Cutting and pasting genes
One of the basic tools that scientists use to study or transfer segments of DNA is a group of chemicals called restriction enzymes.
Restriction enzymes are like molecular scissors. They are used to cut up DNA. They were originally found in bacteria, where they act as a defence mechanism. The bacterial restriction enzymes recognise foreign DNA, such as from a virus, when it enters a bacterial cell, and inactivates it by cutting it up. The handy thing for scientists is that restriction enzymes do not cut DNA randomly - they cut at very specific places.
Bacterial restriction enzymes will work on DNA from other organisms because DNA is chemically identical, whether it comes from bacteria, a rose or a human. The bases of DNA are always the same; it's just the arrangement of the bases that varies.
Different restriction enzymes cut at different sites. Each restriction enzyme recognises a certain DNA sequence, usually about four to six base pairs long, and cuts the DNA within this sequence. For example, the enzyme EcoRI, which is taken from the human gut bacteria E. coli, recognises the genetic sequence GAATTC. It cuts the DNA between the guanine (G) and the adenine (A).
Some enzymes cut straight through both strands of the DNA molecule to produce 'blunt' ends of DNA. Others cut one strand of the DNA molecule in one spot and the second strand slightly to the left or right. This creates two ends with one strand that hangs slightly over; these are called 'sticky' ends.
When cutting and pasting a gene, researchers usually use the enzymes that cut and produce sticky ends. This allows the overhanging strand of DNA to be easily matched up with another strand of DNA .
The cut piece of DNA can be pasted either into a new chromosome, or into a plasmid - one of the small circular DNA molecules found in bacteria. Plasmids are used when transferring a gene from one organism to another.
The DNA of the new chromosome or plasmid is usually cut with the same restriction enzyme that was used to cut the gene out. This means the sticky ends will have the same DNA code as the ends of the piece being pasted in. The overhanging strands of DNA bind together, and another enzyme, called ligase, is used to seal the join.