Question

DNA from the semen sample taken from the crime scene is subjected to RFLP analysis along with DNA from Mrs. Tucker, David Velasquez who is currently incarcerated for the crime, Timothy Spencer who is a suspect, and Mr. Tucker who is also a possible suspect. Analyze the RFLP analysis and write up your hair, blood, and DNA findings.

Answer 1

Gel electrophoresis separates DNI fragments according to their length and charge and provides significant information about suspected individuals. In this example, Timothy Spencer is the author of the crime.

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Restriction enzymes are degradative polymers that recognize specific nucleotide sequences in DNA and cut up the molecules into many fragments of varying lengths.

These pieces are called minisatellites and exhibit the existing variation from one person to the next.

These short sequences can be separated by using gel electrophoresis and will express a pattern of differentiation.

Gel electrophoresis separates fragments according to their size and charge.

So the largest fragments remain in the first places, being the ones that hardly migrate.

Meanwhile, the smallest fragments are the ones that migrate the most, so they are positioned far away in the last places.

These are the restriction fragment length polymorphisms pattern, RFLP.

Each individual has a unique pattern of minisatellites, which makes this person different than the rest.

The method used to identify an individual based on the minisatellites patterns in their DNI is known as DNA fingerprinting.

Although the difference in base sequences between two individuals is very small, fragments produced by restriction endonucleases are of different sizes.

Because restriction enzymes, like EcoRI and Hind III, recognize different sequences they cut the same molecule in different pieces, resulting in a different pattern.  

In the exposed example, we have five profiles,

• one from the semen stain,
• one from Susan,
• one from Velasquez,
• one from Spencer,
• one from Tucker.

We need to find out which of the suspected individuals matches the semen satin pattern.

So we need to compare the pattern of bands of the different lanes with the semen stain pattern of bands.

In resumed terms,

• If the semen stain patterns completely match with another lane pattern, then this is the individual to whom it belongs the semen.

• However, if they do not match, we need to exclude this individual.

If we look carefully at the band pattern, we can see that Timothy Spencer's DNA is the only one that completely matches with the DNA from the semen stain.  

This fact positions Spencer as the author of the crime.

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