Cell Differentiation

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All of the cells in your body, every single last cell, can be traced back to just a single cell (the fertilised egg a.k.a. zygote), and a single DNA code. How is it possible for hundreds of different cell types to arise from that? The zygote undergoes mitosis, and according to that, all daughter cells resulting are genetically identical. Therefore, all cells in our body must be genetically identical. Well, they are*. So how do all these supposed "clone cells" end up being so different from one another, and hence achieve such different purposes within the larger organism?

They differentiate due to selected genes becoming active in certain cells but not others. The initial cells from which others can differentiate are called stem cells. These can be totipotent, pluripotent or unipotent. Totipotent stem cells have the ability to differentiate into any type of cell. Pluripotent stem cells are descendants of totipotent stem cells, and can differentiate into many different kinds of cell, but not all. Unipotent stem cells can only replicate themselves, and so produce just one type of cell.

The genes active in cells determine whether they are skin cells, heart muscle cells, neurons, lung cells, etc.

A tissue is formed of structurally and functionally similar cells. For example, nerve tissue or connective tissue.

An organ is formed of multiple tissues working to achieve a physiological function. For example, the pancreas is involved in blood glucose regulation.

A system is formed of multiple organs which achieve an overarching function in an organism. For example, the circulatory system ensures a constant blood supply around the body.

*Of course you must have worked out so far that biology is never that simple. Somatic cells (i.e. most cells in our body, with gametes - eggs and sperm - excluded) are theoretically clones. Gametes arise from meiosis rather than mitosis, hence they are all very diverse genetically. Spontaneous mutations can arise in some somatic cells, say, in one cell in your knee. That mutation has just made that cell cease to be a clone, or genetically identical, to the other cells. By and large, though, the majority of cells in the body are clones.