Biology Reference
In-Depth Information
Start
Termination
Exon
Intron
Exon
DNA
Regulatory
sequences
Transcription
mRNA
Addition of
5' cap
m
7
Gppp
Cleavage and
addition of poly(A)
tail to 3' end
m
7
Gppp
AAAA
RNA splicing
m
7
Gppp
AAAA
Transport to
cytoplasm
m
7
Gppp
AAAA
Translation
Figure 1.2.
Expression
pathway of a human protein-
coding gene
PROTEIN
1990; Hentze 1991;
Figure 1.2
). A variety of sequences within a typical gene region
are required for normal and appropriate expression to occur. These are described
briefly in subsequent sections.
Polymorphisms
Variation, whatever may be its cause, and however it may be limited, is the
essential phenomenon of evolution. Variation, in fact,
is
evolution. The readiest
way, then, of solving the problem of evolution is to study the facts of variation.
William Bateson (1894)
Materials for the Study of Variation
The term
polymorphism
has been defined (Vogel and Motulsky, 1986) as a 'mendelian
trait that exists in the population in at least two phenotypes, neither of which occurs
at a frequency of less than 1%'. Some DNA polymorphisms are neutral single base-
pair changes detected by virtue of the consequent introduction or removal of a
restriction enzyme recognition site and are accordingly termed Restriction
Fragment Length Polymorphisms (RFLPs). They are inherited as simple
Mendelian traits since two alleles are generated as a consequence of the presence or
