Phosphate groupA phosphorus atom with four oxygens round it. It is joined to the sugar's 5′ carbon. In a strand it also joins the 3′ carbon of the next sugar.
Pentose sugarA five-carbon sugar: deoxyribose in DNA, ribose in RNA. Four of its carbons and one oxygen make the ring.
Nitrogenous baseThe part that carries the genetic code. It is adenine, guanine, cytosine or thymine in DNA, with uracil in place of thymine in RNA. This one is a purine.
The sugar's carbons and bonds: 7 7
1′ carbonThe sugar's first carbon, where the base is joined.
2′ carbonIn DNA it carries only a hydrogen. In RNA it carries a hydroxyl group. That missing oxygen is the deoxy in deoxyribose.
3′ carbonThe carbon that carries the 3′ hydroxyl. The next nucleotide joins there in a strand.
4′ carbonThe last carbon in the ring, next to the ring oxygen. It holds the 5′ carbon outside the ring.
5′ carbonThe one carbon outside the ring, joined to the phosphate group.
3′ hydroxyl groupAn oxygen and a hydrogen on the 3′ carbon. The next nucleotide's phosphate joins here, so a strand grows at its 3′ end.
Glycosidic bondThe bond that joins the base to the sugar's 1′ carbon.
Two kinds of base: 2 2
PurineA base with two rings. A six-sided ring is joined to a five-sided one. Adenine and guanine are purines.
PyrimidineA base with one six-sided ring. Cytosine, thymine and uracil are pyrimidines.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Nucleotide, https://en.wikipedia.org/wiki/Nucleotide : three parts, a nitrogenous base (nucleobase), a five-carbon sugar (ribose or deoxyribose) and a phosphate group; adenine and guanine are purines, cytosine, thymine and uracil pyrimidines; thymine in DNA, uracil in RNA; the glycosidic bond joins base and sugar. (2) Wikipedia, Deoxyribose, https://en.wikipedia.org/wiki/Deoxyribose : the base joined to the 1′ carbon; the 5′ hydroxyl replaced by a phosphate, which joins the 3′ carbon of the next sugar; the hydroxyl at the 2 position replaced by hydrogen. (3) Wikipedia, Nucleoside, https://en.wikipedia.org/wiki/Nucleoside : the glycosidic bond to N9 of a purine or N1 of a pyrimidine; the 3′ hydroxyl accepts the phosphate. (4) OpenStax Biology 2e, 3.5 Nucleic Acids, https://openstax.org/books/biology-2e/pages/3-5-nucleic-acids (used to check, nothing copied): the three parts, the numbered carbons, purines with a double ring and pyrimidines with a single ring. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
The three parts of a nucleotide are a phosphate group, a five-carbon sugar and a nitrogenous base. Start with those three, then number the sugar's carbons and tell the two kinds of base apart.
Read the full guide
Counting the carbons
Start at the ring oxygen and go clockwise: the 1′ carbon holds the base, then come 2′ and 3′, and the 4′ carbon holds the 5′ carbon outside the ring. The phosphate hangs from 5′, and the 3′ hydroxyl is where the next nucleotide joins.
Purine or pyrimidine
Count the rings. Two rings, a six-sided one joined to a five-sided one, make a purine: adenine or guanine. One six-sided ring is a pyrimidine: cytosine, thymine or uracil.
Pure As Gold: the purines are adenine and guanine. Every other base, cytosine, thymine and uracil, is a pyrimidine.
Questions and answers
What are the three parts of a nucleotide?
A phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA) and a nitrogenous base. The phosphate is joined to the sugar's 5′ carbon and the base to its 1′ carbon.
What is the difference between a purine and a pyrimidine?
A purine has two rings, a six-sided ring joined to a five-sided one: adenine and guanine. A pyrimidine has one six-sided ring: cytosine, thymine and uracil.
How is a DNA nucleotide different from an RNA nucleotide?
In the sugar and one base. DNA's deoxyribose has only a hydrogen on the 2′ carbon where RNA's ribose has a hydroxyl group, and RNA uses uracil in place of thymine.