Structure and function of DNA

DNA molecules are incredibly long, but also very thin. One DNA molecule from the chromosome of a mammal may be about 1 m long when unraveled. However, it has to fit in a nucleus of some 5-6 orders of magnitude smaller and is folded up in chromosomes in a highly organized manner. DNA is a linear polymer that is composed of four different building blocks, the nucleotides. It is in the sequence of the nucleotides in the polymers where the genetic information carried by chromosomes is located. Each nucleotide is composed of three parts: (1) a nitrogenous base known as purine (adenine (A) and guanine (G)) or pyrimidine (cytosine (C) and thymine (T)); (2) a sugar, deoxyribose; and (3) a phosphate group (see pp. 20-22 of Molecular Biotechnology for molecular structures of DNA and its components). The nitrogenous base determines the identity of the nucleotide, and individual nucleotides are often referred to by their base (A, C, G, or T). One DNA strand can be up to several hundred million nucleotides in length. T can form a hydrogen bond with A, and C with G; two DNA strands wind together in an antiparallel fashion in a double-helix.

Inside the cell, the DNA acts like an "instruction manual": in its sequence, it provides all the information needed to function, but the actual work of translating the information into a medium that can be used directly by the cell is done by RNA, ribonucleic acid. The structural difference with DNA is that RNA contains a -OH group both at the 2' and 3' position of the ribose ring, whereas DNA (which stands, in fact, for deoxy-RNA) lacks such a hydroxy group at the 2' position of the ribose. The same bases can be attached to the ribose group in RNA as occur in DNA, with the exception that in RNA thymine does not occur, and is replaced by uracil, which has an H-group instead of a methyl group at the C-5 position of the pyrimidine. The molecular structures of uracil and thymine are compared at The RNA has three functions: (a) it serves as the messenger that tells the cell (the ribosomes) what protein to make (messenger RNA; mRNA); (b) it serves as part of the structure of the ribosome, the protein/RNA complex that synthesizes proteins according to the information presented by the mRNA (ribosomal RNA; rRNA); and (c) it functions to bring amino acids (the constituents of the proteins) to the ribosome when a specific amino acid "is called for" by the information on the mRNA to be put in into the protein that is being synthesized; this RNA is called transfer RNA (tRNA).

An important point of emphasis should be that all vegetative cells of one organism contain the same genetic information. Upon division, each daughter cell obtains an "exact" copy of the DNA of the parent However, the specific genes that are expressed at specific times may be very different between different tissues. These differences in gene expression allow for the regulation of development of the organism, and for the development of different tissues. For the most part, DNA-binding proteins (encoded by the DNA) play an important role in the regulation of expression of genes encoded on the DNA.