Gametes are made by a process called gametogenesis, which takes place in specialised reproductive organs: the testes and ovaries (the gonads) in mammals, and the anthers and ovules in flowering plants.In mammals, gametogenesis starts with diploid germ cells in the gonads. The process has three phases:
• Multiplication: germ cells divide repeatedly by mitosis to form many diploid cells.
• Growth: some of these cells enlarge, replicating their DNA and building up cytoplasm ready for meiosis.
• Maturation: the cells divide by meiosis, producing haploid cells that develop into gametes.So both types of cell division are needed: mitosis to make many cells, and meiosis to halve the chromosome number and create variation. The details differ between sperm formation (spermatogenesis) and egg formation (oogenesis), which are covered in later lessons.In flowering plants the order is different. Diploid cells in the anther or ovule divide by meiosis first, producing haploid cells. These haploid cells then divide by mitosis to produce the gametes. Because the cells are already haploid, the gametes produced by this mitosis are also haploid.Worked example: a human germ cell has 46 chromosomes, and after meiosis each gamete has 23. A plant with 2n = 24 produces haploid cells with 12 chromosomes by meiosis, and the gametes formed from them by mitosis also have 12.
• Multiplication: germ cells divide repeatedly by mitosis to form many diploid cells.
• Growth: some of these cells enlarge, replicating their DNA and building up cytoplasm ready for meiosis.
• Maturation: the cells divide by meiosis, producing haploid cells that develop into gametes.So both types of cell division are needed: mitosis to make many cells, and meiosis to halve the chromosome number and create variation. The details differ between sperm formation (spermatogenesis) and egg formation (oogenesis), which are covered in later lessons.In flowering plants the order is different. Diploid cells in the anther or ovule divide by meiosis first, producing haploid cells. These haploid cells then divide by mitosis to produce the gametes. Because the cells are already haploid, the gametes produced by this mitosis are also haploid.Worked example: a human germ cell has 46 chromosomes, and after meiosis each gamete has 23. A plant with 2n = 24 produces haploid cells with 12 chromosomes by meiosis, and the gametes formed from them by mitosis also have 12.