DNA Characteristics

The characteristics of every living creature are dictated by the genetic material, DNA. Nuclear DNA is organized into units called chromosomes, of which there are 46 in the human. The chromosomes are found in pairs and contain the individual units called genes. DNA is a double-stranded helix composed of four bases, two pyrimidines (cytosine and thymine) and two purines (adenine and guanine), that are joined together by ribose and phosphate groups (Figure 1). DNA is formed when the bases are joined through phosphodiester bonds using ribose as the common linkage. The phosphodiester linkage is between the 5' phosphate group of one nucleotide and the 3' OH group of the adjacent nucleotide. This provides a direction (5' to 3') to the chain. The bases are hydrophobic and contain charged polar groups. These features are responsible for the helical shape of the nuclear DNA chain. A double helix forms when the bases of each chain interact through hydrogen bonding.

The DNA base sequence is unique for every protein and peptide that synthesized in the body. The sequence of these bases determines the genotype of the individual for each gene product. Although only four different bases are used for the DNA, it is the sequence of these bases that determines the product being produced. Each gene product is uniquely derived from a specific gene. Although all cells contain the same DNA, not all genes are expressed in every cell; some are particular to specific cell types. Thus, the function of DNA is to determine not only the particular characteristics of the individual but also the properties of each cell through the provision of a multitude of genes, each coding for a particular protein found in that cell. Therefore, it functions to transmit genetic information from one generation to the next in a given species and ensures the identity of specific cell types.

The Human Genome Project has detailed the specific base sequence of nuclear and mitochondrial DNA in the human cell. The identification of each gene and its corresponding controls of expression have not been completely elucidated. Although the nuclear genome has been sequenced, it has not been completely mapped; that is, the location, within the DNA, of each gene (and its promoter region) and the identification of the protein or peptide it encodes

tCiH

H TH

HO HN

kj2"0

X TJ

Figure 1 Representation of a segment of DNA showing the phosphodiester bond that uses ribose as the common link between the bases. A, adenine; C, cytosine; G, guanine; T, thymine; R, ribose.

have not been fully determined. In addition, we do not know all the details of the regulation of gene expression. Some genes have been intensely studied, whereas others have yet to be identified. In contrast, the genome in the mitochondria has been completely sequenced and mapped. It is a very small genome encoding only 13 gene products (components of the mitochondrial respiratory chain) under the control of a single promoter sequence, the D-loop. Despite its small size and apparent simplicity, however, we know even less about the regulation of its expression than we know about some of the nuclear-encoded genes.

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