The opened circle of thread is estimated to be about 1 mm (1000 n) long. It is packed into a cell only about 2 by 8 p in dimensions and it constitutes a single, double helical macromolecule with a molecular weight of the order of two billion.
When it replicates, one or more still obscure mechanisms, called replicators, proceed along the chromosome from a certain fixed point, separating the two strands of the helix as it (they?) go (es) and permitting the formation of complementary strands as previously described.
A number of problems still exist concerning the mechanism. For example, what is the mechanism that triggers replication? How does the replicator unwind the helix through some 300,000 turns during the fission period of a cell like Escherichia coli that can mature and divide within 20 minutes?
This involves some 15,000 turns per minute— a dizzy spin indeed! Further, the chromosome at one point is attached to the cell membrane and, if it is unwinds; it involves some sort of swivel to permit the turning. Bear in mind also that the two helices run in opposite directions (have opposite polarities).
RNA commonly differs structurally from DNA in being single stranded. The exact mode of synthesis of RNA is still under investigation. However, double-stranded RNA and single stranded DNA occur in some viruses. The single-stranded RNA in cellular organisms is of vital importance in protein synthesis and therefore in inherited characteristics, as explained in the following paragraphs.
Since about 1964 it has been observed that in some bacterial viruses the replication of RNA involves the formation, at least transitorily, of two double-stranded forms of RNA in addition to the single-stranded form.
One is spoken of as a replicative form (RF), the other as replicative intermediate (RI). The significance of these findings is clearly of fundamental importance though complete elucidation awaits further investigation.