Video Transcript
Which of the following statements
about restriction enzymes is true? (A) All restriction enzymes cut
DNA, leaving sticky ends, which allows the joining of different fragments of
DNA. (B) Restriction enzymes cut at
specific nucleotide sequences that are also palindromes. (C) Restriction enzymes were
discovered in antibiotic-resistant strains of viruses. (D) A fragment of DNA produced by
the action of one restriction enzyme is always joined with a fragment of DNA
produced by the action of a different restriction enzyme.
Restriction enzymes are important
in combining different sources of DNA to make recombinant DNA. They are useful because they act
like molecular scissors to cut DNA. These enzymes were discovered in
bacteria as a defense against viral infection. So, answer choice (C) is
incorrect.
Restriction enzymes cut DNA at
specific nucleotide sequences called recognition sequences. This particular sequence is
recognized by the restriction enzyme Eco R one. Eco R one cuts DNA like this. This produces two fragments that
each have an overhang of unpaired nucleotides. These are called sticky ends
because they can pair together with their complementary nucleotides from a
compatible sticky end.
By using the same restriction
enzyme, DNA from any source can be combined. This is how genetic engineering can
be used to insert genes into organisms. If different restriction enzymes
are used, then different sticky ends will be produced that are not compatible, so in
this case the DNA fragments can’t be joined. Therefore, answer choice (D) is
incorrect.
Not all restriction enzymes make
sticky ends. The restriction enzyme Eco R five
cuts DNA like this at its recognition sequence. This produces blunt-end cuts, which
aren’t as convenient in combining fragments like sticky ends are. Therefore, answer choice (A) is
incorrect.
One important feature of
recognition sequences is that they are palindromes, meaning they read the same
forwards and backwards like the word “racecar.” On the top strand, the sequence
reads GAT ATC. And on the bottom strand, in
reverse, it also reads GAT ATC. Therefore, the correct answer
choice is (B). Restriction enzymes cut at specific
nucleotide sequences that are also palindromes.
