Knowledge of the genetic basis of health and disease has increased dramatically in the last 20 years. The first draft of the sequence of the human genome was published in 2001, and the Human Genome Project was completed in 2003.1,2 From this project has come a detailed map of genes and genetic markers and a clearer understanding of how genes function. The intent of this chapter is to provide a basic understanding of gene structure and function and a glossary of terms that should be helpful in incorporating genetic screening and testing into a practice of obstetrics and gynecology. For those interested in a more comprehensive resource on basic molecular genetics, we suggest either Gelehrter et al or Nussbaum et al.3,4
ORGANIZATION OF THE GENOME
Within the human nucleus are 3 billion base pairs of DNA, contained in which are approximately 25,000 to 30,000 genes. The DNA is tightly wrapped around proteins called histones to form what are termed nucleosomes. Nucleosomes are organized into solenoid structures and looped around a nonhistone protein scaffold to form chromatin, which makes up chromosomes. Each chromosome is composed of densely packed nontranscribed DNA located near the centromere (heterochromatin), and less densely packed and transcribed DNA (euchromatin). Chapter 2 provides a more detailed discussion of the structure of chromosomes.
About three-quarters of the genome is unique, single-copy DNA, and the remaining one-quarter is made up of various forms of repetitive DNA. Less than 10% of the genome encodes genes. Initially, it was thought that the repetitive DNA, and much of the single-copy DNA, had no function. However, recent studies suggest that this “noncoding” DNA may be where the various “switches” are located that control gene function. Although more than 99% of DNA is identical in all humans, the small variations, known as polymorphisms, have been key to understanding the genetic basis of many diseases thought to have a genetic component, such as heart disease, diabetes, and other adult-onset disorders.
In addition to the nuclear genome, each cell contains a mitochondrial genome, approximately 16,500 nucleotides in length. The mitochondrial genome contains 37 genes, and these genes encode 13 essential mitochondrial proteins, 22 transfer RNAs (tRNAs), and 2 ribosomal RNAs (rRNAs). The mitochondria genome does not encode all of the proteins that make up mitochondria; the remaining proteins are encoded by the nuclear genome. Each mitochondrion usually contains multiple copies of mitochondrial DNA, and each healthy cell consists of several hundred mitochondria. If all the mitochondria in a given cell contain the same DNA sequence, this is called homoplasmy, while populations of mitochondria with differing DNA sequences give rise to heteroplasmy.
Each gene is a unique series of four purine (adenine, guanine) and pyrimidine (thymine, cytosine) bases. The nucleotides that make up these genes are composed of a ...