DNA  stands for Deoxyribonucleic Acid. 


ALL animal and plant cells contain DNA in the nucleus, it is a very long molecule, a POLYMER and contains the coded information required to make organisms. Try to imagine having an alphabet with ONLY FOUR letters (with the occasional use of a 5th), but being able to make ALL of the words you will EVER need with that alphabet….. these are the four words:



Each of the molecules above is known as a "base". Bases combine with Phosphate groups (from Phosphoric Acid) and a molecule of Deoxyribose (a sugar) to form a large unit known as a Nucleotide.


Here is an example of a Nucleotide:



This Nucleotide is ADENOSINE MONOPHOSPHATE (AMP) and is built from a Phosphate Group, a Sugar (Deoxyribose) and a Base (in this case Adenine).


So, how do these units build up?


A polymerisation takes place, building the nucleotide units and then the chain which becomes DNA. The type of polymerisation is a "condensation" as a by-product of the reaction is a molecule of water. This happens in a couple of places.


Step 1 - an Ester link is created by the ejection of water in the first condensation:





Step 2 - a nitrogenous base links with the hydroxyl group on Deoxyribose to eject another water molecule, forming the Nucleotide.




Polymerisation of Nucleotides to form a chain releases further water molecules as the remaining hydroxyl group of the Deoxyribose ring links with the OH group on the Phosphoric Acid linking the two nucleotides together with a Phosphodiester Bond:





The polynucleotide chain continues to form. Another polynucleotide chain forming in the opposite direction will see two "sugar-phosphate" backbones forming like the up/down rails of a ladder with the nucleotide bases pointing inwards.


At this point, imagine two combs lying side by side, but with their teeth pointing towards each other?


We are on the way to the final shape of DNA. If the combs above were joined "tooth to tooth" and then twisted in opposite directions from each end, we would have a double helix shape.....voila....DNA!



OK so these "teeth" link with Hydrogen bonds, but is there any special way?. Well ...... there are DOUBLE and TRIPLE hydrogen bonds between bases, and it is these bonds which hold the molecule together in its double helix form. The bases 'link' according to this pattern:



There are a number of ways that you can memorise which pair of bases will DOUBLE bond and which will TRIPLE bond, my preference is to remember:


 "82" 


which is also "A-T-2" telling you that the double bond occurs between Adenine and Thymine. The triple bond must then occur between the other base pair.