Cosmids

Question 1

What are cosmids?

Answer 1

• Hybrid of a plasmid and bacteriophage and can carry DNA fragments of up to 45 kb.
• It is made up of a cos site (cohesive end) inserted into a plasmid
• Can replicate like plasmids but packaged like phage lambda

Question 2

What are cosmids made of?

Answer 2

• It should have a cos site, an origin of replication site, a selectable marker e.g. Ampicillin resistance (ampR) and recognition site for a restriction enzyme e.g. BamHI
• Colonies are formed on selective media e.g. Antibiotic containing media

Question 3

How to recognise a cosmid:

Answer 3

• Presence of selectable marker and a plasmid origin of replication (ori)
• Small in size to support DNA between 40-45 kb
• Cohesive ends of bacteriophage lambda (cos site)
• One or more unique restriction sites for cloning