For example why does a skin cell has a copy of how to make blood cells if their only job is just to do skin cell things?
In: Biology
Life isn’t “developed” like a computer program which has different parts for different applications. What is responsible for what; and where these things are located on the genome is random. Everything is mixed up.
So you can’t just take all the skin cell things and separate them from the blood cell things, because in the best case it would mean cutting out and rearranging 1000s of tiny snippets. Moreover, the presence of DNA alone isn’t the whole picture. Skin and blood cells might use the same gene, but one uses it more actively, therefore its function is more pronounced. Or they use the same gene to make different protein via alternative splicing.
Because you began as just one cell, and all your cells are descended from that one cell.
When the cells eventually specialize into one type of cell, they deactivate the genes they don’t need. Actually removing the unneeded genes would be far more complicated, and not necessary.
I think, and I may be wrong. All cells come from stem cells which are like cells without a specific job yet and so the stem cell has the data/possibility to perform any job. And once it gets selected to become a blood cell or skin cell it just still has the data for the whole DNA even though it’s not using it. But that’s just my supposition
Your ancestors billions of years ago were single cells that only had two was of dividing. Each of the two cells either got a whole copy (normal division) or they each got half (used to make sex cells). When your later ancestors hundreds of millions of years ago started hanging around together as multicellular organisms, those two types were too deeply set to change and other work arounds were used instead where cells don’t express all their genes.
Because humans “happend to be” the way we are.
You are right that its completely unnecessary for all cells to have a complete DNA.
However humans (like other multicellular beings) were originally made by copiees of the same cell doing different things – and “erasing” the unnecessary code precisely – while keeping whats necessary – is more effort than “just copy everything and disregard what isnt important to you”.
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