Each phage head holds about 44 Kbp of double stranded linear DNA. A phage encoded endonuclease initially cuts the DNA at a pac site. One phage head is filled with DNA, then the DNA is cut by the endonuclease. The next head is filled with DNA beginning with the remaining end. These reactions continue processively until 3-5 phage heads are filled with DNA.
Note that the phage genome is about 42 kb, approximately 2 Kbp less than the length of one headful of DNA. Thus, the DNA packaged into each phage head is terminally redundant and the ends are circularly permuted.
Check out this link for a nice review of P22 assembly.
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Last modified September 9, 2004