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Title
International cooperation and technology transfer
Author
Mussio, Luigi

chains (Wodicka et al., 1997; Lockhart et al.,
1996). The signal detection is by reaction of
color-forming enzymes on nylon filter
membranes (Chen et al., 1998) or by laser-
inducted fluorescence (Wodicka et al., 1997;
Bernard et al., 1996) on microscope glass slides
(Schena et al., 1995, 1996; DeRisi et al., 1997;
Lockhart et al., 1996).
The single-color image, for the quantitative
analysis of one cellular population, and the dual
color image, for the semi-quantitative analysis of
two cellular populations (by the characterization
of the color components of the signal: hue,
saturation and brightness), are used. The aim is
to highlight the differential expression of genes.
Cellular populations mean either different tissue
cells or identical cells, which are subjected to
different conditions (development phase,
environmental factors, toxic substances, etc.).
The gene expression profile derived from the
analysis of the images in combination with the
genetic information retrieved from specialized
data banks, will give the opportunity of
characterizing new gene functions and
discovering even novel genes on the basis of
image results and in the absence of any
information about the gene sequence (Schena et
al., 1995, 1996).
2. BIOLOGICAL ASPECTS
The DNA chain (deoxyribonucleic acid)
constitutes the genetic material of each living
cell; it contains genes, which regulate the life
program of any living organism.
The fundamental structural unit of DNA is the
nucleotide; this unit is composed of the pentose
sugar, deoxyribose, a phosphate group and one
nitrogenous base (among the following four
bases: the purines adenine A and guanine G and
the pyrimidines thymine T and cytosine C). The
together bounded nucleotides create the DNA
chain.
In 1953 Watson and Crick suggested the double
right-handed helix structure of DNA in which
two polinucleotide chains (running in opposite
directions) unite by means of weak bonds. The
type of bond, which can occur between the
nitrogenous bases, determines this pairing
(known as Watson and Crick’s pairing). The
bond stability contributes to decide which
specific pairs can form; more precisely the
purine adenine always pairs with the pyrimidine
thymine and the purine guanine always pairs
with the pyrimidine cytosine. This reaction is
called hybridization reaction. It is evident how it
is possible, by using a single DNA chain (or a
DNA fragment), to go back to the
complementary chain (or fragment).
Each living cell owns the whole genetic
inheritance of the organism, which means that
the entire DNA, coding the development
program of the living being, is located in the cell
nucleus. Each cell owns the totality of the living
being’s genes, and what makes the cell different
from the other ones (or even from itself in
different phases of its life) is the expression
profile of its genes (which genes express
themselves and how).
The cellular DNA distributes itself along
structures called chromosomes. Genes are
protein-coding DNA pieces and the position of
gene (or some other marker), on the respective
chromosome, is defined locus.
Information about genes is contained in the
sequence of the four bases (A, C, G, T) along the
exons (pieces of protein-coding gene sequence).
Exons are usually separated by introns (not-
coding zones). Gene sequencing means
determining the order of the nucleotide bases
along the DNA region of a gene.
Gene transcribes itself into messenger RNA
(mRNA) that is a single polinucleotide chain,
which is identical to one of the two DNA chains
of gene (which is called “coding strand”). During
the splicing phase, introns are eliminated and the
obtained mRNA chain translates into the
particular amino acidic sequence according to
the Genetic Code: indeed each mRNA codon
(three-base group) corresponds to a particular
amino acid in a one-sense correspondence.
Cellular expression profile is determined by
measuring the mRNA amount produced by gene
transcription.
Complementary DNAs (cDNAs) are greatly used
as probes. The probes are single radioactively or
immunologically labeled DNA chains allowing
the detection of complementary DNA sequence
by hybridization in combinatorial chemistry for
gene expression studies.
The cDNAs are synthesized from mRNA
templates by reverse transcription. Pieces of
coding DNA sequences defined ESTs (Expressed
Sequence Tag) can also be used for expression
studies.