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Chapter
7 |
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Genome
Organization
and Expression
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| The characteristics of an organism are encoded
in its DNA. In the eukaryotic nucleus, most of this information
resides within thousands of genes and is organized into
linear chromosomes. A few genes are also encoded by DNA
found in plant mitochondria and plastids. The entirety
of this genetic material is referred to as an organism’s
genome. The study of the organization and regulation of
the nuclear genome is the fundamental basis for genetic
and molecular genetic research. In the 1850s Gregor Mendel
developed the science of genetics by proposing that particular
factors (now called genes) gave rise to specific traits.
Mendel’s pea experiments were the first to correlate physical
traits (phenotype) with heritable components (genotype).
About 50 years after Mendel’s work, genes were postulated
to reside on chromosomes in the nucleus; it took another
50 years for scientists to identify the molecule responsible
for transmitting genetic information, deoxyribonucleic
acid (DNA). During the century after Mendel, elucidation
of the organization of the nuclear genome included the
discovery that genes are aligned on chromosomes and can
be inherited in linkage groups and, moreover, that genetic
recombination within a linkage group plays a role in the
generation of new phenotype combinations. The analysis
of genetic recombination also gave rise to the ability
to map genes to their relative positions in a chromosome.
The role of a gene is to encode a product that contributes
to the phenotype of that organism; often, however, only
a small percentage of an organism’s genome actually encodes
gene products. The intervening and flanking noncoding
sequences are integral parts of a typical gene and contribute
to some of the diversity in genome size observed among
similar organisms. The intervening sections of noncoding
DNA are called introns, and the sections of coding sequence
are referred to as exons. In addition, repetitive noncoding
DNA sequences contribute to the character and function
of specialized structures in chromosomes such as the centromere
and telomere. Another specialized class of DNA sequence
that can make up a significant portion of the genome is
the transposable elements. Transposable elements of sections
of DNA move, or transpose, from one site in the genome
to another, carrying genetic information with them as
they transpose. Not all genes of a nuclear genome are
expressed all the time. Selective expression gives rise
to cellular differentiation during development and enables
cells to respond to environmental signals accordingly.
Genes are divided into two fundamental regions. The coding
region is transcribed into the gene product, and the regulatory
region contains a variety of sequence elements that function
in the recruitment of protein transcription factors, which
facilitate transcription. Another feature of the genome
that plays a role in the selective expression and regulation
of genes is the chromatin structure associated with a
given gene. Chromatin refers to the complex of DNA and
protein that makes up the chromosome and organizes the
genome within the nucleus. In addition to these large-scale
roles, the fine-scale features of chromatin influence
gene regulation by manipulating the accessibility of a
gene promoter to the factors required for initiating transcription.
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