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Which of the following statements describing restriction endonucleases
is true?
a. They always yield overhanging single-stranded ends
b. They recognize methylated DNA sequences
c. They recognize triplet repeats
d. They cleave both strands in duplex DNA
e. They always yield blunt ends
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The answer is d. (Murray, pp 488“504. Scriver, pp 3“45. Sack, pp
41“45. Wilson, pp 151“186.) Restriction endonucleases are produced by
prokaryotes for cleaving both strands of foreign DNA. The host cell™s DNA
is not degraded because the recognition sites are specifically methylated.
The endonucleases recognize specific short symmetrical sequences known
as palindromes. These cleavage sites contain twofold rotational symmetry
in that the sequence is identical but antiparallel in the complementary
strands. In some cases, single-stranded cohesive ends on each of the complementary
strands are produced, while in other cases double-stranded
blunt ends are formed. Modern analysis of DNA structure is highly dependent
upon the use of different restriction endonucleases that permit the
specific hydrolysis of DNA into large polynucleotides.
