What Is the Structure of DNA and RNA?
Learn how DNA and RNA are built from nucleotides, including their sugars, bases, bonds and chains, with the key differences explained for AQA A Level Biology.
DNA and RNA are nucleic acids made from chains of nucleotides. Each nucleotide contains a pentose sugar, a phosphate group and a nitrogen-containing base. The key structural difference is that DNA has two nucleotide chains twisted into a double helix, while RNA is usually a shorter, single chain.
DNA structure
A DNA nucleotide contains deoxyribose sugar and one of four bases: adenine, thymine, cytosine or guanine. Nucleotides join by condensation reactions, which release water and form phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next. This produces a sugar-phosphate backbone along each chain.
The two DNA chains are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine, and cytosine pairs with guanine. The order of bases carries genetic information. For example, wherever one chain has an adenine base, the base opposite it on the other chain must be thymine. This pairing allows each existing chain to act as a template when DNA is copied. Remember that hydrogen bonds connect the two chains, whereas phosphodiester bonds link nucleotides within each chain.
RNA structure
An RNA nucleotide contains ribose sugar and one of four bases: adenine, uracil, cytosine or guanine. RNA has uracil instead of thymine. Its nucleotides are also linked by phosphodiester bonds, but RNA is typically a short, single polynucleotide chain rather than a double helix. A polynucleotide is simply a chain of many nucleotides. RNA helps transfer genetic information to ribosomes, which contain RNA as well as protein.
To compare the structures, check sugar, base and number of chains: DNA has deoxyribose, thymine and two chains; RNA has ribose, uracil and usually one chain. In an exam, do not say that DNA and RNA are made of amino acids. Their building blocks are nucleotides; amino acids are the building blocks of proteins.
