To see how DNA unzips and zips up again, get a bunch of kids together, and two pieces of rope like long jumpropes. Start with five of the kids holding on to one jumprope - two boys and three girls, in any order. These kids are an RNA strand and they are going to try to reproduce themselves. All the rest of the kids just stand around near them.
Now the RNA strand - the five kids holding the rope - begins to run through the other kids, trying to catch a kid. When they catch a boy, he holds hands with another boy, and when they catch a girl, she holds hands with one of the girls. When they have all five spots filled, the new kids hold on to the other jumprope with their free hands, and now the ten kids together are a strand of DNA - one DNA molecule (except that real DNA molecules have four different kinds of stairs, not just two, and they are thousands of stairs long, not just five).
Now this DNA strand needs to make more DNA. It has to unfasten (stop holding hands), and each of the two strands now tries to catch more kids to build a new strand. When all (or almost all) of the kids are in strands, the game is over.