Origins of replication Every plasmid has one essential feature:
the origin of replication. To make a circular, double stranded
DNA replicate in E. coli, the origin of replication is the only
thing you need. If that’s all you have, you’d call the thing a
“cryptic” plasmid—which just means it replicates in the cell but
has no selectable marker. However, there are many different
origin types, and different ones are used for different things.
Rather than referring specifically to the origin, most people
will refer to “types of plasmid”. Usually, what they are talking
about is just what origin "part" is present in the plasmid, but
some "types of plasmid" are a combination of parts needed for
replication. I'll go through the most common ones here. A really
important concept in thinking about plasmids is compatibility
groups. In a nutshell, if you put two different plasmids into
the same cell with the same origin of replication, they will
compete for replication resources to their detriment. Sometimes
this will resu