Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 2417.4 Genes and the Genome
How much do you have in common with a small,
spiny fish like that shown in Figure 7.29? Studies
of DNA from such diverse organisms as pufferfish,
fruit flies, yeast, and humans demonstrate a number
of shared patterns in the way hereditary information
is organized at the molecular level. For instance,
there are similarities in how individual genes—
specific sequences of DNA that have the potential to
be expressed to guide an organism’s development
— are organized. There are also similarities in the
organization of that organism’s genome— the sum
of all the DNA carried in its cells.
Figure 7.29Although pufferfish are separated from humans
by millions of years of evolution, their DNA contain thousands
of genes that are nearly identical to genes found in humans.
Genes
The gene is the major functional sub-unit of DNA.
Each chromosome in any living cell carries a
particular set of genes. The specific number, type,
and arrangement of genes are unique to each
species, but even organisms that are only distantly
related may carry very similar genes. The pufferfish
genome, for example, includes genes that are
almost identical to the human genes associated
with Huntington’s disease and Alzheimer’s disease.
Although Mendel was unaware of the existence
of DNA or chromosomes, his factors of inheritance
underlie the traditional definition of a gene.
According to this definition, a gene is the portion
of inherited information that defines one particular
trait of an organism’s physical characteristics. The
more precise functional definition of a gene has
evolved since then, keeping pace with researchers’
growing understanding of the role of DNA in
directing development.
In the 1940s, after DNA had been identified as
the material of heredity but before its structure had
been discovered, George Beadle and Edward Tatum
studied patterns of development in a particular
species of Neurosporabread mould. They
identified a number of different strains of this
mould that had different nutritional needs. The
wild variety of Neurosporacould be grown on an
agar medium containing minimal nutrients. Various
mutant strains would grow only on a medium that
contained additional nutrients such as particular
sugars or amino acids.
Beadle and Tatum hypothesized that while the
wild variety of Neurosporacan synthesize all the
amino acids it requires from the minimal medium,
different mutations disrupt the metabolic pathways
by which the mould synthesizes the proteins it
requires for development. With further research,EXPECTATIONS
Relate the evolution of functional definitions of the term “gene” to advances
in scientific understanding of the structure and function of DNA.
Describe how the structure of genes varies among different organisms.
Identify two features of the eukaryote genome that allow for greater
developmental complexity than is allowed by the prokaryote genome.