biology

• Individual nucleotides of DNA are made up of 3 components

- Deoxyribose sugar

- Phosphate group

- Organic base

• Single ring base - so it can be cytosine and thymine
• or it can be a double ring base - Adenine and guanine

- the 3 components are joined together by a condensation reaction ---> 1 nucleotide

- two mononucleutides are combined to form a dinucleoutide

• Linking of mononucleutides forms a long chain called a polynucleutide 

DNA STRUCTURE

• DNA is made up of 2 strands (joined by hydrogen bonds)
• DNA can be thought of as a ladder in which the phosphate group and the deoxyribose alternate to form the uprights and organic base pairs to form the rungs

PAIRING OF BASES

• Organic bases contain nitrogen and are of 2 types
• double ring structures have longer molecules than single ring bases
• Therefore if the DNA is to be of the same length then there has to be one double ring base and one single base ring
• As a result A  + T bind together ... 2 hydrogen bonds
• and G + C bind together ... 3 hydrogen bonds
• THEREFORE, THESE BASE PAIRINGS ARE COMPLIMENTORY
• The ratio of Adenine- Thymine : Guanine-Cytosine vary in species

double helix

- ladder like arrangement

- 2 polynucleutide…