The structure of mononucleotides, DNA and RNA

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The structure of mononucleotides, DNA and RNASpec 2.9

In short

A mononucleotide is a pentose sugar (deoxyribose or ribose) joined to a phosphate group and a base. Condensation reactions link mononucleotides by phosphodiester bonds into polynucleotides. DNA is two antiparallel polynucleotide strands held in a double helix by hydrogen bonds between complementary bases: A with T, and C with G.

A mononucleotide has three parts: a pentose sugar, a phosphate group and a nitrogen-containing base. In DNA the sugar is deoxyribose; in RNA it is ribose. DNA has four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). In RNA, thymine is replaced by uracil (U).

Mononucleotides join in condensation reactions. A phosphodiester bond forms between the phosphate group of one nucleotide and the sugar of the next, releasing water. Many nucleotides in a chain make a polynucleotide, with a sugar–phosphate backbone and bases sticking out to the side.

The DNA double helix

A DNA molecule is made of two polynucleotide strands wound into a double helix.
The strands are antiparallel: they run in opposite directions (one runs 5′ to 3′, the other 3′ to 5′).
Complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). The base pairs lie across the middle of the helix.
Many hydrogen bonds hold the two strands together. Each bond is weak, so the strands can be separated for replication and transcription, but together they make DNA stable.
The sugar–phosphate backbones protect the bases on the inside.

Comparing DNA and RNA
Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Two polynucleotide strands, double helix	One polynucleotide strand
Size	Very long	Relatively short
Types	—	Messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA)

Common mistake:

Phosphodiester bonds join nucleotides along one strand; hydrogen bonds join bases between two strands. Do not mix them up, and do not say hydrogen bonds link A with G.

Two antiparallel polynucleotide strands with sugar-phosphate backbones, phosphodiester bonds, base pairs A-T with two hydrogen bonds and C-G with three, 5-prime and 3-prime ends, and the same molecule as a double helix. — The double helix: antiparallel strands joined by **hydrogen bonds** between complementary bases.

A DNA mononucleotide with phosphate, deoxyribose and a nitrogenous base, and an RNA mononucleotide with phosphate, ribose and the base uracil. — A DNA nucleotide contains deoxyribose; an RNA nucleotide contains ribose and the base **uracil**.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between DNA and RNA?

DNA is a double helix made of two polynucleotide strands, with deoxyribose and the bases A, T, C and G. RNA is a single polynucleotide strand with ribose, and uracil instead of thymine. DNA stores genetic information in the nucleus; mRNA, tRNA and rRNA are involved in protein synthesis.

What does semi-conservative replication mean?

In semi-conservative replication each new DNA molecule contains one original strand and one new strand. The helix unwinds, each strand acts as a template, and DNA polymerase joins complementary nucleotides to it. The Meselson and Stahl experiment, using heavy and light nitrogen, gave evidence for it.

What is the genetic code and why is it degenerate?

The genetic code is the set of rules by which base triplets code for amino acids. It is a non-overlapping triplet code. It is degenerate because there are 64 possible triplets but only 20 amino acids, so most amino acids are coded for by more than one triplet.

All 5 questions on DNA, the genetic code and gene expression