Question

6) DNA containing bacteriophages can use several types of DNA replication during their growth cycle to produce DNA templates termed concatamers that are used for packaging. Give one example to explain how concatamers are formed.

Answer 1

As mentioned, bacteriophages can use several types of methods for the DNA replication at the time of their growth cycle when there is a need for new template DNA molecules. One of the mechanism termed as rolling circle replication is employed by several bacteriophages such as bacteriophage lambda.

The bacteriophage lambda has a genome made up of double-stranded DNA molecules which are linear in nature. When this phage infects a host cell it incorporates its genetic content with that of the host. It is to be noted that the dsDNA if the phage lambda has cohesive ends due to which the DNA circularizes with its incorporation with the host genome. In order to grow in number the phage has to divide and for that it will have to replicate its DNA, now since the DNA is circularized after incorporation with the host, the mechanism of Rolling circle replication or RCR is employed.

The rolling circle DNA replication will have four steps in which it will be over replicating the circular DNA, before getting into those steps remember few things: The elongation of the DNA strand takes place from 3' end to the 5'end. The 3' end of the primer has a -OH which helps in the elongation step or the formation of the phosphodiester bond. This 3' -OH is usually made available by the primer.

Here, in case of the phage DNA, the DNA double strand is circularized, in the first step there would be a formation of nick in here (nick is the gap created by breaking of the phosphodiester bond) the nick is on the external strand which is open now as a result of that nick.

In the next step,2, Due to nick the strand will have two ends, a 3' end, and a 5' end. There would be intact circular strand (remember the internal strand), this 3' end will be elongated continuously while displacing the 5' strand.

Imagine this as if the outer nicked strand has a gap, at the gap the 5' strand is continuously opened when the 3' end is elongated utilizing the inner intact strand as a template.

In the third step this opened 5' strand which happens to be displaced will be elongated by the formation of the Okazaki fragments, as the replication is proceeding in the 3' to 5' end this 5' displaced strand will act as the lagging strand and the replication on this strand will happen in the form of the Okazaki strand as in case of eukaryotes.

In the fourth step, this way the unnicked inner strand is replicated (think this of as opening a cello-tape from the roll, where you pull apart the tape from a roll, the tape that you are pulling is the nicked external strand whereas the inner intact roll is the inner unnicked strand), also the completely displaced external strand is also replicated by Okazaki model.

When the multiple rounds of rolling circle replication take place the resultant DNA is called a concatemer where the several full-length DNA sequences comprising of the entire genome of the phage will be linked end to end. At these end to end linking sites, there are nucleotides present, which are recognized by specialized enzymes that break the bonds at that particular site and then pack the single copy of the entire genome inside a phage.