VPB 112: General Veterinary Biochemistry (1+1)

CHEMISTRY OF DNA

• DNA is also built from 4 nucleotides. But, it is different from RNA in 2 important ways.
• The pyrimidine uracil in RNA is replaced in DNA by the pyrimidine - thymine.Uracil and thymine are very similar in structure. In thymine, the pyrimidine ring has a methyl group attached to the 5th carbon atom in the ring.
• The sugar ribose is present in RNA nucleotides and deoxyribose is present in DNA nucleotides.
• Deoxyribose is a ribose molecule that has a hydrogen atom on the 2nd carbon atom instead of a hydroxyl group.
• The attachment of deoxyribose to the two purines (adenine and guanine) and to the two pyrimidines (cytosine and thymine) yields 4 nucleoside.
• Because the sugar is deoxyribose instead of ribose, the nucleosides are called deoxy nucleosides.
• The 4 deoxynucleosides of DNA are deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine (because, thymidine occurs only in DNA, it always contains the deoxy ribose and the deoxy – prefix is omitted).
• A deoxynucleotide is a deoxynucleoside with PO4 group (deoxynucleotide = deoxynucleoside phosphate).
• The four deoxynucleotides are deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxycytidine monophosphate (dCMP) and thymidine monophosphate (TMP).
• The four deoxynucleotides are also called as deoxyadenylic acid, deoxyguanylic acid, deoxycytidylic acid and thymidylic acid.