Re-culturing empty plasmid - (Dec/11/2012 )

Greetings

I am new in molecular biology lab. I was given an ecoli + empty pgsa plasmid and my sv ask me to reculture them.

My questions are:

1) What is an empty plasmid?
2) When I reculture them, my sv ask me to add antibiotics. What is the purpose of doing that?

Thank you for all the brilliant answers!

Empty plasmid is just circular plasmid, without our interest gene. Check the antibiotic of the plasmid. For example, pGEMT vector is carrying ampicilin resistance gene. To cultivate this plasmid, you have to add 50 or 100 ug/ml ampicillin in the medium for plasmid transformed E. coli. Otherwise, the plasmid will not replicate. To cultivate the E. coli with plasmid we have to add appropriate anitibiotic in the medium.