9cell to another. Gene expression can be detected by various techniques described in
Chap. 2. The discovery that eukaryotic genes are not contiguous sequences of DNA
but consist of coding sequences (exons) interrupted by intervening sequences
(introns) led to a more complex view of gene expression. The temporal, develop-
mental, typographical, histological and physiological patterns in which a gene is
expressed provide clues to its biological role. Malfunctioning of genes is involved
in most diseases, not only inherited ones.
All functions of cells, tissues and organs are controlled by differential gene
expression. As an example, red blood cells contain large amounts of the hemoglobin
protein that is responsible for carrying oxygen throughout the body. The abundance
of hemoglobin in red blood cells refl ects the fact that its encoding gene, the hemo-
globin gene, is actively transcribed in the precursor cells that eventually produce red
blood cells. In all other cells of the body, the hemoglobin gene is silent. Accordingly,
hemoglobin is present only in red blood cells. It is now well established that dif-
ferential gene expression results in the carefully controlled (or regulated) expres-
sion of functional proteins, such as hemoglobin and insulin.
Gene expression is used for studying gene function. Genes are now routinely
expressed in cultured cell lines by using viral vectors carrying cDNA, the transcrip-
tion of which yields the gene’s mRNA. RNA-RNA interaction can induce gene
expression and RNA can regulate its activities without necessarily requiring a pro-
tein. The protein produced from mRNA may confer specifi c and detectable function
on the cells used to express the gene. It is also possible to manipulate cDNA so that
proteins are expressed in a soluble form fused to polypeptide tags. This allows puri-
fi cation of large amounts of proteins that can be used to raise antibodies or to probe
protein function in vivo in animals. Knowledge of which genes are expressed in
healthy and diseased tissues would allow us to identify both the protein required for
normal function and the abnormalities causing disease. This information will help
in the development of new diagnostic tests for various illnesses as well as new drugs
to alter the activity of the affected genes or proteins.
DNA Sequences and Structure
The human genome project has provided the genetic sequence of the entire human
genome and identifi ed the need for further work to study the biological function of
genes. X-ray crystallography has been used to determine the 3D structures of nearly
all the possible sequences of DNA at atomic level and create a map of DNA struc-
ture. This will help to explain function of genes and gene expression, which often
occurs through variations in DNA structure and may provide answers to questions
as to why some DNA structures are inherently prone to damage or mutation and
how DNA is able to repair itself. An understanding of DNA structure and its rela-
tionship to genetic sequences will advance applications in molecular diagnostics,
gene therapy, nanobiotechnology and other areas of biomedicine.
Molecular Biological Basis of Personalized Medicine