Question

Virtually every multicellular organism, including you, goes from a single-celled fertilized egg to a large, multicellular organism.

• How does mitosis explain this process? Is all growth through mitosis?
• If every cell in your body is a copy of that original fertilized egg, how is it that some cells are so different than other cells? What are some examples of differentiated cells, and how does mitosis help explain their differences?

Answer 1

Mitosis is the mode of cell division in all vegetative cells,starting from the fertilised egg ,that is, the zygote. The only exception is cell division mode in germ cells,that is,for ovum and sperm production . So all growth(increase in the number of cells) in vegetative cells is through mitosis.

Thus mitosis helps to form 2 daughter cells from a single cell but the fate of each of those cells depend on gene expression. For example, once an embryo is embedded in the uterus,2 things occur-

1. The genes that keep the embryo in fully potent state are turned off.
2. The tissue-specific genes are turned on .   Once this occurs,the cell will program in only a controlled and specific manner.

Some examples of differentiated cells are liver cells,muscle cells,etc. All the cells have arisen from the zygote but got differentiated only when certain genes were turned on and accordingly specific proteins were synthesized. Cell differentiation in turn depend on cell signalling. Until and unless a signal(like a transcription factor) interacts with our DNA i.e, specific genes, differentiation will not start.