DNA, RNA, ATP and the genetic code

Last time

Before we start: last time, in DNA replication and gene mutations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Four letters, one instruction book

A whole human being is described using just four letters. A, T, C and G. Strung together in the right order, they make the instruction book for every protein. To read that book you need to know what the letters are, how they pair, and how groups of three become amino acids.

The nucleotide

A nucleotide has three parts: a pentose sugar, a phosphate group, and a nitrogenous base. In D N A the sugar is deoxyribose. In R N A it is ribose, which has one more oxygen atom. There are two kinds of base. Adenine and guanine are purines, with a double ring. Cytosine, thymine and uracil are pyrimidines, with a single ring. Nucleotides join by condensation. A phosphodiester bond forms between the phosphate of one nucleotide and the sugar of the next.

DNA

A D N A molecule has two polynucleotide strands wound round each other as a double helix. The strands run in opposite directions, so they are antiparallel. Bases pair across the middle. Adenine pairs with thymine by two hydrogen bonds, and cytosine pairs with guanine by three. A purine always pairs with a pyrimidine, so the width stays constant. This is called complementary base pairing. D N A is stable and long, with hydrogen bonds easy to break during replication but strong enough in number to hold the helix.