Deoxyribonucleic Acid (DNA) consists of two polymers with backbones consisting of sugars and phosphate groups. These two polymers are connected to each other through hydrogen bonds between nucleobases attached to the sugar group on the backbone. Please see the diagram below:

There are four types of nucleobases found in DNA, they are Cytosine, Guanine, Adenine, and Thymine. The four bases are complimentary, meaning that when DNA is formed and the nucleobases hydrogen bond, Cytosine can only pair with Guanine and Adenine can only pair with Thymine. The diagram above is a bit vague, so lets view the molecular components of DNA one by one.
First the sugar group from the DNA backbone is Deoxyribose. Deoxyribose (actually 2-Deoxyribose) is basically a Ribose minus an oxygen (thus Deoxy). Technically it replaces the hydroxyl group (OH) with a hydrogen at the second carbon (thus 2-Deoxy). Please see image below:

The Phosphate group (see below) connects to the sugar group to form a polymer.
  
 
Finally there are the 4 nucleobases which connect to the sugar group and "stick out" so that they can hydrogen bond:

Put them together and you form single stranded DNA:

Now if you take two single stranded DNA and line them up with complimentary base pairs hydrogen bonding, you get double stranded DNA

You can see above that the reason that DNA nucleobases are complimentary has to do with the hydrogen bonding, like two puzzle pieces that fit together.

DNA is so large that pieces of it can be chemically interesting. One often hears about DNA sequences. These are the order in which the nucleobases are found in the DNA. Next time we will discuss DNA sequencing and the chemical origins of inherited traits.
http://www.albany.edu/~rp858838/
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