DNA Structure

DNA in cells exists as a double-stranded helix.

A strand of DNA is a chain of nucleotides joined by covalent bonds between the phosphate of one nucleotide and the pentose sugar of the adjacent nucleotide.

A nucleotide consists of a phosphate group bound to a pentose (a five carbon sugar; in DNA the sugar is deoxyribose) with a nitrogenous base (A, T, C, or G) also bound to the pentose.

		The carbons in the pentose are numbered to make it easier to talk about them.   The phosphate attached to the number 5 carbon of the pentose is called a 5' ("five prime") phosphate.   The hydroxyl group (O-H) attached to the number 3 carbon is called the 3' ("three prime") hydroxyl.
		Because the nucleotides in a strand of DNA are always joined together in the same way (5' phosphate to 3' hydroxyl) the strands have direction, sort of a head and a tail, if you want to think about it that way.    Of course the proper way to think about it is "DNA strands have a 5' end and a 3' end".

The two strands of a double stranded DNA molecule are held together by hydrogen bonds between specific nitrogenous base pairs: adenines on one strand form hydrogen bonds with thymines on the opposite strand (A-T) and guanines on one strand form hydrogen bonds with cytosines on the opposite strand (G-C).

The strands are oriented in opposite directions.

This is the only way the bases can line up across from each other to form the hydrogen bonds that hold the strands together.

Because the two strands of a double stranded DNA molecule are oriented in opposite directions a double stranded DNA molecule is said to be antiparallel.

Now what about that "helix" thing?  Look at the figure above and notice how the double stranded molecule is offset, or staggered, like a stairstep from the top left to the bottom right of the figure.

In real life this causes the double stranded molecule to coil like a spiral staircase.  This is "helical".