Biology Reference
In-Depth Information
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
allele 8
allele 10
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
TCTA
Figure 2.6 The structure of a short tandem repeat. This example shows the structure of two alleles
from the locus D8S1179. 1 The DNA either side of the core repeats is called flanking DNA. The alleles
are named according to the number of repeats that they contain - hence alleles 8 and 10
Minisatellites
Minisatellites are located predominantly in the subtelomeric regions of chromosomes
and have a core repeat sequence that ranges in size from 6 bp to 100 bp [30, 31].
The core repeats are represented in some alleles thousands of times; the variation in
repeat number creates alleles that range in size from 500 bp to over 30 kb (Figure 2.5).
The number of potential alleles can be very large: the D1S7 locus, for example has
a relatively short and simple core repeat unit of 9 bp with alleles that range from
approximately 1 kb to over 20 kb - which means that there are potentially over 2000
different alleles at this locus [28].
Minisatellites were the first polymorphisms used in DNA profiling [32, 33]
and they were successfully used in forensic casework for several years. The
use of minisatellites was, however, limited by the type of sample that could be
successfully analysed, because a large amount of high molecular weight DNA was
required. Interpreting minisatellite profiles could also be problematic. Their use in
forensic genetics has now been replaced by microsatellites, which are also known
as STRs.
Short tandem repeats
STRs are currently the most commonly analysed genetic polymorphism in forensic
genetics. They were introduced into casework in the mid-1990s and are now the
main tool for just about every forensic laboratory in the world - the vast majority
of forensic genetic casework involves the analysis of STR polymorphisms.
There are thousands of STRs that can potentially be used for forensic analysis. STR
loci are spread throughout the genome, including the 22 autosomal chromosomes and
the X and Y sex chromosomes. They have a core unit of between 1 bp 2 and 6 bp
and the alleles typically range from 50 bp to 300 bp. The majority of the loci that are
used in forensic genetics are tetranucleotide repeats, which have a 4 bp repeat motif
(Figure 2.6).
1 Note: The considerations for describing STR structure have been documented by the International Society for Forensic
Genetics (ISFG) [29].
2 Note: STRs with repeating core units of one base pair are not used in forensic analysis.
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