DNA is made up of DNA nucleotides, which are composed of deoxyribose sugar, phosphate and base.

DNA nucleotide

DNA has a sugar-phosphate backbone. Which is formed as the phosphate of one nucleotide is attached by a strong chemical bond to the deoxyribose sugar of the next.

There are four bases that a DNA nucleotide could have: (A) adenine, (T) thymine, (G) guanine and (C) cytosine.

These bases follow the base pairing rules: A always pairs with T, and G always pairs with C, i.e. bases A and T are complementary and bases C and G are complementary.

The bases are held together by hydrogen bonds.

DNA is double stranded — meaning it is made up of two strands of nucleotides.

DNA also has an antiparallel structure, which means the strands run in opposite directions to each other (3’ to 5’ versus 5’ to 3’), i.e. one is inverted relative to the other.

The carbons on the deoxyribose sugar of a DNA nucleotide can be numbered as shown below.

Deoxyribose sugar with carbon atoms numbered

The ends of a DNA strand are derived from these numbers — the 3’ end of a DNA strand is that which ends in deoxyribose sugar, and the 5’ end is that which ends in phosphate.

DNA structure

The base sequence of DNA forms the genetic code.

DNA is organised differently in different organisms and in different parts of organisms — there are specific ways it is organised in each.

Prokaryotes are organisms with a single cell that lack a true membrane-bound nucleus, and other membrane-bound organelles.

Prokaryotes have a single, circular chromosome and smaller, circular plasmids.

Eukaryotes are organisms made up of cells that have a true membrane-bound nucleus and other membrane-bound organelles.