DNA is a polymer. The monomer units of DNA are nucleotides, and the polymer is known as a “polynucleotide.” Each DNA nucleotide consists of 3 components 1) 5-carbon sugar (deoxyribose), 2) a nitrogen containing base attached to the sugar, 3) and a phosphate group.

There are four different types of nucleotides found in DNA, differing only in the nitrogenous base (Nucleobases). The nucleobases are adenine (A), guanine (G), cytosine (C), and thymine (T). The sugars and phosphates of the nucleotides bond strongly together to form a “backbone” of the double helix to which these four bases connect, forming the “rungs”.

The skeleton of adenine and guanine is purine, hence the name purine-bases. A purine has 9 atoms that make up the fused rings (5 carbon, 4 nitrogen). The skeleton of cytosine and thymine is pyrimidine, hence pyrimidine-bases. A pyrimidine has 6 atoms (4 carbon, 2 nitrogen). All ring atoms of both purines and pyrimidines lie in the same plane.

BASE PAIRING:

Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite strand, and G forms 3 hydrogen bonds with C on the opposite strand. dA-dT and dG-dC base pairs are the same length, and occupy the same space within a DNA double helix with uniform diameter. dA-dT and dG-dC base pairs can occur in any order within DNA molecules.

This simplicity is useful when the DNA replicates. The enzyme helicase triggers the unwinding and opening up of the double helix structure. Another enzyme, DNA polymerase, matches up each newly unbonded base with its complementary base. When replication is complete, there are two identical copies of the original DNA molecule. As hydrogen bonds are not covalent, they can be broken and rejoined easily. The two strands of DNA in a double helix can therefore be pulled apart like a zipper. This process called melting forms two single-stranded DNA molecules (ssDNA) molecules. Melting occurs at high temperature, low salt and high pH (low pH also melts DNA, but DNA becomes unstable due to acid depurination, so low pH is rarely used.

The stability of the double stranded DNA form depends not only on the GC-content (% GC basepairs) but also on sequence and also length (longer molecules are more stable). Long DNA helices with a high GC-content have stronger-interacting strands, while short helices with high AT content have weaker-interacting strands. In the laboratory, the strength of this interaction can be measured by finding the temperature necessary to break the hydrogen bonds or their melting temperature (also called Tm value).

ANALOGUES: The most common application of nucleobase analogues are fluorescent probes. In medicine, they are used as anticancer and antiviral agents.

The information in DNA is stored as a genetic code made up of these four nucleobases. Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same across populations. The order, or sequences, of these bases are the building blocks of an organism.

January 17th, 2014

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