What it's made of
DNA is a polynucleotide - it's made of lots of joined nucleotides.
Each nucleotide is made from a pentose sugar, a phosphate group and a nitrogenous base.
The sugar is a deoxyribose sugar.
Each nucleotide has the same sugar and phosphate. The bases can vary though.
There are four possible bases - adenine A, thymine T, cytosine C and guanine G.
Forming a double helix
The nucleotides join up between the phosphate group of one nucleotide and the sugar of another, creating a sugar-phosphate backbone.
Two DNA polynucleotide strands join together by hydrogen bonds between the bases.
Each base can only join with one particular partner - this is specific base pairing.
A always pairs with T. G always pairs with C.
The two strands wind up to form the DNA double-helix.
Why it's good at its job.
DNA contains all the instructions needed to grow and develop from a fertilised egg to a fully grown adult.
The DNA molecules are very long and are coiled up very tightly, so a lot of genetic information can fit into a small space.
DNA molecules have a paired structure, which makes it easier to copy itself. This is called self-replication.
It's important for cell division and for passing genetic information through generations.
The double-helix structure means DNA is very stable in the cell.
Eukaryotic cells contain linear DNA molecules that exist as chromosomes, each made up of one long molecule of DNA.
The DNA has to be wound up so it can fit into to nucleus.
The DNA molecule is wound around proteins (called histones)
Histon proteins also help to support the DNA.
The DNAis then coiled up very tightly to make a compact chromosome.
Prokaryotes also carry DNA as chromosomes - but the DNA molecules are shorter and circular.
The DNA isn't wound around proteins - it condenses to fit in the cell by supercoiling.
Good luck on your exams!