system
Given the following post, respond as if you're commenting on Reddit:
human
Why do cells have a copy of the whole genome instead of only the genes they need?

It isn't energetically expensive to copy ALL the DNA when the cell undergoes mitosis? Neurons use a very different set of genes than, for instance, hepatic cells, or muscular cells. Why not just have a copy only of the genes they need?
gpt
First of all, since an organism evolving multicellularity would already have a mechanisms for silencing genes in response to it's environment, these mechanisms are probably the easiest way for different cell types to evolve. This is what we see when we compare the silencing mechanisms in say yeast, to humans. Yeast have only one cell type, but the systems they use to turn off say, their sporulation genes, are related to what we use to switch off brain genes in our muscles. Elimination of DNA would require some complex, reliable targeting mechanism not present in single cells. No sense evolving wings from scratch if you can just repurpose your arms.

It's also worth noting that some ciliates (single celled organisms unrelated to plants animals or fungi) DO eliminate some of their genome, when they create a second, larger 'Macronucleus' (this nucleus does most of the work in the cell, but is never replicated. they have a second smaller germline nucleus which is used to reproduce).

This mechanism hasn't evolved in complex organisms, and one could speculate that a complex organism has too much to lose by re-arranging it's DNA. Broken DNA can lead to random re-arrangements and deletions, so one slip up could lead to a cancerous cell and kill the whole organism. 

Remember as well that the DNA exists in only 2 molecules per cell -replicating it might not offer a large energy saving compared to maintaining a complex system of proteins to do the genome elimination.

Also note that genes can get re activated - such as when amphibians regenerate lost limbs. Simpler animals are even better at this, so it may have been important even if humans have lost it.

It's possible of course that if we could run evolution on again a few thousand times we'd end up with complex genome eliminating organisms some or even most of the time. Evolution just builds on the first solution it hits. This isn't necessarily the only solution it could have hit, or the best possible.