Why a cell contains all DNA when it only needs a few genes?
Why does a cell contain all DNA when it only needs a few genes ?


I interpret this question to mean "why does every cell contain the whole human genome even though it doesn't use it all, i.e. why does a liver cell contain the genes for eye color?"

If that is what you meant, then we can easily answer the first question in your actual text:


  Is that what they called junk?


No, that is not what is called "junk DNA". "Junk DNA" can refer to many concepts, but generally it refers to parts of DNA that serve no function in the body: they are not transcribed into proteins, they don't serve to regulate other genes, and if you remove them or change their sequence this will have no effect on the overall organism. This means it refers to parts of DNA that are used nowhere in the body, not parts that are used in one place but not in others.

How much DNA is "junk", how useless it actually is, and why we have it are complex questions that don't all have known answers, but it is completely different from your headline question.

I don't have the time to look up a detailed answer to your first question so I will leave that to others, but here are some elements that might help:

1) I think cells use more genes than you realize. I don't know the proportion of genes that are used for the cell to live as a cell vs those that are used for the specific tasks this cell needs to do beyond maintaining itself and reproducing, but I'm pretty certain there is no cell in the body that "only needs a few genes". I would even guess all cells in the body use most of the genes they contain.

2) not all cells contain an unmodified copy of the whole genome. Some cells lose their nucleus, i.e. their DNA entirely (red blood cells); others multiply some genes that they use a lot.

3) having said all that, I would guess that liver cells indeed don't need the genes for eye color, yet they might have them. Consider that cells reproduce by copying themselves, and copying their whole genome. Is it more of a waste to keep the copy as it is, or to go around before or after the copying cutting out all the bits this specific cell won't use? Do I go around editing the code of my text editor and recompiling it to get rid of all the functionalities I don't use? I might save some space on my hard drive but it is very obviously not worth my time to do that either. If the extra DNA doesn't do enough harm, there is no reason it would be taken out; indeed the wasteful thing would be to do so.