Question

Which can be used as a vector for transfer of DNA segments?
(a) Bacterium
(b) Plasmid
(c) Plasmodium
(d) Bacteriophage

• A. a, b and d
• B. b and d
• C. a and c
• D. a only

Answer 1

The correct option is B b and d

Vector
Vectors are the DNA molecules which can carry the foreign DNA fragment into the host cells for cloning. Examples include plasmids and bacteriophages.

Features of vector
The following are the common features of vector:

• Origin of replication (ori) - It is a specific nucleotide sequence that initiates replication. Due to this the vector can undergo self replication giving rise to multiple copies of the desired gene in the host cell.
• Selectable marker - It must be present for the identification of the transformants and non-transformants.
• Sites for the action of restriction endonucleases- Recognition sites for restriction enzymes help in the cleavage of the vector helping in inserting the gene of choice.
• Carrying capacity - They must carry DNA inserts into the host cells.

Bacterium
A bacterium as a whole cannot be used as a vector as it is a cell. It is used as a host in genetic engineering. Hosts are the organisms in which the genes of interest are expressed. For example, Escherichia coli acts as host and produces the human protein insulin.

Plasmid
Plasmids are extranuclear, circular, double stranded, DNA that can self-replicate, independent of the nuclear DNA of the cell. They can be used as a vector as it has all the features of an ideal vector.

$Plasmodium$
$Plasmodium$ is a sporozoan. It is the causative agent of malaria. It cannot be used as a vector as it is a cell, because a gene that is inserted directly into a cell usually does not function.

Bacteriophage
Bacteriophage is a virus that can infect a bacterium. The self replicating property of the bacteriophage and the property that allows it to easily infect the host cell allows it to be used as a vector.

Final answer:
d. b and d