Biomedical Engineering Reference
In-Depth Information
unique sequences and repetitive sequences. Many of
the repetitive DNA sequence elements are arranged
in tandem and are known as satellite DNA. Three
types of satellite DNA can be distinguished on the
basis of the level of repetition and the repeat-unit
length (Table 14.2). Not only is satellite DNA dis-
persed throughout the genome, it is highly variable
and provides a valuable tool for genetic individual-
ization. An example of this is shown schematically
in Fig. 14.6. In this case the RFLPs detected are due
to variations in the number of repeat units (VNTR
polymorphisms) between restriction sites, rather
than changes in the location of the restriction sites,
as discussed in the previous section.
VNTR polymorphisms are of importance to
clinical geneticists because a number of important
hereditary diseases are associated with alterations
in the degree of repetition of microsatellites (for
reviews see Bowater & Wells 2000, Gutekunst
et al.
2000, Usdin & Grabczyck 2000). Probably the best
example of such a disease is Huntington's chorea,
which is caused by an expansion of a CAG trinu-
cleotide repeat in exon 1 of the gene coding for a
protein of unknown function, which has been
named huntingtin. Expansion beyond 40 repeat
units correlates with the onset and progression of
the disease (for review see Reddy
et al.
1999).
10 kb
10 kb
M
N
P
7 kb
M
A
7 kb
Gene map of
parents
Chromosomes of parents
Mother
+
+
Father
10 kb
7 kb
Brother
+
+
Fetus
Gene map of
brother and fetus
Family studied by gene mapping
Fig. 14.5
An example of prenatal diagnosis using restriction
fragment length polymorphism (RFLP) linkage analysis. The
parents are both carriers for a deleterious gene (A): one of
their chromosomes carries this determinant, the other its
normal allele (N). One of the parental chromosomes carries
a polymorphic restriction enzyme site P, which is close
enough to A or N for them not to be separated in successive
generations. On the chromosome which does not contain this
site (-), a particular restriction enzyme cuts out a piece of DNA
10 kb long which contains another locus (M), for which we
have a radioactive probe. On the chromosome containing the
polymorphism (
Forensic applications of VNTRs
), a single base change produces a new site
and hence the DNA fragment containing locus M is now only
7 kb. On gene mapping of the parents' DNA using probe M,
we see two bands representing either the
+
The existence of VNTR polymorphisms is of great
utility in paternity testing and criminal investigations,
since they allow ready comparison of DNA samples
in the absence of detailed genetic information by the
generation of a DNA profile or fingerprint. In prin-
ciple, a multilocus DNA fingerprint can be generated
either by the simultaneous application of several
probes, each one specific for a particular locus, or by
applying a single DNA probe that simultaneously
detects several loci. When DNA profiling was first
developed ( Jeffreys
et al.
1985a), multilocus probes
were used and these were derived from a tandemly
repeated sequence within an intron of the myo-
globin gene (Fig. 14.7). These probes can hybridize
to other autosomal loci - hence their utility. The
first criminal court case to use DNA fingerprinting
was in Bristol, UK, in 1987, when a link was shown
between a burglary and a rape. In the following
chromosomes.
A previously born child had received the deleterious gene
A from both parents and on mapping we find that it has the
++
+
or
−
chromosome arrangement, i.e. only a single 7 kb band.
Hence the mutation must be on the + chromosome in both
parents. To identify the disease in a fetus in subsequent
pregnancies, we shall be looking for an identical pattern,
i.e. the 7 kb band only. (Reproduced courtesy of Professor
D. Weatherall and Oxford University Press.)
Variable number tardem repeat
(VNTR) polymorphisms
In higher eukaryotes, genes and their associated
introns occupy only a small proportion of the
genome. The intergenic DNA, which makes up the
majority of the genome, is composed of a mixture of
