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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 . |
