Читать книгу Infants and Children in Context - Tara L. Kuther - Страница 92

Most cells in the human body reproduce through a process known as mitosis in which DNA replicates itself, duplicating chromosomes, which ultimately form new cells with identical genetic material (Sadler, 2018). The process of mitosis accounts for the replication of all body cells. However, sex cells reproduce in a different way, through meiosis. 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. The chromosome pairs align and DNA segments cross over, moving from one member of the pair to the other, essentially “mixing up” the DNA. Crossing over thereby creates unique combinations of genes (Sadler, 2018). The resulting cell consists of only 23 single, unpaired chromosomes. Known as gametes, these cells are specialized for 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, and it is estimated that individuals can produce millions of genetically different gametes (National Library of Medicine, 2019).