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Southern Blot
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Why would the same sequence show up in different gel bands in different individuals? The Southern Blot procedure breaks DNA into fragments using a restriction enzyme, runs the fragments on a gel, and then uses a specific labeled sequence as a probe. The DNA sequences are spaced out on the gel by length. The shortest sequence travels the furthest distance and the longest sequence travels the shortest distance. The pattern of bands is thus entirely due to the number and location of the restriction sites where the DNA is cut. The addition or loss of a single restriction site can place the target sequence in a new band on the gel even when the rest of the DNA is unchanged.

View the animation below, then complete the quiz to test your knowledge of the concept.





1The smaller the DNA fragment
A)the closer to the origin it will appear
B)the brighter color it produces with ethidium bromide
C)the faster it migrates during separation by electrophoresis
D)the slower it migrates during separation by electrophoresis


2The purpose of the Southern Blot test is to
A)look for a specific nucleotide sequence in the DNA being tested
B)to determine how closely two organisms are related
C)to identify the size of the fragment that contains the sequence
D)a and c
E)a, b, and c


3The steps involved in the Southern Blot test should be performed in the following order
1 = x-ray film
2 = electrophoresis
3 = digestion with restriction enzyme
4 = ethidium bromide
5 = radioactive probe
A)3, 2, 4, 5, 1
B)3, 4, 2, 5, 1
C)3, 2, 5, 4, 1
D)2, 4, 3, 5, 1


4The DNA probe will bind to identical nucleotide sequences on the test sample.
A)True
B)False


5The position of the bound radioactive probe is made visible on x-ray film.
A)True
B)False





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