Coming from GENETICS
==DNA STRUCTURE== DNA or '''Deoxyribose Nucleic Acid''' is made up of three simple chemical groups repeated millions of times. These three chemical groups; '''phosphate''', '''sugar''' and ''base'' (or nitrogenous base) together form the simple, repeated unit of DNA; a '''nucleotide'''. ===Nucleotide:=== [image:http://i.imgur.com/DEqVS1Xl.jpg?1] The phosphate & sugar groups are strung together end to end to form the '''back-bone''' of the DNA molecule where the sugar molecule of one nucleotide attaches to the phosphate of the next nucleotide. However, DNA is double stranded –it is made up of two of these nucleotide strings. DNA is often described as a twisted ladder where the sides (back-bones) of the ladder are made up of repeating sugar – phosphate groups. The runs / steps are made up of the bases. DNA contains 4 different types of bases A (Adenine), T (Thymine), C (Cytosine), G (Guanine). These bases can pair with each other by forming hydrogen bonds. It is these bonds that hold the two nucleotide strands together. Base pairing always follows these simple rules: *'''A''' (Adenine) always pairs with '''T''' (Thymine) *'''C''' (Cytosine) always pairs with '''G''' (Guanine) The two nucleotide strands are actually twisted around each other giving DNA its β€˜''double helix''’ structure. [image:http://i.imgur.com/wZDaHkFl.jpg?1] If you look very carefully at the DNA structure you will notice that the two strands are '''anti-parallel''' -they run in opposite directions (look carefully at the sugar molecules on each strand). =Extra for experts= ''The following is not normally assessed at a high school level'' ==PYRIMIDINES & PURINES== [image:http://i.imgur.com/eR6MfsD.png?1] '''Pyrimidines''' are are a special group of chemicals with a single ring structure. Cytosine (C) & Thymine (T) are both pyrimidines. '''Purines''' are are a special group of chemicals with a double ring structure. Guanine (G) & Adenine (A) are both Purines. ==Bonding== [image:http://i.imgur.com/L9NiFWn.png?3] The structure of pyrimidines and purines helps to ensure the correct pairing of bases within the DNA molecule. Each base pair should consist of one pyrimidine and one purine, held together by hydrogen bonds. If paired correctly, each base pair will consist of three 'rings'. If two purines paired there would be four 'rings' and they wouldn't fit or would distort the structure of the DNA molecule. Two pyrimidines would not easily bond as their single ring structures would not normally span the distance between the two strands. Guanine (G) and Cytosine (C) form three hydrogen bonds while Adenine (A) and Thymine (T) form only two. The number and orientation of these bonds also helps to ensure that bases are paired correctly. Sequences rich in AT base pairs are often found before genes. In these sections the two strands are more easily pulled apart (fewer hydrogen bonds). This helps the enzymes involved in gene expression access the following gene.
Credit: Ben Himme, Amy Parry