When two nucleotides are joined by phosphodiester bond, the resulting dinucleotide has, at one end, a phosphate group on carbon 5 that remains free, and can be attached to a hydroxyl group on carbon 3 of another nucleotide. At the other extreme, the hydroxyl group of carbon 3 is also free, available to react with the phosphate of carbon 5 of another nucleotide. Thus, long nucleotide chains can be formed that will always have a free 5 'phosphate group at one end and a free 3' hydroxyl group at the other.
The nucleotide sequence of a nucleic acid is written from left to right, from the 5th to 3rd carbon end.
A short-stranded nucleic acid is called an oligonucleotide (generally up to 50 nucleotides) and if its length is greater, a polynucleotide .
The DNA of mitochondria and chloroplasts is distinct from nuclear DNA, much like the DNA of prokaryotes. This DNA forms a nucleoid that lacks a nuclear envelope, and is also associated with other proteins.