Why are there two origin of replications in plasmids - (Dec/05/2014 )

Hi,

I'm new here. And I keep looking around for a specific answer for why there two origin of replications in plasmids? 

Thank you!

P.S. I know this might be such a simple question to answer.

What plasmids are you thinking of? Some have multiple replication origins which are functional in one or another species (such as E. coli/yeast shuttle plasmids). Some, such as the rolling circle plasmids, have a positive origin, which copies the plasmid producing a long single stranded DNA strand, and a separate negative origin, which initiates to produce the complementary strand. You'll have to be more specific about the plasmid for this to have a solid answer.

Sorry. For example, pet28a has two origin of replication (f1, PBR3286)

The pBR322 origin is active in replicating the plasmid under normal circumstances. The F1 origin is only active if the cell is co-transformed with a phage, such as the helper phage M13K07. Then, the F1 origin initiates a single stranded copy of the plasmid, which can be packaged into M13 phage particles. This is pretty much unused today, but was very important in the early days of sequencing, when the sequencing protocols required single stranded DNA substrates.