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The essence of every characteristic of a species,
from morphology to timing of senescence, resides
in information stored in the species’ DNA: its
genome. All of that information, encoded in thousands
of genes in even simple organisms, requires some
sort of organization. In eukaryotes, the first
level of organization is the chromosome (Fig.
7.1). With so many genes and so few chromosomes
(five, for example, in Arabidopsis), a vast array
of genes must reside on any one chromosome. How
all of these genes are organized is an important
question of molecular genetic research. This chapter
will focus on the informational organization of
the genes within chromosomes. Our current understanding
of genome organization and expression reflects
the capabilities and limitations of widely varied
current technologies. Merging the data from these
technologies into a complete picture of genome
structure is a major research goal. Most of an
organism’s genes reside on nuclear DNA. However,
plastids and mitochondria also contain DNA that
encodes some of the gene products required for
organelle function and reproduction. Several traits
encoded by organelle genes are in commercial use
in plant crops, such as cytoplasmic male sterility
in sorghum and maize. Traits arising from organelle
genes can be clearly recognized by their uniparental
inheritance patterns, because in many species
mitochondria and chloroplasts are inherited only
through the maternal contribution to the seed.
In the 1950s, plant geneticists found evidence
that pieces of genetic material could move from
place to place in the DNA. These mobile genetic
elements were subsequently found to be ubiquitous
components of genomes, providing the possibility
for genomic changes on a larger scale than the
accumulation of single-point mutations and thereby
speeding evolutionary change. Our examination
of genome organization and genetic regulation
begins with a historical perspective and proceeds
to a state-of-the-art view of the function of
single genes (Fig. 7.2). The
historical perspective conveys the development
of genetic theory and illustrates the continued
value of “classic” approaches. However, our ability
to characterize the chromosome and the genes contained
therein with increasingly finer resolution has
developed tremendously since the time of Mendel.
Elucidating the workings of genetics has been,
and continues to be, one of the most fruitful
of biological disciplines, changing the way we
view our world and think about ourselves as biological
organisms. In 1958 George Beadle, Joshua Lederberg,
and Edward Tatum were awarded the Nobel Prize
for their work in gene regulation. Since then,
almost half of all the Nobel Prizes awarded for
Chemistry and Medicine or Physiology have been
for innovations in our understanding of genetics
and gene regulation.
Figure
7.1
Electron
micrograph of a metaphase chromosome, showing
the structure of a chromosome in its most compact
state. C, centromere.
Figure
7.2
Levels
of genetic and molecular organization: Chromosomes
(especially those in metaphase) can be characterized
by structural features, such as the position of
the centromere, and by banding patterns seen in
the presence of certain cytological dyes. A genetic
map can be created for a chromosome by assigning
genes to their relative positions on that chromosome.
Smaller sections of the genome can be precisely
ordered into a physical map by using molecular
tools such as restriction fragment length polymorphism
(RFLP) analyses (see Fig. 7.18). Individual genes
within the physical map can be analyzed as to
their specific sequence of nucleotides by using
DNA sequencing (not shown). Functional analyses
evaluate the importance of certain sequence features
to the transcriptional regulation of that gene
(see Box 7.5).
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