A piece of alien DNA is transferred into a host organism. It is not linked to an origin of replication and does not become part of a chromosome of the host. What is most likely to happen to this DNA?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → It will not be able to multiply itself in the progeny cells of the host.
A piece of DNA that enters a new organism has no machinery of its own to copy itself. It multiplies only when it becomes part of a chromosome or is linked to a sequence called the origin of replication, the point from where replication starts. This is why, in genetic engineering, alien DNA is always joined to a vector that carries an ori.
Option (1) It will not be able to multiply itself in the progeny cells of the host. — Correct. With no ori and no place in a chromosome, the alien DNA is not copied when the host cells divide, so it is lost in the progeny cells.
Option (2) It will multiply and be inherited along with the host DNA. — Incorrect. The alien DNA is inherited along with the host DNA only after it gets integrated into a chromosome of the host. The question says this has not happened.
Option (3) It will be copied as many times as the host plasmids, since it uses the host DNA polymerase. — Incorrect. The copy number of a plasmid is set by its own origin of replication. DNA polymerase starts copying only at an ori, so DNA without an ori is not copied, however many plasmids the host has.
Option (4) It will make multiple copies once the host cells are given a heat shock. — Incorrect. A heat shock (ice, then a brief warm spell, then ice again) helps competent bacteria take up recombinant DNA. It does not make DNA multiply inside the cell.
NCERT Extract:
Principles of Biotechnology – “… Do you know the likely fate of a piece of DNA, which is somehow transferred into an alien organism? Most likely, this piece of DNA would not be able to multiply itself in the progeny cells of the organism. But, when it gets integrated into the genome of the recipient, it may multiply and be inherited along with the host DNA. This is because the alien piece of DNA has become part of a chromosome, which has the ability to replicate. In a chromosome there is a specific DNA sequence called the origin of replication, which is responsible for initiating replication. Therefore, for the multiplication of any alien piece of DNA in an organism it needs to be a part of a chromosome(s) which has a specific sequence known as ‘origin of replication’.”