Restriction Enzyme Digestion
Restriction Enzyme Digestion
Description:
Restriction enzyme digestion describes the process used to cut, or digest, large pieces of DNA into smaller pieces. DNA is cut by combining it with a special type of enzyme, a restriction enzyme, which "recognizes" a unique DNA sequence. The mixture is then incubated under laboratory conditions to maximize digestion. During digestion, the enzyme locates places along the DNA with the unique sequence and cuts the DNA at that site. 
Some molecular tests are based on restriction enzyme digestion. For example, many HbS (sickle cell anemia) DNA tests utilize an enzyme that only cuts DNA sequences of CTGAG (the restriction enzyme is called DdeI). The mutation that causes HbS is a single base pair change (from A to T) at position 6 of the beta-globin chain gene. In people without HbS, the normal beta-globin gene sequence is G-A-G. Individuals with HbS, however, have a gene sequence of G-T-G.
After copying the DNA from fetal amniocytes using PCR, the amplified DNA is exposed to the restriction enzyme. If the HbS mutation is present, the DNA will not be cut and a single large fragment of DNA will remain, the original PCR product of 233 base pairs. Conversely, if the mutation is not present, the restriction enzyme will cut the DNA into 2 pieces. One piece will be 178 base pairs and the second will be 55 base pairs.
