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I know that DNA can be used to encode information, using ACGT as a quaternary instead of binary encoding.
Could a functional genetically-modified human walk around with a lot of information stored in their DNA? What would be the side effects of this, and would the data stay stable through that human's life?
By extension, could two humans modified in the same way parent children, some of which would also have the stored information?
Obviously DNA is a very fragile construction. The most obvious manifestation of this is a very well-known and prevalent disease known as cancer.
We would need multiple redundant copies of the stored DNA information in the test subject to avoid information loss, as we have with our functional DNA. These copies ideally would be spread throughout a variety of different cells, making it probably the best to insert several modified cells containing our genetically encoded information in a developing human embryo to let it multiply later. (Obviously, ethical issues would abound here.)
Our next question is where this data would be encoded. Obviously, for us to have a functional human, our data can't overwrite existing DNA. One place someone could insert information would be in the telomeres of chromosomes, which are long strips of redundant genetic information that are slowly stripped away as a cell replicates over and over again. Obviously, as the person ages, more and more information would be lost due to the telomeres' length shortening, making this an undesirable solution in the long term. However, to store data in the short term, this could potentially work.
As for parenting, there could be some issues with data retrieval. Meiosis is a complicated process that involves some genetic switcheroos between various chromosomes in a process known as crossing over, and data could be mixed up or split apart in that process. (After all, the whole point of reproduction is to induce genetic variety in offspring, not to prevent genes from changing.) So I wouldn't bet on all of the information surviving.
Could a functional genetically-modified human walk around with a lot of information stored in their DNA?
Yes. I mean, obviously: unmodified humans already walk around with a lot of stored information their DNA.
would the data stay stable through that human's life?
DNA is subject to random damage all the time, and the repair processess are not always perfact. Cells are regularly recycled, too, so you'd need to store copies of the information in multiple cells throughout the body to ensure than you could reliably find and then correctly reassemble the information you wanted and correct for any encoding errors that may have arisen.
Have a read of Christian Bök's efforts to encode poetry in the genome of Deinococcus Radiourans. Bacteria have far less junk DNA than eukaryotes... the excess weight affects their competetiveness, so it gets pruned more agressively, but a load of junk doesn't make a big different to the weight of a big eukaryotic cell.
What would be the side effects of this
If you are careful to store this information in non-coding regions of DNA, you'll probably be just fine. Exactly which bits of your genome are actually junk and which bits aren't isn't always obvious at first glance though. You'd need to be careful not to accidentally scribble all over some subtle bit of non-protein-coding but functionally-critical bit of DNA.
It's possible that non coding DNA can be used to encode information, as stated in other answers. However, random mutation might complicate reading out after some times. Human DNA contains 725 megabytes of data, as pointed out here
The 2.9 billion base pairs of the haploid human genome correspond to a maximum of about 725 megabytes of data
That gives enough space for quite some text.
About your last point
By extension, could two humans modified in the same way parent children, some of which would also have the stored information?
I am afraid not. Upon mating, each parent provide one half of the DNA. Thus, in your case, one parent would provide half of a book, and the other parent would provide the half of ANOTHER book. The results ought to be hilarious, at best.
And Gandalf stood on the path of the Balrog, shouting with all his mighty "Thou shall not pass, light of my life, fire of my loins. My sin, my soul."
Yes, we could do this with existing technology if we were so inclined. The easiest way to do it and ensure that the message isn’t lost is to add the information at the one-cell stage just after fertilization. This way the message will be inherited by every cell in the human’s body. This will be fine because the human genome is vast and there is ample space to encode whatever you would like without seriously impacting anything as long as you are somewhat selective in where you insert it. In principle, though there’s no reason you couldn’t write your own custom chromosome specifically for maintaining messages.
While any individual cell might have a mutation that damages the message the changes will be different in every cell so pooling many cells together for sequencing (which is already how we do it) will give an average readout that will correspond to the original message.
Ideally, there would be no side effects of this. Of course, there are no guarantees because how exactly the genome works is an ongoing field of study, but based on our current understanding there’s no reason to think there would be negative consequences to even a large insert if it’s done correctly.
If two humans modified with the same message in the same place were to have children the results depend on whether the modification was placed on only one chromosome on the parents or both. Since the message is in all cells in the humans it will also be in their gametes. If both parents have two copies of the message then all of their children will also have two copies of the message. If both parents have one copy of the message then ¼ of their children will have 2 copies, ½ will have one copy, and ¼ will have no copies.
