Appendix A: 

 Primer on Molecular 

 Genetics 



On the average: 



• 4-ba.se recognition sites will yield pieces 256 bases long, 



• 6-base recognition sites will yield pieces 4,000 bases long, and 



• 8-base recognition sites will yield pieces 64.000 bases long. 



Since hundreds of different restriction enzymes have been characterized. DNA can be 

 cut into many different types of smaller fragments. 



Electrophoretic techniques (Fig. 6) can be used to .separate DNA fragments according 

 to length with a 1 -base-pair resolution up to 1.000 base pairs, with 1% resolution up to 

 I million base pairs, and with 5% resolution up to 10 million base pairs. This great 

 separation power forms the basis of most methods for handling and analyzing 

 DNA molecules. 



Fig. 6. Schematic results of 

 electrophoresis of DNA. The 



fragments resulting from restriction 

 enzyme cutting can be separated 

 by gel electrophoresis. The smallest 

 fragments move most rapidly 

 toward the positive electrode. DNA 

 fragments can be detected by uv 

 absorbance, fluorescence, radio- 

 activity, or other labels. The 

 electrophoresis pattern itself can be 

 of interest, since variations in the 

 pattern from a given chromosomal 

 region can sometimes be associ- 

 ated with variations in genetic traits, 

 including susceptibility to certain 

 diseases. This kind of "genetic 

 linkage" has been used to establish 

 the locations of important genes. 



Intact DNA molecule (sizes in base pairs) 



7,000 



1,000 



600 



7.000- 



1,500 



-► 3.900- 



3,900 



Cut with 

 restriction 

 enzyme 1.500- 



1.000- 

 600 ■ 



electrophoresis 



(-^) 



fragment sizes in number of bases 



126 



