This preview shows page 1. Sign up to view the full content.
Unformatted text preview: such a primer-template pair is found, DNA polymerase will begin adding bases to the primer to create a complementary copy of the template. 3. Ligases. Ligases bind molecules together. For example, DNA ligase will take two strands of DNA in proximity and covalently bond them into a single strand. DNA ligase is used by the cell to repair breaks in DNA strands that occur, for instance, after skin cells are exposed to ultraviolet light. 4. Nucleases. Nucleases cut nucleic acids. For example, restriction endonucleases will “search” a strand of DNA for a predetermined sequence of bases and, when found, will cut the molecule into two pieces. EcoRI (from Escherichia coli) is a restriction enzyme that will cut DNA after the G in the sequence GAATTC—it will almost never cut a strand of DNA anywhere else. It has been suggested that restriction enzymes evolved to protect bacteria from viruses (yes, even bacteria have viruses!). For example, E. coli has a means (methylation) of protecting its own DNA from EcoRI, but an invading virus with the deadly GAATTC se...
View Full Document
- Fall '08