T4 and other bacteriophages commonly use a method of DNA replication and packaging called "headful packaging."

Question

Explain how viral genomes are replicated and packaged in this process and hypothesize how headful packaging might affect evolution of BOTH the bacteriophages and their prokaryotic hosts.
What will be an ideal response?

Answer 1

Answer: A linear concatemer is first formed after recombination of several copies of the viral genome. An endonuclease then cuts out particular regions of the concatemer to form individual linear replicated genomes. Although the genomes will all contain the same genes, this circular permutation process generates the genes in a different sequence order. In addition, the concatemer is not cut at specific sequences but is instead cut into linear segments of DNA just long enough to fill the phage head. The phage head typically holds slightly more than a genome length of DNA, thus the mechanisms generates terminal repeats at each end of the DNA genome. Headful packaging may allow for the mixing and recombination of phage genes because not every page head has an identical genome. This might increase the rate of evolution of the phage. In addition, this mechanism of packaging is not sequence specific, thus host DNA could be packaged into the phage head instead of viral DNA. When the phage infects another bacterial cell, it would inject new bacterial genes into the host cell. Without a full complement of phage genes, the phage would not replicate within the new host cell, but the new bacterial genes could add to or change the genetic material of the host. Therefore, headful packaging could increase the evolution of the prokaryotic host as well.