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Lab Day ____________________________DNA Profiling Using STR Analysis
It has become very popular on TV crime dramas to show investigators sending samples off for DNA and chemical analysis. Over the years many analysis techniques have evolved, early on scientists used RFLP and Southern blotting of DNA to work up DNA fingerprints of samples. Currently scientists use a technique called STR analysis (pronounced star). STR stands for simple tandem repeat and rather than looking at genes for known proteins like hemoglobin or insulin, STR looks at the part of the DNA that doesn’t actually code for proteins.
“The gene sequences that code for proteins or are transcribed into rRNA or tRNA molecules make up only about 1.5% of the human genome… The vast majority of the human genome does not code for proteins” (Reece et al. 2011, p 435). The rest of the genome is sometimes referred to as junk DNA. Over the years scientists have realized that a portion of that junk DNA is used to control production of proteins but a good deal of it has no apparent function and consists of nonsense sequences. A short tandem repeat is a unit of 2 to 5 nucleotides that repeats itself a number of times. The number of times the STR repeats itself can vary from person to person. Also remember that humans are diploid so that for any given STR, a person can be homozygous so that they have the same number of repeats at a locus or they could be heterozygous so that they have a different number of repeats at a given locus.
The number of times a STR repeats itself can vary from several hundred thousand repetitions to only several repetitions. Researchers using STR loci have found sites that have relatively few repeats. Indeed while more than 8000 STR loci have been identified there are only 13 common sites used for STR analysis. At each locus instead of having just 2 alleles like many genes students first learn about, STR loci have many alleles at each locus. As a…