1: Nature affects your genes throughout your life when you start getting older, your genes change and you change in puberty until you are an adult, but In my opinion you mostly change by Nurture because everything you chose to do throughout life affects you and your body, for example if you only eat junk food it will make your body weaker and you will be more open to a diseases, but if you eat very healthy you will have more chances of surviving a disease.
2: DNA needs to be copied correctly for your body to have the same process of things and for avoiding trouble inside the body. If DNA doesn't copy correctly 3 main things could happen, It might be good for evolution and make great changes throughout history, it may not be a problem at all or It could be a bad thing for every single cell in your body if it keeps on copying badly it could continue harming future generations of the family, in any case people should be aware of what they do in life for avoiding harm.
3: Sexual reproduction.
Advantage: Every offspring will be born different and some would be stronger or weaker than others and there could be more opportunities for change and evolution. Also the offspring would have more protection from the mother.
Dis advantage: The mother of the offspring may suffer from the pregnancy because it may be too hard for her, it takes more time for the offspring to be born and in the life of the offspring takes time for it to form perfectly healthy and correctly.
Advantage: The offspring reproduces really quickly and more and more offsprings would be born and there would be more of that species.
Dis advantage: The offsprings are exactly the same and if there is a harmful disease it would wipe out every single species that the disease can affect, also there is almost no chances of evolution for that species.