Short Tandem Repeat (STR) Analysis Using PCR and Gel Electrophoresis

Short Tandem Repeat (STR) Analysis Using PCR and Gel Electrophoresis

In forensic investigations, Forensic Biologists use short tandem repeat (STR) analyses to help identify individuals that had been in or around a crime scene.  Like identifying double muscled (DM) cows, the analysts use 20 regions of the DNA that have repeating units to generate an allelic profile to compare to known profiles of victims and potential suspects.  Research and answer the following questions about Polymerase Chain Reaction (PCR) and gel electrophoresis as it pertains to forensic investigations in your own words.


1.      Explain what happens in each of the three fundamental stages of PCR and the temperatures at which these stages are typically held. (6 pts


2.      The Taq DNA polymerase is used to add nucleotides to the template strand during PCR. What characteristic makes the Taq polymerase ideal when used in PCR? (1 pt.)



3.      PCR is an exponential copying of the template strands and can be represented by the function: y = a * 2n, where a is the initial number of template copies and n is equal to the number of cycles PCR has gone through.  How many DNA fragments would be produced after:

a.      15 cycles? (1 pt.)

b.      13 cycles with 13 starting template strands? (1 pt.)

c.       29 cycles with 32 starting template strands? (1 pt.)


4.      Use the shapes/ text box features in Word to draw and label a diagram of a gel electrophoresis unit. Make sure to include:

a.      The wells (1 pt.)

b.      The anode and cathode (and their charge) (1 pt.)

c.       A ladder with fragments at 10, 50, 100, and 500 base pairs (bp) (1 pt.)

d.      Sample 1: with fragments at 75 bp and 75 bp (1 pt.)

e.      Sample 2: with fragments at 50 bp and 165 bp (1 pt.)

f.        Sample 3: with fragments at 100 bp and 250 bp (1 pt.)


5.      Analyze the gel below and answer:

a.      Which samples are heterozygous? (1 pt.)

b.      Which samples are homozygous? (1 pt.)

c.       Which suspect alleles match those of the DNA found on the murder weapon?  How do you know this? (1 pt.)

d.      How does STR analysis allow for investigators to exclude certain individuals from being possible suspects? (2 pts).

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