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DNA stands for deoxyribonucleic acid.
It is a long molecule that is made up of lots of smaller units. These units are called nucleotides.
DNA is a double stranded molecule. The two strands are joined together by hydrogen bonds and twisted together. The resulting shape is know as a double helix.
dna-testing-lg.jpg

Each nucleotide is made of three main components. A sugar a phosphate and a base.
In the picture above, you can see that DNA looks a bit like a twisted ladder. The sugars and phosphates make up the sides of the ladder and the bases make up the rungs.
Below is a picture of one nucleotide.

nucleotide.jpg

There are 4 different types of bases. Their names are adenine (A), thymine (T), cytosine (C) and guanine (G)
Nucleotides join together in a chain to build up each strand of a DNA molecule.
The order of the bases that are in this chain is very important. It is the order of the bases that determines the structure of the proteins that our DNA codes for.

When a DNA molecule is being made, the two strands are held together by hydrogen bonds. These bonds are formed between the bases.
But bonds can only form between particular bases.

Adenine will bond with Thymine

Cytosine will bond with Guanine

These are the only bonds that can form. A typical DNA strand may look something like the one below.

DNA_helix.jpg
DNA coils tightly and forms larger structures called chromosomes.