Cell division is an important part of life. For organisms to grow or reproduce, their cells must divide. In animals, including humans, there are two main ways in which cells divide. In most of our tissues, cells divide through mitosis. Mitosis is how embryos can grow into fetuses that can grow into fully developed humans. Mitosis creates new cells that have genes that are nearly identical to the cells they came from. These cells have two copies of DNA and so are described as diploid.
Sex cells can divide in a different way, called meiosis. In meiosis, one cell divides to make new cells that are very different genetically. These cells have only one copy of DNA and so are described as haploid. Meiosis is what allows most animals to have offspring with genes that are different from their own. In humans, egg cells are made in the ovaries before birth and sperm cells are made in the testes after puberty.