Biological information exists as three fundamental forms: DNA, RNA, and protein. Each is accompanied by a wide degree of structural and functional complexity. Resulting in part from the major strides made by researchers in the development of recombinant DNA technologies and from the unprecedented successes of the Human Genome Project, we are now capable of providing direct and indirect molecular diagnostic assays for many human diseases at the level of the nucleic acid. The human genome is thought to comprise 30,000-40,000 genes neatly packaged into a total of 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (X or Y) (Fig. 1). These 46 chromosomes represent the diploid number, found in most cells in the body. A haploid genome, found in sperm and egg cells (gametes), contains half of the total genetic material, or 23 chromosomes.
The haploid genome contains approx 3 x 109 basepairs (bp); on average, then, each chromosome contains 1,000-1,500 genes dispersed within 1.3 x 108 bp.
The majority of human DNA exists within the nucleus of the cell. Through interactions with specific histone and other nuclear proteins, DNA is condensed into chromosomal structures. The packaging efficiency provided by these DNA:protein interactions is analogous to the packaging efficiency necessary to place approximately several hundred miles of cable wire (DNA) into the center of a basketball (nucleus). Some of these protein: DNA interactions function in structural roles, whereas others regulate expression of genes. Of the 3 x 109 bp of genomic DNA, protein-coding sequences (exons) comprise only 3-5% of the DNA. The remaining 95-97% is noncoding and includes intervening sequence (introns) and repetitive sequences; in fact, up to 50% of this noncoding DNA is some sort of repetitive sequence (Table 1) (10). Although this DNA has been called "junk" sequence, evidence is building that a significant amount of this DNA has important structural and regulatory roles (9). Another source of human DNA resides within the cell's mitochondria; the mitochondrial DNA consists of approx 16,500 bp that are unique to this organelle (8).
DNA is considered the "blueprint" of living organisms, as it contains the information needed to create the vast array of proteins required for cellular function. The DNA sequence in an individual is the genotype; characteristics (physical, biochemical, and physiological) arising as a result of the DNA sequence interacting with the environment are referred to as the individual's phenotype. An alteration in the DNA sequence (genotype) can ultimately result in a change in the amino acid sequence of the protein encoded by that sequence; this change can render the protein nonfunctional (resulting in a specific phenotype). As our understanding of the correlation of sequence variations with altered protein functions continues to grow, nucleic acid testing will continue to play an increasing role in diagnostic laboratory medicine.
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