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CHAPTER
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G G G Genetics
Bradley E. Aouizerat
Nearlyy 6 decades ago, Watson and c lollleagues discovered the secret
(G)), deoxyadenine (A), deeoxxythymine (T), and deoxycytosine
Ne ar d go Wa ts on a nd c g c re t (G d xy in A ), d o (T ), nd y to si ne
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quantities necessary to program a living cell. This discovery set transmitting information withinn cells. DNA, located in the nu-
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much of it guided by this original discovery. of the information necessary for a living organism to grow and
Research is a slow process, often with years between each sen- function. The DNA molecule resembles a twisted ladder, usually
sation, and even today, the DNA revolution remains largely be- described as a double helix. The rungs are repeating units called
hind laboratory doors, in the form of scientists’ ever-increasing nucleotides, which are the quantum building blocks of DNA. Nu-
understanding of the mechanisms of life. But a few powerful in- cleotides are composed of one sugar-phosphate molecule (the linear
ventions—forensic DNA examination, DNA-based drug discov- strands or outer rails of the DNA ladder) and one base (Fig. 4-2).
ery, and specific disease susceptibility mutation screening—have DNA in eukaryotic cells consists of two nucleotide strands joined
enjoyed a significant active contribution to society (see Display by weak chemical bonds between the two bases, forming base
4-1 for definitions). pairs. Therefore, a base pair constitutes a “rung” on the ladder of
DNA underlies almost every aspect of human health. Obtain- the DNA. The four bases organize in two fundamental pairs, A
ing a detailed picture of how genes and other DNA sequences with T and C with G. One rail of a DNA ladder is a single strand
function together and interact with environmental factors ulti- of DNA that is denoted by a sequence of nucleotides (e.g., ACGT-
mately will lead to the discovery of pathways involved in normal GCTGACCTGACGTAGGGCATA), which has complementary
processes and in disease pathogenesis. Such knowledge will have bases on the opposite rail, forming complementary nucleotide
a profound impact on the way disorders are diagnosed, treated, strands (e.g., TATGCCCTACGTCAGGTCAGCACGT). Within
and prevented and will bring about revolutionary changes in clin- the regions of DNA that express information, these strings of nu-
ical and public health practice. Some of these transformative de- cleotides are organized into three unit “words” termed codons.
velopments are described herein. These codons are organized into groups called exons. Ultimately,
How do scientists study and find these genetic mutations? They these exons form sentences, or genes. Genes encode all of the nec-
have available to them a growing battery of tools and technologies essary information to produce a messenger molecule composed of
to compare a DNA sequence isolated from a healthy person to the ribonucleic acids (RNA), which are composed of four other nu-
same region of DNA extracted from an afflicted person. Advanced cleotides: guanine (G), adenine (A), cytosine (C), and uracil (U).
computer technologies, combined with the explosion of genetic Thus, DNA sentences are transcribed into RNA messages, which
data that is currently generated from the various whole-genome se- are single-stranded complementary copies of DNA. Once
quencing projects, enable scientists to use these molecular genetic processed, these messages leave the nucleus and enter another cel-
tools to more accurately diagnose disease and to design new thera- lular compartment where they are threaded into cellular machin-
peutic interventions. This chapter reviews some common princi- ery, which translates the information into its final state, the protein.
ples that geneticists—scientists who study the inheritance pattern Proteins are required for the structure, function, and regulation of
of specific traits—can use to inform clinical practice. the body’s cells, tissues, and organs. This process, where DNA is
transcribed into RNA and subsequently translated into proteins,
represents the central dogma of molecular biology. It is worth not-
DNA ing that a small subset of genes expresses RNA without being trans-
lated into proteins. These other forms of RNA play crucial roles in
Molecular genetics is the study of the units, or segments, of DNA the biology of the cell.
that pass information from generation to generation. These mol- Humans have approximately 3 billion base pairs of DNA in
ecules, our genes, are long sequences of deoxyribonucleic acid, or most of their cells. This complete set of genes is called a genome. The
DNA. Just four chemical building blocks or bases, deoxyguanine exact sequence of the bases is different for everyone, which makes
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