Palindrome blunt ends

Two enzymes (Kpnl and Pst in the list in Table 26-2) form 3 -cohesive ends rather than 5 -cohesive ends. In addition, there are three (Alul, EcoRV, and HaellT) that cut at the local twofold axis they form no cohesive ends but leave blunt ends (flush ends). Blunt end fragments are also much used in genetic engineering. "Linkers" that provide cohesive ends can be added.119 The Sfil endonuclease cuts between two 4-bp palindromes in a 13-bp recognition sequence (Table 26-2).120... 

Fig. 17.1. Action of restriction enzymes. Note that the DNA sequence shown is a palindrome each strand of the DNA, when read in a 5 to 3 direction, has the same sequence. Cleavage of this sequence by iicoRI produces single-stranded (or sticky ) ends or tails. Not shown is an example of an enzyme that generates blunt ends (see Table 17.1).

EXAMPLE 8.17 Among the most important enzymes of recombinant DNA technology are the restriction enzymes. These are endonucleases that cleave DNA only at specific sequences of bases (called restriction sites). Typically, restriction sites are palindromic, in other words, the sequences are the same in the 5 — 3 and 3 — 5 strands. Restriction enzymes are produced by bacteria as an antiviral defense, and they cleave the DNA of viruses (bacteriophages) that infect them. However, they do not cleave host bacterial DNA. Fig. 8-17 shows the restriction sites of three common restriction enzymes, BamHI, EcoRI, and PvuII. Because BamHI and EcoRI cleave their restriction site asymmetrically, they produce overhangs in the cleaved DNA, called sticky ends. Conversely, PvuII cleaves symmetrically, producing blunt ends. 

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