My people are traveling in a generation ship. For some reason they can't make the ship radiation-proof. Their solution: genetically modify their descendants to be radiation proof. How could one modify a cell to resist radiation? Is it plausible to modify our genetic structure to be radiation-proof?
Assume they have a level of tech about fifty to a hundred years ahead of our current level.
Use Checksums.
A Checksum is a computer science tool used to prevent storage or replication errors. In your case you would have some sort of algorithm that would translate a DNA sequence into a unique code. You'd then store this value. Then when the DNA replication step happens, it is first checked against the stored Checksum. If it's incorrect, the cell then needs to poll its neighbors for the "good" DNA value, and replace it.
Potential Issues:
Rip it off from something that worked millions of years to get it.
The bacterium deinococcus radiodurans is an extremophile. It can survive cold, dehydration, vacuum, acid, and ionizing radiation. It is, in fact, an organism with some of the highest radioresistance known. For instance, this guy can take an acute dose of 5,000 Gy. For a 50% chance of death a human can only take 4.5 Gy. To give it a 37% chance of life it has to be hit with 15,000 Gy.
How does it do this?
It has multiple copies of its genome and a unique rapid repair mechanism for its DNA.
It usually repairs breaks in its chromosomes within 12–24 hours through a 2-step process. First, D. radiodurans reconnects some chromosome fragments through a process called single-stranded annealing. In the second step, multiple proteins mend double-strand breaks through homologous recombination. This process does not introduce any more mutations than a normal round of replication would.
Not only that, but they can take DNA from other cells if theirs is too damaged, they'll even repair it first.
D. radiodurans is capable of genetic transformation, a process by which DNA derived from one cell can be taken up by another cell and integrated into the recipient genome by homologous recombination. When DNA damages (e.g. pyrimidine dimers) are introduced into donor DNA by UV irradiation, the recipient cells efficiently repair the damages in the transforming DNA as they do in cellular DNA when the cells themselves are irradiated.
Other Uses
These bacteria are so good at repairing their DNA that they have been considered for long term information storage, capable of surviving a nuclear holocaust.
In 2003, U.S. scientists demonstrated D. radiodurans could be used as a means of information storage that might survive a nuclear catastrophe. They translated the song "It's a Small World" into a series of DNA segments 150 base pairs long, inserted these into the bacteria, and were able to retrieve them without errors 100 bacterial generations later.
How to rip it off?
The DNA repair mechanism from this bacteria has been replicated to assemble DNA fragments into chromosomes, the ultimate goal is to assemble a synthetic lifeform by the Craig Venter Institute. It would not be impossible then, that engineered humans could include these mechanisms in their own cells.
If your starship encounters a very strong source of radiation such as a gamma-ray burst, game over. ("The crew are dead, killed by a radiation leak. The only survivors are Dave Lister, who was in suspended animation at the time of the disaster, and his pregnant cat...")
Otherwise, the crew will undergo long-term exposure to moderate levels of radiation. The main danger from this is errors in DNA replication, which can cause cancer and/or birth defects in the following generation.
The human body already has elaborate mechanisms to ensure DNA replication fidelity, destroy tumours when they appear, and stop a non-viable embryo from being implanted and carried to term. If you could make these mechanisms better, you would have a cure for cancer; so they are a very active field of research.
Unfortunately for us, it turns out that curing cancer is hard; but in a science-fiction scenario, you could postulate some amazing breakthroughs in cancer prevention. These would serve to protect your starship crew from the worst effects of background radiation.
As I wrote in my answer to Radiation sudden stop the key to surviving radiation is to efficiently repair DNA as @bowlturner has mentioned as well.
The most notable example of this ability is Deinococcus radiodurans. You might be aware that there is already bacterial DNA in the human genome so it's quite likely humans could be genetically engineered to gain deinococcus radiodurans' radiation-resistance.
Edit:
Luckily, I just had another idea regarding your problem so I can contribute some more beside just citing my self.
Here we go:
If the generation ship is not part of your plot you could also have an automated ship with robots on stand-by. Instead of humans you transport only their DNA in small radiation-proof container. After an automatic landing on your target planet the robots will get activated and create your population by in vitro fertilization in artifical wombs and raise them to adolescence.
This way you only have to conserve the DNA plus ova&spermatozoa – or build them artificially.
You will not need a circular economy on your ship, i.e. no complex infrastructure and no resources to fuel the infrastructure so that your space-requirements and maintenance costs are very, very low.
Of course the premise of the whole idea is mind uploading / transhumanism is not technically feasible or still to scary compared to letting humanity cease to exist only temporarily ;-)
There are two things you could do about the radiation, reduce it or reduce its effects.
The reduce approach depends on the type of radiation and the energy of the radiation. The basic idea is cladding the human's surface with a material which has a very high reduction such as lead or even a composite.
