The Structures of DNA and RNA Molecules
DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix
The backbones run in opposite 5′ → 3′ directions from each other, an arrangement referred to as antiparallel
One DNA molecule includes many genes Only certain bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)
This is called complementary base pairing
This feature of DNA structure makes it possible to generate two identical copies of each DNA molecule in a cell preparing to divide RNA, in contrast to DNA, is single stranded
Complementary pairing can also occur between two RNA molecules or between parts of the same molecule
In RNA, thymine is replaced by uracil (U) so A and U pair
While DNA always exists as a double helix, RNA molecules are more variable in form