Sulfur and phosphorous can violate the octet rule and have an expanded octet, so molecules of these elements can contain more than eight valence electrons. In contrast, nonmetal elements in period 2 can't have an expanded octet because they don't have d orbitals. These elements must obey the octet rule and can hold only up to eight valence electrons. Period 2 elements can fill their outer orbitals by covalently bonding with one other atom of the same element.
the double helix of dna is, like its name, implies, in the shape of a helix which is essentially a three-dimensional spiral. the double comes from the fact that the helix is made of two long strands of dna that are intertwined—sort of like a twisted ladder.