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
You and your friends are working to assemble a lego village, with each person building their own structure
DNA is the manual that you all use, which contains the blueprint for every single building. Naturally, you (the cell) would only use a few pages of the manual while your friends use other pages.
At the start of the project, you and your friends communicate to determine who is building what. Cells can do a similar thing during development to tell its neighbouring cells what to be (what pages to use) – this starts as early as when you are only comprised of 8-cellsl
It all goes back to when you were conceived. The fertilized egg begins to divide. The cells start out undifferentiated, they could turn into anything. Then as the cells work through the genetic instructions they start designating certain cells for performing certain functions, making them “differentiated”. Once differentiated those cells basically turn off all the parts of their DNA that doesnt work towards fulfilling their new role. So a muscle cell and a nerve cell may have a copy of the same instruction manual but each is working from different chapters.
As an adult a stem cell divides creating a new undifferentiated cell. That cell then responds to signals being sent out from surrounding cells. Skin for example. It is made of many layers of skin cells which are being produced on the bottom layer,then gradually get pushed up by newer cells and eventually reach the surface, die and flake off.
The stem cell is like your mom or dad, and they just made you and they tell you “you can be anything you want” and then the other skin cells are life and they say “no you’re going to be a skin cell”, so you become a skin cell.
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.**
This music video does a surprisingly good job explaining the concept behind the differentiations: https://www.youtube.com/watch?v=ydqReeTV_vk
One thing no one has really covered is how cells know what they should be.its not like they have a map with a big “you are here” sign that tells them, ooh, I’m a kidney cell.
It’s all down to ratios. Let’s say you have a cell named Bob. Bob is destined to be an elbow cell. Rob is bathed in stuff. Some of this stuff is concentrated, some is diffuse. The ratio between these concentrations is unique depending where the cell finds itself.
So Bob says, hmm, the mix of stuff I’m bathing in must mean I activate these bits of my DNA. I guess that makes me an elbow cell.
There’s a show on Netflix called The 9 Months that Made You that goes through each stage of develop and exactly what happens in these stages, as well as what happens when something goes wrong (disabilities, birth defects, etc). I’d highly recommend it! It’s super interesting and totally awesome 🙂
Pluripotent stem cells – plural for many, potent for potential
There are different conditions that need to be met for each cell to split, kind of like evolution into niche corners of an ecosystem
Essentially as the body grows it reaches stages of development where those conditions exist and the cells diverge (differentiate)
The others here seem to be much more well versed on this subject, but I’m not sure that they answered your actual question. As 2 cells turn into 4 and continue to multiply, each one down the line starting from the first will secrete chemical signals that will be more concentrated near the cell secreting them and less as you move farther away. The different levels of each of the different chemical signals from each cell triggers the newly created cells to become a specific thing (arm, leg, kidney).
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6 would eventually become the right arm because it sees very little of 1 and 7 and a lot of 3 and 9. Something like that
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