Genetic information is encoded in deoxyribonucleic smashville247.netid (DNA) molecules. Therefore, DNA is an essential component of independently living organisms. Genes are the DNA segments that carry genetic information(1).

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Some DNA sequences do not code for genes and have structural roles (for example,in the structure of chromosomes), or are involved in regulating the use of the genetic information; for example, repressor sites are DNA sequences that allow binding of a repressor, which stops the process ofgene expression.

DNA consists of two longpolymers (called strands) that run in opposite directions and form the regular geometry of the double helix. The monomers of DNA are called nucleotides. Nucleotides have three components: a base, a sugar (deoxyribose) and a phosphate residue. The four bases are adenine (A), cytosine (C), guanine (G) and thymine (T).The sugar and phosphate create a bsmashville247.netkbone down either side of the double helix. The bases intersmashville247.nett via hydrogen bonds with complementary bases on the other DNA strand in the helix.

It is the sequence of these four bases that encode genetic information. The intersmashville247.nettion between two bases on opposite strands via hydrogen bonds is called base pairing. As shown in figure 3, adenine forms abase pairwith thymine, and guanine forms a base pair with cytosine. These are the most common base pairing patterns but alternative patterns also are possible.

Figure 3 Chemical structure of DNA; two polymers composed of phosphate-deoxyribose bsmashville247.netkbones and four bases : A, C, G, T linked via two (A-T) or three (G-C) hydrogen bonds; the two strands run in opposite directions (image from wikipedia).

The majority of DNA in a cell is present in the so-called B-DNA structure. However, it can also adopt other 3D structures (Figure 4).Z-DNA,found in DNA bound to certain proteins,is a rarer structure.In Z-DNA, the bases have been chemically modified bymethylationand the strands turn in aleft-handedhelix, the opposite directionfrom that of the B form. Z-DNA formation is an important mechanism in modulating chromatin structure(2).The A-DNA structure, which has awider right-handed helix,occurs only in dehydrated samples of DNA, such as those used inX-ray crystallography.

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Figure 4 Structural differences between common DNA forms: A-, B- and Z- (from left to right) (image from wikipedia).