DNTP

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dNTP stands for deoxyribonucleotide triphosphate. Each dNTP is made up of a phosphate group, a deoxyribose sugar and a nitrogenous base. There are four different dNTPs and can be split into two groups: the purines and the pyrimidines.

Contents

Purines

dATP, (deoxyadenosine 5'-triphosphate), and dGTP, (deoxyguanine 5'-triphosphate), make up the purines.

Adenine and guanine, the bases which feature in the purines, both have a double ring structure.

Pyrimidines

dTTP, (deoxythymidine 5'-triphosphate), and dCTP, (deoxycytidine 5'-triphosphate), make up the pyrimidines.

Thymine and cytosine, the bases which feature in the purines, both have a single ring structure.

DNA Structure

The double helix structure of DNA is made up of the dNTPs, much like monomer units in a polymer. If you unwound DNA and imagined it as a ladder, the phosphate groups alternating with the deoxyribose sugar groups would make up the sides of the ladder, (hense the phrase "sugar phosphate backbone"), and the bases would form the rungs of the ladder. An important feature of DNA is that two bases attach by hydrogen bonds to form one rung. This bonding holds the two strands of DNA together. Pyrimidines cannot pair together, nor can purines. Base pairing can only take place between T-A or G-C. This was discovered by Erwin Chargaff. In his experiments, he extracted DNA from nuclei and recorded the amounts of the four nucleotides present. he found that the percentage of T was always equal to the percentage of A. The same relationship was found between G and C. He also found that the percentage of A or T plus the percentage of G or C added to 100%. These findings greatly aided Watson, Crick, Franklin and Wilkins.

Hydrogen Bonding

There are three hydrogen bonds between G and C bases, but only two between A and T bases.

Only two form between A and T because carbon 2 of the adenine base has little more electronegativity than its attached hydrogen so does not induce a delta negative charge on it which means that the oxygen on the carbon 2 of thymine cannot form a hydrogen bond with the hydrogen.

This, therefore, means that it takes more thermal energy to denature DNA which is more G-C rich than DNA which is A-T rich.

Forming the DNA Backbone

As mentioned earlier, the backbone of DNA is made up of a strand of alternating phosphate and deoxyribose sugar groups. The bond that forms between adjacent nucleotides on one strand of the dna is the phosphodiester linkage. This is formed when either the 5' or 3' carbon atom of one deoxyribose sugar bonds to an oxygen on the first phosphate. This is a condensation reaction which removes diphosphate from each nucleotide. This is why the nucleotides found in DNA are actually dNMPs, (deoxyribonucleotide monophosphates)[1].

References

  1. Carroll S, Doebley J, Griffiths A, Wessler S. (2000) Introduction to Genetic Analysis, tenth edition, New York: W.H.Freeman and Company. Pages 284 to 287

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