Replication Fork


Origin of Origin of replication replication

the origin used during transfer. All plasmids must have a vegetative origin since they must all divide to survive. But only those plasmids that can transfer themselves have a special transfer origin.

The relationship between certain plasmids and viruses is illustrated by their DNA replication mechanisms. Rolling circle replication is not only used by transferable plas-mids but also by many viruses (Fig. 16.07). Some manufacture many double stranded molecules of virus DNA. These viruses use the dangling strand as a template to synthesize a new strand of DNA. They just keep rolling and synthesizing and end up with a long linear double stranded DNA many times the length of the original DNA circle. This is chopped into unit genome lengths and packaged into virus particles. (Some of these viruses convert the DNA into circles before packaging, whereas others package linear DNA and only circularize their DNA after infecting a new cell when it is time to replicate again.)

Other viruses contain single stranded DNA. These viruses leave the dangling strand unpaired. They continue rolling and end up with a long linear single-stranded

Nick one strand

Unroll dna

FIGURE 16.06 Rolling Circle Replication

During rolling circle replication, one strand of the plasmid DNA is nicked, and the broken strand (pink) separates from the circular strand (purple). The gap left by the separation is filled in with new DNA starting at the origin of replication (green strand). The newly synthesized DNA keeps displacing the linear strand until the circular strand is completely replicated. The linear single-stranded piece is fully "unrolled" in the process.

New DNA synthesized

New DNA synthesized

Complete new strand

Old strand fully unrolled

DNA (Fig. 16.07B).This is cut into unit genome lengths and packaged as before. When these viruses infect a new cell, they synthesize the opposite strand, so converting their single strand to a double stranded DNA molecule.

Antisense RNA is involved in regulating the copy number of many plasmids.

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