Cloning Vectors

A cloning vector can replicate a desired gene in a host. This is the main goal of gene cloning.

An antibiotic is used as a selective pressure in cloning experiments. The cloning vector carries an antibiotic-resistance marker, and cells that receive the vector can survive and grow on a medium containing the matching antibiotic. Cells that do not receive the vector usually lack this resistance and are prevented from growing under the selection condition.

Thus the antibiotic helps researchers select cells carrying the marker, which is option A. It does not join DNA strands; DNA ligase performs that function. It does not determine the insert sequence, and it does not create the origin of replication, or ori. Antibiotic selection identifies suitable transformed cells after the transformation step.

Ori replication control can determine copy number. A low-copy ori can give fewer plasmid copies.

A high-copy vector produces many copies of the plasmid in each host cell. Therefore, plasmid isolation from the cells gives a higher DNA yield, which is useful for cloning and downstream analysis. Producing only one copy per cell is closer to a low-copy vector. Exam tip: high-copy refers to more plasmid copies per cell, not to more cloning sites.

The direct answer is A: a low-copy vector is useful when an insert may be unstable or toxic. Copy number means how many plasmid molecules are maintained in each host cell. A high-copy vector produces many copies, which is useful for obtaining much DNA, but a harmful or unstable insert may stress the host, be rearranged, or be lost. A low-copy vector keeps fewer copies, reducing the burden and sometimes improving stability and cell survival. Option A is correct. Option B is wrong because a low-copy vector still contains DNA; it is not used when no DNA is wanted. Option C is wrong and unsafe: burning a host has nothing to do with vector copy number. Option D concerns freezing water and is unrelated to molecular cloning. Low copy does not guarantee stability in every case, but it is a sensible strategy for toxic or unstable inserts. Memory cue: fewer copies, less cellular burden.

Simple plasmids are convenient for small inserts and BAC can carry large inserts. This is a broad comparison.

A screening marker like lacZ can distinguish recombinants and non-recombinants. This speeds up identification.

A cloning vector carries insert replicates in host and helps selection. These are easy exam functions.

A cloning vector carries foreign DNA into a host and helps make copies. It has parts like ori marker and cloning site.

In final revision treat vector as a DNA carrier and ori marker cloning site as core features. This is an exam-safe summary.

The lacZ reporter is commonly used in blue-white screening, a selection and screening method associated with cloning vectors. The lacZ gene can encode a portion of β-galactosidase. When it remains functional and the appropriate substrate is present, colonies produce a blue colour; insertion of foreign DNA into the relevant cloning region can disrupt lacZ activity, so recombinant colonies usually appear white. Thus option A is correct because lacZ helps distinguish probable recombinant colonies from non-recombinant vector colonies. The colour is a screening indication, not absolute proof, so promising colonies still require confirmation. Water purification, fat digestion and host drying are unrelated to reporter-gene function.

The governing concept is insertional inactivation used in recombinant DNA screening. In many cloning vectors, the multiple cloning site, or MCS, is placed within a reporter sequence such as the lacZ region. If foreign DNA is inserted at that site, the inserted sequence interrupts the reporter gene or its reading frame, so the reporter product may no longer be formed or may lose activity. This change can help distinguish recombinant clones from non-recombinant ones, for example through blue-white screening. Option A is correct. The other options describe unrelated effects: water availability, conversion of DNA into protein, and agar chemistry do not explain reporter-gene disruption.

The governing concept is sequence fidelity: the biological function of a cloned gene depends on its nucleotide sequence. A mutation may alter a codon, create a premature stop signal, change a regulatory region, or shift the reading frame. Any of these changes can modify or eliminate the encoded product and may reduce the usefulness of the clone. Thus option A is correct because a sequence mutation can change gene expression or the function of the resulting protein. Sequencing is therefore used to verify the intended insert before further work. A mutation does not automatically improve the host, convert agar into DNA, or remove the need for selection.

A restriction enzyme with one recognition site in a vector makes a unique cut and converts the circular plasmid into a linear molecule with defined ends. This controlled opening provides a predictable location where the desired insert can be joined, often using compatible sticky ends or suitable blunt ends. Because the vector is not cut at many unrelated positions, essential replication or selectable-marker regions are less likely to be disrupted, and the intended recombinant structure is easier to obtain and analyse. The enzyme does not automatically make protein, destroy all vector DNA, or stop electrophoresis. Therefore option C is correct.

A selectable marker is a gene that allows cells carrying the vector to be identified or selected, commonly through resistance to a particular selection condition. Before restriction digestion, the vector map should be examined to determine whether the chosen enzyme has a recognition site inside that marker. If the marker is cut and disrupted, transformed cells may no longer show the expected selectable phenotype, making it difficult or impossible to identify recombinant colonies using that selection. The enzyme is not being coloured, gel is not made sweet, and DNA is not converted into RNA by this check. Hence option A is correct.

The governing concept is protection of DNA integrity during visualization before recovery. Traditional fluorescent DNA stains are often viewed under ultraviolet light, but UV exposure can damage DNA by causing lesions or strand-related injury. Such damage may reduce the efficiency of downstream ligation, cloning, amplification, or transformation. A stain compatible with blue-light visualization allows the desired band to be located without relying on UV, so the recovered DNA may retain better biological activity. It does not convert DNA into protein, methylate an enzyme, or prevent the gel from running. Blue light is not an absolute guarantee of zero damage, but it can reduce UV-associated risk; therefore option A is the best answer.