Question

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how are damaged or dead cells replaced?

Explanation:

Damaged or dead cells are replaced through the process of cell division. In multicellular organisms, mitosis is the key mechanism. Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus. For example, in human skin, new skin cells are constantly being produced through mitosis to replace the dead skin cells that are shed. Stem cells also play a crucial role. They are undifferentiated cells that can divide and differentiate into specific cell types. When there is damage, stem cells can be activated. They divide (through mitosis) and then differentiate to become the type of cell needed (e.g., in bone marrow, hematopoietic stem cells divide and differentiate to replace damaged blood cells).

Answer:

Damaged or dead cells are replaced through cell division (specifically mitosis in most somatic cells) and the activity of stem cells which can divide and differentiate into the required cell types.