Hydrogen bonds.
The nucleotides in a base pair are complementary which means their shape allows them to bond together with hydrogen bonds. The A-T pair forms two hydrogen bonds. The C-G pair forms three. The hydrogen bonding between complementary bases holds the two strands of DNA together
DNA is cytoplasmic (extranuclear) in Prokaryotes but intranuclear in Eukaryotes.
Glassy igneous rocks (i.e. obsidian) are cooled very rapidly.
It is a rare genetic mutation that both the maternal and paternal partners have to be at least a carrier for the albinism gene.