Читать книгу Lifespan Development - Tara L. Kuther - Страница 104

Most cells in the human body reproduce through a process known as mitosis in which DNA replicates itself, permitting the duplication of chromosomes and ultimately the formation of new cells with identical genetic material (Sadler, 2015). The process of mitosis accounts for the replication of all body cells. However, sex cells reproduce in a different way, through meiosis (see Figure 2.1). First, the 46 chromosomes begin to replicate as in mitosis, duplicating themselves. But before the cell completes dividing, a critical process called crossing over takes place. Chromosome pairs align, and DNA segments cross over, moving from one member of the pair to the other. Crossing over creates unique combinations of genes (Sadler, 2015). The cell then continues to divide. As the new cells replicate, they create gametes, containing only 23 single, unpaired chromosomes. Gametes are the cells of sexual reproduction: sperm in males and ova in females. Ova and sperm join at fertilization to produce a fertilized egg, or zygote, with 46 chromosomes, forming 23 pairs with half from the biological mother and half from the biological father. Each gamete has a unique genetic profile. It is estimated that individuals can produce millions of versions of their own chromosomes (National Library of Medicine, 2017).