Which of the following statements is true about plasmids and bacteriophages in the context of genetic engineering?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Plasmid is an autonomously replicating circular extra-chromosomal DNA.
A vector is a DNA molecule that carries a foreign gene into a host cell. Plasmids and bacteriophages are the common vectors because they can replicate inside bacteria on their own, independent of the bacterial chromosome. A plasmid is a small circle of DNA that lies outside the bacterial chromosome.
Option (1) Plasmids and bacteriophages are unable to replicate independently within bacterial cells. — Incorrect. The opposite is true. Plasmids and bacteriophages can replicate independently within bacterial cells. This is the very property that makes them useful as vectors.
Option (2) Vectors are not engineered to assist in the linking of foreign DNA with bacterial DNA. — Incorrect. The vectors used today are engineered so that foreign DNA links to them easily and recombinants can be picked out from non-recombinants.
Option (3) Plasmid is an autonomously replicating circular extra-chromosomal DNA. — Correct. A plasmid is an autonomously replicating circular extra-chromosomal DNA. It copies itself without the help of the chromosome, so any gene joined to it is copied too.
Option (4) Bacteriophages are always present in equal numbers to plasmids within bacterial cells. — Incorrect. Their numbers differ. Bacteriophages have a very high copy number per cell, while plasmids may have only one or two to 15-100 copies per cell. They are not present in equal numbers.
NCERT Extract:
Principles of Biotechnology – “… The construction of the first recombinant DNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid (autonomously replicating circular extra-chromosomal DNA) of Salmonella typhimurium.”
Cloning Vectors – “You know that plasmids and bacteriophages have the ability to replicate within bacterial cells independent of the control of chromosomal DNA. Bacteriophages because of their high number per cell, have very high copy numbers of their genome within the bacterial cells. Some plasmids may have only one or two copies per cell whereas others may have 15-100 copies per cell.”
Cloning Vectors – “… Vectors used at present, are engineered in such a way that they help easy linking of foreign DNA and selection of recombinants from non-recombinants.”