Answer:1. The zygote typically undergoes a series of mitotic divisions to form a ball of identical cells called a blastula. The number of divisions that occur during this process can vary depending on the organism, but in humans, for example, the zygote undergoes about 4-5 divisions to form a blastula with about 100-150 cells.
2. The process by which cells become specialized is called differentiation. During development, cells receive signals from their surroundings that trigger specific genetic programs, causing them to adopt specialized functions and morphologies. These signals can come from other cells, extracellular matrix molecules, and various signaling molecules. The specific combinations of signals received by each cell determine its fate and the type of specialized cell it will become.
3. Dead cells in the body are replaced through a process called cell turnover, which involves the continual production of new cells from stem cells or precursor cells. In some tissues, such as the skin and the lining of the gut, cells are replaced relatively rapidly, while in other tissues, such as the brain and the heart, cell turnover is much slower. Stem cells are capable of dividing and differentiating into multiple cell types, allowing them to replenish the body's supply of specialized cells as needed. In addition to stem cells, some tissues also contain dedicated progenitor cells that can give rise to specific cell types.