes place. In humans, there is one complete round of DNA replication during the S phase, regardless of whether the cell is preparing for mitosis or meiosis.
This means that before a cell undergoes mitosis (the process of somatic cell division) or meiosis (the process of gamete formation), each chromosome is duplicated. After replication, each chromosome consists of two sister chromatids joined together. This ensures that when the cell divides, each daughter cell will receive an identical set of chromosomes.
In summary, during the S phase of the cell cycle, every human cell undergoes one round of DNA replication, leading to the doubling of genetic material in preparation for either mitosis or meiosis.