Question

Which of the following statement is/are incorrect?

• A. Cosmids contain gene coding for viral proteins.
• B. Cosmids replicate like plasmids.
• C. They have cos sites with 12 nucleotide bases which prevent them from circularising.
• D. Cosmids have antibiotic resistance marker genes.

Answer 1

Answer: (A), (C)

The correct options are
A Cosmids contain gene coding for viral proteins.
C They have cos sites with 12 nucleotide bases which prevent them from circularising.

Cosmid vectors are a fusion of plasmids and lambda phage DNA. Phage genome has cos sites which help it to circularise inside host bacterial cell. Leaving the cos sites intact, most of the viral genes are removed from the phage genome and a plasmid is introduced within the cos site. This plasmid contains an antibiotic resistance marker gene and ori site. Cosmids can be used to clone DNA inserts up to 45kb. The cosmid itself however doesn't have an antibiotic resistance gene or an ori and hence cannot self-replicate like a plasmid.