Structure of DNA and RNAEdexcel International A Level Biology: Revision notes

Nucleic acids are polymers made of mononucleotides. Every mononucleotide has three parts joined together:

• a pentose sugar (five carbon atoms)
• a phosphate group
• a nitrogenous base

In DNA the sugar is deoxyribose. In RNA it is ribose, which has one more oxygen atom than deoxyribose.

The bases are adenine (A), cytosine (C), guanine (G) and thymine (T) in DNA. In RNA, uracil (U) replaces thymine, so RNA contains A, C, G and U.

Mononucleotides join in a condensation reaction, which releases a molecule of water. The bond forms between the phosphate group of one nucleotide and the sugar of the next, and it is a phosphodiester bond.

Repeating this builds a polynucleotide: a long chain with a sugar–phosphate backbone and a base projecting from each sugar. The backbone is held by strong covalent bonds.

A strand of $n$ nucleotides has $n - 1$ phosphodiester bonds. Adding water (hydrolysis) breaks these bonds and releases the nucleotides.

In double-stranded DNA the bases of the two strands pair in a fixed way, called complementary base pairing:

• adenine pairs with thymine (two hydrogen bonds)
• cytosine pairs with guanine (three hydrogen bonds)

In RNA, adenine pairs with uracil.

Because pairing is fixed, the amount of A equals the amount of T and the amount of C equals the amount of G in double-stranded DNA.

Worked example: a double-stranded DNA sample is 28% adenine. Thymine is also 28%, so A + T = 56%. The remaining 44% is C + G, so cytosine is 22% and guanine is 22%.

A DNA molecule has two polynucleotide strands held together by hydrogen bonds between complementary bases. The two strands twist round each other into a double helix.

The structure is stable because the sugar–phosphate backbones are held by strong covalent bonds and the very large number of hydrogen bonds hold the strands together. The hydrogen bonds are individually weak, so the strands can still be separated when needed.

A C–G pair has three hydrogen bonds and an A–T pair has two, so a region rich in C–G pairs needs more energy to separate.

RNA is usually a single polynucleotide strand and is much shorter than DNA, for example a messenger RNA is a copy of just one gene. It contains ribose and uracil.

• DNA: deoxyribose, bases A, T, C, G, double-stranded (double helix), very long
• RNA: ribose, bases A, U, C, G, single-stranded, shorter

Both are polynucleotides with a sugar–phosphate backbone joined by phosphodiester bonds.