Key Terms:
- The genome is the complete set of sequences in the genetic material of an organism. It includes the sequence of each chromosome plus any DNA in organelles.
- Nucleic acids are molecules that encode genetic information. They consist of a series of nitrogenous bases connected to ribose molecules that are linked by phosphodiester bonds. DNA is deoxyribonucleic acid, and RNA is ribonucleic acid.
- A gene (cistron) is the segment of DNA specifying production of a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).
- An allele is one of several alternative forms of a gene occupying a given locus on a chromosome.
- A locus is the position on a chromosome at which the gene for a particular trait resides; a locus may be occupied by any one of the alleles for the gene.
- Linkage describes the tendency of genes to be inherited together as a result of their location on the same chromosome; measured by percent recombination between loci.
The hereditary nature of every living organism is defined by
its genome, which consists of a long sequence of
nucleic acid that provides the information
need to construct the organism. We use the term "information" because the genome
does not itself perform any active role in building the organism; rather it is
the sequence of the individual subunits (bases) of the nucleic acid that
determines hereditary features. By a complex series of interactions, this
sequence is used to produce all the proteins of the organism in the appropriate
time and place. The proteins either form part of the structure of the organism,
or have the capacity to build the structures or to perform the metabolic
reactions necessary for life.
The genome contains the complete set of hereditary
information for any organism. Physically the genome may be divided into a number
of different nucleic acid molecules. Functionally it may be divided into genes. Each gene is a sequence within the nucleic acid
that represents a single protein. Each of the discrete nucleic acid molecules
comprising the genome may contain a large number of genes. Genomes for living
organisms may contain as few as <500 genes (for
a mycoplasma, a type of bacterium) to as many as >40,000 for Man.
In this Chapter, we analyze the properties of the gene in
terms of its basic molecular construction. Figure 1.1
summarizes the stages in the transition from the historical concept of the gene
to the modern definition of the genome.
The basic behavior of the gene was defined by Mendel more
than a century ago. Summarized in his two laws, the gene was recognized as a
"particulate factor" that passes unchanged from parent to progeny. A gene may
exist in alternative forms. These forms are called alleles.
In diploid organisms, which have two sets of chromosomes,
one copy of each chromosome is inherited from each parent. This is the same
behavior that is displayed by genes. One of the two copies of each gene is the
paternal allele (inherited from the father), the other is the maternal allele
(inherited from the mother). The equivalence led to the discovery that
chromosomes in fact carry the genes.
Each chromosome consists of a linear array of genes. Each
gene resides at a particular location on the chromosome. This is more formally
called a genetic locus. We can then define the
alleles of this gene as the different forms that are found at this
locus.
The key to understanding the organization of genes into
chromosomes was the discovery of genetic linkage.
This describes the observation that alleles on the same chromosome tend to
remain together in the progeny instead of assorting independently as predicted
by Mendel's laws. Once the unit of recombination (reassortment) was introduced
as the measure of linkage, the construction of genetic maps became
possible.
On the genetic maps of higher organisms established during
the first half of this century, the genes are arranged like beads on a string.
They occur in a fixed order, and genetic recombination involves transfer of
corresponding portions of the string between homologous chromosomes. The gene is
to all intents and purposes a mysterious object (the bead), whose relationship
to its surroundings (the string) is unclear.
The resolution of the recombination map of a higher
eukaryote is restricted by the small number of progeny that can be obtained from
each mating. Recombination occurs so infrequently between nearby points that it
is rarely observed between different mutations in the same gene. By moving to a
microbial system in which a very large number of progeny can be obtained from
each genetic cross, it became possible to demonstrate that recombination occurs
within genes. It follows the same rules that were previously deduced for
recombination between genes.
Mutations within a gene can be arranged into a linear order,
showing that the gene itself has the same linear construction as the array of
genes on a chromosome. So the genetic map is linear within as well as between
loci: it consists of an unbroken sequence within which the genes reside. This
conclusion leads naturally into the modern view that the genetic material of a
chromosome consists of an uninterrupted length of DNA representing many
genes.
A genome consists of the entire set of chromosomes for any
particular organism. It therefore comprises a series of DNA molecules (one for
each chromosome), each of which contains many genes. The ultimate definition of
a genome is to determine the sequence of the DNA of each chromosome.
The first definition of the gene as a functional unit
followed from the discovery that individual genes are responsible for the
production of specific proteins. The difference in chemical nature between the
DNA of the gene and its protein product led to the concept that a gene
codes for a protein. This in turn led to the discovery of the complex
apparatus that allows the DNA sequence of gene to generate the amino acid
sequence of a protein.
Understanding the process by which a gene is expressed
allows us to make a more rigorous definition of its nature. Figure 1.2 shows the basic theme of this book. A gene is a
sequence of DNA that produces another nucleic acid, RNA. The DNA has two strands
of nucleic acid, and the RNA has only one strand. The sequence of the RNA is
determined by the sequence of the DNA (in fact, it is identical to one of the
DNA strands). In many, but not in all cases, the RNA is in turn used to direct
production of a protein. Thus a gene is a sequence of DNA that codes for an
RNA; in protein-coding genes, the RNA in turn codes for a protein.
No comments:
Post a Comment