It just is; there really isn't a "why". It's a result of the way chemistry and physics work. By the way, it's not just human DNA that forms a double helix; it's a property of DNA generally.
One represents a genetic junk pile, the other refers to the abortion survivor.
Francis Crick and James Watson
yes
It refers to the DNA molecule that is shaped like a ''twisted ladder'' with the outsides (made of deoxyribose and phoshate) twisted.