To appreciate the techniques involved in cloning one must understand the differences between the two cell types, somatic cells and gametes (or germline cells), and the two types of cell division, mitosis and meiosis. The bulk of the human body consists of somatic cells that contain two copies of each chromosome (a threadlike structure that carries the genes) and that are called diploid. In humans the somatic cells contain twenty- three pairs of chromosomes (forty-six in total). One member of each chromosome pair comes from the mother and the other from the father. The chromosomes carry the genes that determine the characteristics of each individual. The genes too come in pairs, one from the mother and one from the father.
Somatic cells divide by a type of cell division known as mitosis (see Figure 64.1). Before a cell can divide it must grow and duplicate its chromosomes. During the first part of the cell cycle, the Gl phase, the cell just grows. During the next stage, the S phase, the chromosomes are duplicated, but the duplicates, known as sister chromatids, remain attached to each other. In the G2 phase the cell grows again and prepares to divide. Finally, during the M phase, or mitosis, the cell divides, and the two sister chromatids of each chromosome separate, one passing to each of the two daughter cells. The daughter cells each have 46 chromosomes and are genetically identical. Some cell types, for example, nerve cells, do not divide. They pass from the Gl phase into a dormant phase known as GO.
The second type of cell, the gametes, are the eggs and sperm. The cells that give rise to the eggs are known as primary oocytes, and those that