I've been learning about PCR and plasmids. I understand that the reason for having both a forward and a reverse primer is to extract and amplify the specific piece of DNA between these two primers.
What I'm struggling to understand is that why after the first replication, a fully circular DNA strand isn't formed and hence, why isn't a fully circular DNA strand formed for each replication.
For the following diagram , let the strands be labelled strand 1, strand 2, strand 3, strand 4 from top to bottom. (Diagram from Lehninger Principle of Biochemistry, 5th Ed).
For example, in the diagram here. When cloning a plasmid I assume strand 1 is fully circular, so strand 2 (which was transcribed from strand 1) should also be fully circular and in the diagrams it should show DNA to the left of the site of interest.
Does the transcription stop somewhere along the line, preventing it from copying the whole strand? Or does the strand never join up into a circle, so that the DNA that would appear to the left of strand 2 has been copied, but doesn't join up to the site of interest so wasn't included in the diagram.
Thanks for your help.