All cells in a living organisms share the same DNA but develop into different types of cells. What mechanism tells these cells which type of cell to become? How does it work?
In: Biology
This is a huge topic and a lot of is is still unknown.
I don’t think I’m able to explain this for a 5 year old but I’ll try to make it as simple as possible.
A fertilized cell (egg+sperm) is called a zygote. This cell is called “totipotent”, which means it can develop into any other cell type including tissues surrounding the embryo. The zygote starts dividing and becomes pluripotent, which means it can form all cell types of the body but not the extraembryonal tissues. Different stem cells are then dividing and differentiating into different tissues, e.g. there will be stem cells than can build the skin or stem cells forming different organs. At some point, a cell is fully differentiated (e.g. the cells on the outermost layer of your skin) and cannot divide anymore.
As for how this all works:
On DNA level, there are different mechanisms to switch genes on and off. This is called “gene expression”.
DNA can be packaged to make it inaccessible and thereby switch genes “off”. Also, DNA can be chemically marked for activation or inactivation. Some of these marks can be copied to daughter cells when a cell divides which explains how cells can keep their identity when they divide. This is called epigenetics.
Also there’s proteins present in a cell called “transcription factors”. These go to target genes and switch them on. When a cell divides, the two daughter cells can inherit the proteins of their progenitor. However there is also asymmetric cell division. Some proteins build a gradient within a cell, so when the cell divides, one offspring can stay a stem cell and the other one can differentiate into a more specialised cell type.
In 2012, Shinya Yamanaka was awarded a Nobel prize for generating inducible pluripotent stem cells (iPS). The trick he used was to introduce a combination of transcription factors into a differentiated cell. These transcription factors then reverted the cell back to the pluripotent state by resetting the epigenetic state of the cell. This means the profile of genes that were switched on and off in the iPS cell was comparable to a stem cell.
**TL;DR: A cell divides, stuff in the dividing cell it distributed between the two daughter cells. For stem cells, one daughter cell gets different stuff than the other making one stem cell and one skin/brain/liver/whatever cell. “Stuff” in this context means proteins and chemicals floating around in the cell or sitting directly on DNA. There’s still lots we don’t know about this.**
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