Brainstorm

[Mister Agenda]

But why at the moment of conception? Do we consider a sperm quasi-human, prior to this?

I think we should consider whether it can suffer and maybe think(or be self-aware) in any way before we actually define it as a person and consider it with the same dignity we consider other humans. Otherwise we are merely talking about its potential to become a human being.

Because at the moment of conception a brand new set of human DNA is formed. A sperm is just a sex cell from the man's body, it isn't its own separate entity. 

To address your last paragraph though, a human fetus begins to feel pain about midway through the second trimester... at least as far as we can tell, though it could be before that. But I don't think the ability to feel pain should be the indicator to being human. If you are comprised of human DNA, you are a biological human being, whether you have the ability to feel pain or not, imho.

An identical twin (or clone) does not have a brand new set of human DNA, just a copy of someone else's. What effect does this have on their status as a human entity?