Cloning Vectors

The governing concept is the use of a cloning vector in recombinant DNA technology. A vector is a DNA molecule, commonly a bacterial plasmid or a modified viral DNA, that carries a selected gene into a suitable host cell. It usually contains an origin of replication and may include selectable markers, so the inserted DNA can be copied and the cells containing it can be identified. Therefore, option A is correct: the vector transports the desired gene into the host and helps maintain or replicate it. Option B describes proteolytic enzymes, option C is unrelated to gene transfer, and option D describes a function associated with photosynthetic pigments rather than vectors.

The governing concept is the nature and location of plasmids. A plasmid is a small, usually circular, double-stranded DNA molecule that exists separately from the main bacterial chromosome. It commonly occurs in bacteria and can replicate independently when the host provides suitable conditions. Some plasmid-like elements occur in other organisms, but the standard school-level answer is bacterial cells. Thus option A is correct. Mature human red blood cells lack a nucleus and do not normally contain plasmids; a viral coat is a protein structure, not the usual location of plasmid DNA; and the outside of a plant root is not a cellular location for plasmids. Plasmids are also important cloning vectors.

The governing concept is the usefulness of plasmids as cloning vectors. Many bacterial plasmids possess their own origin of replication, allowing them to be copied separately from the host chromosome. This independent replication can increase the number of copies of an inserted gene and makes recovery or expression of that gene easier. Consequently, option A is correct, although the exact copy number depends on the particular plasmid and host conditions. Option B is false because a plasmid is itself DNA and can carry an inserted DNA fragment. Option C is false because plasmids are especially common in bacteria, not only humans, and option D is false because a plasmid does not invariably kill its host cell.

A cloning vector is a DNA molecule used to carry and replicate a foreign DNA fragment inside a suitable host cell. A good vector must contain an origin of replication, or ori. This is the specific sequence from which DNA replication begins, allowing the inserted DNA and the vector to be copied in the host. It also commonly contains selectable markers and suitable cloning sites, but ori is the feature asked here. Option A is therefore correct. Leaf colour and water storage are unrelated biological functions, and a vector must contain DNA rather than lack it.

A selectable marker is a gene that gives transformed host cells a distinguishable survival or growth characteristic, often resistance to a particular antibiotic. After transformation, cells are grown under selective conditions so that cells carrying the plasmid can be identified or enriched. Without such a marker, transformed and non-transformed cells may look alike, making selection and screening much more difficult, though other molecular tests could still be used. Option A is correct. The absence of a marker does not make DNA burn, remove cellular water or automatically increase the physical size of a gene.

A cloning vector is a carrier that delivers desired DNA into a host. For exams remember it as a gene carrier.

Ori is the origin of replication. Without ori the vector cannot properly make copies in the host.

A selectable marker helps identify cells that have received the vector. Remember it as a selection tool.

A cloning site is the place in a vector where foreign DNA is inserted. Treat it as the insert entry point.

Plasmids are small circular DNA molecules and are very useful as cloning vectors. Remember plasmid as a common vector.

Plasmids are small extrachromosomal DNA molecules found in bacteria and are usually double-stranded circular DNA. Therefore, option B is correct. Linear DNA may occur in some organisms or special cases, but the usual plasmid form is circular. Exam tip: Remember plasmids as small, circular DNA molecules that can replicate independently.

A cloning vector must make multiple copies of itself inside the host cell so that the foreign DNA inserted into it is also replicated. This requires an origin of replication (ori). Breaking down proteins, performing photosynthesis, or freezing water are not functions of a cloning vector. Exam tip: Remember ori, a selectable marker, and a cloning site as key vector features.

A small vector is easier to isolate and introduce into a host. In exams treat small size as a useful feature.

An antibiotic resistance gene helps select transformed cells. Therefore it is an example of a selectable marker.

In transformation recombinant or plasmid DNA is introduced into a host cell. It is an important step in cloning workflow.

The foreign or desired DNA fragment is called insert DNA. It can be joined into a vector by ligase.

A multiple cloning site contains many restriction enzyme sites. It increases options for inserting DNA.

A selectable marker helps distinguish transformed and non-transformed cells. Without it selection becomes difficult.

Ti plasmid is linked with Agrobacterium and is useful for gene transfer in plants. Remember it as a plant vector.

Agrobacterium is known for its ability to transfer DNA into plants. Its Ti plasmid is useful in biotechnology.

Copy number tells how many plasmid copies can be present in one host cell. It can affect DNA yield.

The direct answer is option A: pBR322. A cloning vector is a DNA molecule used to carry a desired DNA fragment into a host cell and allow its maintenance or copying. A useful plasmid vector generally has an origin of replication, selectable marker genes, and suitable cloning sites. pBR322 is a well-known bacterial plasmid cloning vector containing an origin of replication and selectable markers such as antibiotic-resistance genes. Option A is correct because it is an actual plasmid vector used in recombinant-DNA work. Option B, amylase, is an enzyme that digests starch; it is a protein product, not a DNA carrier. Option C, insulin, is a hormone or protein, not a vector. Option D, chlorophyll, is a photosynthetic pigment, not a DNA molecule that carries an insert. A common mistake is to confuse any biotechnology-related substance with a vector. The word vector means a carrier, so in cloning it must be a suitable nucleic-acid molecule, commonly a plasmid, virus, BAC, or related construct. Memory cue: pBR322 is a classic plasmid carrier with replication and selection features.

pUC is a plasmid cloning vector known for its high copy number. Many copies of the plasmid can be present in a bacterial cell, making isolation of recombinant DNA easier. Fat digestion is an enzyme-related function, while light absorption and cell-wall formation are not functions of a pUC vector. Exam tip: remember pUC as a high-copy-number plasmid cloning vector.

When an insert disrupts a marker gene and makes it inactive it is called insertional inactivation. It is useful in screening.

A restriction site allows the vector to be cut at a controlled position. This makes insert DNA joining easier.