Claudio Gutierrez

DNA usually replicates itself in a semi conservative way, that is, each new molecule getting one half of old material and one half of new one. This is the case, e.g., of Escherichia coli, whose replication process goes through the typical “eye form”, down to the separation of a new chromosomal circle. Surprisingly, the lambda phage replicates itself in a non conservative manner, since most of the material of the produced genomes (originating from a single phage) is assembled from brand-new nucleotides. This exploit is performed through the incredibly efficient method known as rolling-circle strategy. Once a circular form is adopted right after entering the body of the attacked bacteria, the phagic genome is going to produce a protein replication cluster which will assemble itself at a point of the DNA circle called ORI (origin). An enzyme of this cluster will produce an incision in the external strand of the circle. One of the nucleotides of that strand, ended in 3’, will thus be separated from another of them, ended in 5’ (see image on the right). Both nucleotides already detached from their corresponding complementary bases of the internal circular strand, replication will begin, starting from the loose ends of the external one. Different things, however, will happen at each of the two terminals.

Due to the antisense characteristics of DNA, polymerases are capable of synthesizing in a continuous way starting only from terminal 3’, never from terminal 5’. In this particular case, two polymerase molecules must do different jobs at the two ends of the cut strand. The polymerase in charge of the loose terminal 3’ will have the easiest job; it will successively add new nucleotides in position 5’ to 3’ so that terminal 5’ of the new nucleotide will join terminal 3’ of the strand. In order to select the new nucleotide among the four available types, it will use as template the internal circular strand, thus securing complementariness with the exposed nucleotide. This replication, stemming from terminal 3’ of the cut, will expand the original chain in a continuous way, offering a progressively extended template to the synthesis job of the other polymerase, at the other end of the external strip. This second polymerase will have the heaviest job. Above all, it will have to continue progressively separating the two strands (and, consequently, making the internal circle roll), so that both processes of synthesis can carry on. Secondly, it will have to place new complementary nucleotides over the external strip detached from the circle, working “against the grain,”³⁵⁵ in a discontinuous way, according to a complicated method known as Okazaki fragments synthesis. In its discontinuous job, this polymerase must begin the assemblage of each new group of nucleotides using as a primer a small RNA sequence familiarly known as “oligo”, abbreviation for “oligonucleotide” (few nucleotides). Another enzyme, a ligase, will then be in charge of removing the oligo and darning the seam between the separately synthesized sequences.

The final result of this process will be an indefinitely long double helix of DNA, formed by the chaining of many linear copies of the phagic genome, bound to separate from each other and eventually connect in circular fashion at adhesive COS loci. Meanwhile, multiple hollow heads of the phage will have been forming in the bacterial plasma, each one bound to catching exactly one copy of the phagic genome, by sectioning it out from the continuous strand at the COS loci. Each head will be filled with 50 kilobases (a complete linear copy of the genome). The virus tails, independently built and swimming in the cellular broth, will join the heads as they become filled up. An admirable industrial process worthy of a Henry Ford's stature genius, designed however in automatic fashion, through out countless generations, by the blind algorithm of natural selection.