DNA Fingerprinting and Gel Electrophoresis Notes

DNA Fingerprinting and Gel Electrophoresis Notes February 2, 2012

DNA fingerprints Do you remember what a restriction enzyme is? • Made by cutting DNA in very specific places. • How do we cut DNA? • Restriction Enzymes

What are Restriction Enzymes? • Proteins • In nature: Used by bacteria to cut viral DNA • They “restrict” the growth of viruses • http://www.dnalc.org/resources/animations/restriction.html

Restriction Enzymes • Cut strands of DNA at specific nucleotide sequences • There are many different restriction enzymes that each cut DNA at different nucleotide sequences • Usually occurs at a palindrome 5’ GAATTC 3’ CTTAAG

What is a palindrome in English? • Palindromes can be read backwards and forwards the same way. • Give me an example of a name that is a palindrome: • Hannah • It’s a little more tricky in DNA because the strand itself isn’t what must be backwards and forwards…

DNA palindromes • A DNA palindrome reads exactly opposite its complementary strand • Ex- CAGAC is not a palindrome but CCCGGG is. Why? • Let’s look at the complementary strand: • CAGAC CCCGGG • GUCUG GGGCCC

So back to DNA fingerprinting…

I. DNA Fingerprinting • DNA is cut using ____________________. • Because no two individuals have identical DNA, no two individuals will have the same length fragments

Once the restriction enzyme has successful cut the DNA into fragments, the next step in analyzing the DNA is to use a gel electrophoresis

II. Gel Electrophoresis • Gel electrophoresis is a technique used to separate the DNA fragments according to their size • DNA fragments are placed in wells on a sheet of agarose gel • Gel is slightly porous and allows fragments to move through the gel at a rate relative to their size

An electric current is applied to the sheet • DNA is negatively charged and will move in an electric field toward the positive pole • Which size fragments do you expect to move faster through the gel? • Smaller fragments move through the gel at a faster rate compared to the larger pieces

Bands will be produced on the gelatin where the fragments accumulate • The more fragments that accumulate in one spot, the thicker the line will be in the gel • DNA bands must be stained to make them visible

III. Uses for DNA Fingerprinting and Gel Electrophoresis • Used in forensics as a way to match a sample from a crime scene to a suspect’s DNA • Used in paternity cases where the father is unknown • Each fragment in the child’s DNA must match at least one of the parent’s fragments

Used to analyze genes to test for presence of different alleles • If a person was homozygous for a trait, the DNA would be cut by the same restriction enzyme in the same place because alleles are the same • Fragments would be at the same point when run through a gel electrophoresis Allele from Dad Allele from Mom

If a person was heterozygous for a trait, the DNA would be cut by the same restriction enzyme in different places because the alleles are different • Fragments would not be at the same point in the gel electrophoresis Allele from Dad Allele from Mom

DNA Fingerprinting and Gel Electrophoresis Notes