+----------------------+------------------------+----------------------+
| Material | Halving Thickness [cm] | Halving Mass [g/cm²] |
+----------------------+------------------------+----------------------+
| Lead | 1 | 12 |
| Steel | 2.5 | 20 |
| Concrete | 6.1 | 20 |
| Packed soil | 9.1 | 18 |
| Water | 18 | 18 |
| Lumber or other wood | 29 | 16 |
| Air | 15 000 | 18 |
+----------------------+------------------------+----------------------+
The problem with cladding your human is that he gets very heavy very quick. Even with composite materials and a thin layer you will see this problem arise. An average male has a surface area of 1.9 square meters, this times the mass of lead would result in 22.8 kilos to halve the radiation once.
There is a second problem. Thin layers of material will scatter high energy rays into multiple rays causing quite the opposite of what you want, they would cause even more ionization than when they were a single ray.
Instead of radiation 'proof', use that advanced technology to 'repair' the damage radiation does to genes. Most immediately dangerous radiation that will kill you in a short period of time can be blocked (at least down to tolerable levels) fairly easily. The rest of it is dangerous when dealing with causes of cancer (mostly). Especially in cells that reside in the body for a long time, such as the eggs in women's ovaries.
If you can design a retro-virus that would correct damage to the DNA it could prevent a lot of the problems caused by radiation. It would also be able to dramatically reduce aging as a side benefit.
http://www.ultramet.com/chemical_vapor_deposition.html
some years back I read an article on atom by atom materials .
with nano materials one could make reflective and refracting materials ,
metal objects shinny as copper in Discovery magazine yet with ceramic or glass like
properties ie will shatter.
X or Gama-rays in refract and reflect some of it away from ship, and or absorb and
re-radiative it. IE Infrared , or as light. or other more desirable wavelength.
one could have translucent nano glass shielding for plants and with cosmic rays have
light radiate on them instead. let desirable light in , else have it converted to light plants can use. on a long space flight capturing energies that are freely available is a need. so phosphor or other impregnated nano materials that protect your hydroponic bay and recycle unwanted energies ie x-rays to plant lighting etc useful.
also conceivably one could make better space suits and lightweight/flexible radiation shielding materials
"TONS of Lead shielding" yet light as silk and wearable, soft nano fabrics that reactor techs could ware like any other clothing.
We yet do not have this but someday soon someone will find a means of adding some micro-crystalline miracle compound and make your Levi Jeans very radiation prof.
Point being materials research can make things we cant even conceive of. some today tech below that's mind blowing.
http://www.sciencedaily.com/releases/2010/01/100120113556.htm Plug your iPod into your T-shirt for power?
comments I'm simply stating with new Materials , we dont have yet radiation could be mitigated and or absorbed and transformed to useful energies. and I tried to provide some current examples.
while not a biological solution having radiation resistant clothing via new material be the metamaterials and nano or combination of them.
an overall radiation management gambit mixing of the various answers
some of the new materials made from atom by atom deposition have odd or unique properties , ie a sheet of copper that shatters like glass,
point being if someone found a light weight metal composition for radiation armor that is lightweight like aircraft aluminum but blocks more radiation than pounds of lead 3-4 times as thick and at 1.100th or less the weight would make a starship all that more survivable.
bio-radiation repair will be needed tech,
however if one can avoid or mitigate it not having to absorb as much radiation due to new materials also good.
there is nano particle treatments that makes cotton waterproof. it seems 10-100 years down the like that being able to treat cotton to be as protective to radiation exposure isn't all that inconceivable, not only useful to spaceflight personnel but first responders or even xray techs , or nuclear response personnel as well.
(also so used to html.... and there is no common markdown from sight to sight ie stack exchange github etc good old html , kinda markdown challenged)
and forgive my haste on the original section there isnt a post as a draft answer option, I was thinking kinda rapidly for what new wonder materials is likely possible in the near future that would help..
A:absorb & Change radiation to something useful IE electricity , or recovered as photons of light IE special nano-glass , ie a hydroponics bay designed to use a suns light in planetary orbits or take as much starlight/radiation in and convert it to plant light.
B: Enhanced radiation shielding with new types of metals or forced atomic structuring of the crystalline structures. a ship needs to be light weight , however a material with 6 feet? of lead mitigation/refraction/reflection/re-radiation as heat/etc. or etc but yet extremely light, would be a boon. CVD or similar tech is in it's infancy now, but also not inconceivable. some newer radiation armoring of the hull would be useful esp light weight and great protection.
C: new clothing or treatments for clothing , comfortable clothing ie cotton or other comfortable materials , that provides shielding or converts cosmic rays etc to light we can sea etc. when possible , having some enhanced protections without a radion suit which is bulky and not terribly comfortable in "normal" conditions , that would provide a great deal of protection if an event occurred, which could buy time to get into a radiation shield area, IE spacewalk etc.
normal everyday treated clothing should provide comfort and high amount of radiation protection without users having to be as concerned.
IE if you had normal fabrics that offered a lead apron's worth of protection yet the same as your t-shirt or sweatshirt ie lightweight , and comfortable as daily wares that would help offer good protection , most would hardly notice.
