Cloning Vectors

An empty vector may survive selection but lacks the desired gene. Therefore screening is needed.

The origin of replication (ori) of a cloning vector must be recognized by the host cell's replication machinery. Only then can the vector, along with its inserted DNA, replicate and produce many copies inside the host. Conversion of a vector into a protein is not its role; that idea is associated with expression of a gene. Exam tip: remember that ori enables replication, whereas a selectable marker helps identify transformed cells.

The direct answer is A: a cell prepared to take up external DNA. A competent cell has been treated or selected so that DNA from outside can enter it. In transformation, the external plasmid or recombinant vector is brought near the cell, and suitable conditions help it cross the cell envelope. The cell can then maintain or express the introduced DNA, depending on the vector and host. A is correct. B is wrong because competence does not mean that a cell only digests protein. C is wrong because making water is not the definition of competence. D is wrong because colouring fat has no connection with DNA uptake. Competence may be natural in some microorganisms or induced in laboratory cells; it does not mean that every cell automatically accepts every DNA molecule. Exam cue: competent = “ready for DNA entry,” and transformation = uptake of external DNA.

Insert direction can affect gene expression or analysis. Directional cloning helps control orientation.

Two different restriction enzymes generate different, non-compatible sticky ends at the two ends of the vector and insert. Therefore, the insert can ligate into the vector in only one orientation, which is the main purpose of directional cloning. If identical sticky ends were used, the insert could enter in either orientation. Exam tip: Link the use of two enzymes with a fixed insert orientation in directional cloning.

To make a recombinant vector the insert is joined with the vector. DNA ligase helps in this joining.

An antibiotic resistance gene allows bacteria to grow on the related antibiotic. This selects transformed cells.

The direct answer is D: it is always a starch-digesting enzyme. The question asks for the incorrect statement. A cloning vector is a DNA carrier used to transport foreign DNA into a host and often to help it replicate or be selected. It can therefore carry foreign DNA, so A is correct. It can contain an ori, the origin of replication, so B is correct. It can contain a selectable marker, such as a gene allowing identification of cells that received the vector, so C is correct. D is incorrect because a vector is a DNA molecule or DNA-based carrier, not always an enzyme; it is certainly not defined as a starch-digesting enzyme. Some vectors may carry genes for various proteins, but that does not make the vector itself that enzyme. Memory cue: vector = carrier, ori = copying, marker = selection; it is not automatically a digestive enzyme.

The direct answer is A: carrying desired DNA into the host. Gene cloning requires a DNA fragment to enter a suitable host cell and usually to be copied there. A cloning vector acts like a biological vehicle: the desired gene is inserted into it, the recombinant vector is introduced into the host, and the vector’s features help the DNA remain or replicate. Thus, without a vector, carrying the desired DNA into the host and obtaining many copies becomes difficult in the context of this question. A is correct. B, digesting protein, is an enzymatic activity and is not the main role of a cloning vector. C, freezing water, is a physical process unrelated to gene cloning. D, colouring fat, is also unrelated and does not help DNA transfer. Some DNA can enter cells by other methods, so “without a vector” does not mean transfer is absolutely impossible in every technology; it means the normal vector-based cloning task is made difficult. Exam cue: vector means a vehicle for the gene.

Ori (origin of replication) is the sequence from which vector replication begins. Its nature and replication-control mechanism influence the number of vector copies maintained in a cell. A selectable marker helps identify transformed cells, but it does not directly control copy number. Exam tip: associate Ori in a cloning vector with replication and copy number.

YAC stands for yeast artificial chromosome. It contains functional yeast chromosome elements, such as a centromere, telomere sequences, and an origin of replication, allowing it to behave like a small artificial chromosome in yeast. Because of this chromosome-like organization, YACs can accommodate very large DNA inserts, larger than those normally carried by ordinary plasmids. Option A is correct. pUC plasmids are typically small, high-copy cloning vectors and are not designed for very large fragments. Pepsin and amylase are digestive enzymes, not vectors, so they cannot replicate or maintain a DNA insert.

Ti plasmid is useful with Agrobacterium in plant transformation. Remember it as a plant gene transfer vector.

Ori is for replication marker is for selection and cloning site is for insertion. These are basic parts of a vector.

A cloning vector is a DNA molecule that carries a foreign DNA fragment into a host cell and allows it to replicate there. Plasmids and bacteriophages are common examples. A restriction enzyme cuts DNA, so it is not a vector. Exam tip: identify a vector by its role in delivering foreign DNA into a host and enabling replication.

After a recombinant vector is made it can be introduced into a host cell. This is called transformation.

Cloning vectors are DNA carriers that help foreign DNA replicate in a host. Remember their core parts with functions.

Ori helps replication but a marker is needed for selection. Without a marker transformed cells are difficult to identify.

The ori (origin of replication) is the sequence from which replication of a vector begins. A marker gene may help in selecting transformed cells, but without an ori the vector cannot replicate and be maintained stably in the host cell. Therefore, option B is correct. A marker gene cannot perform the function of an ori. Exam tip: Remember the key vector components—ori for replication, a selectable marker for selection, and a cloning site for insertion of DNA.

When a foreign DNA insert is introduced within a marker gene, it can interrupt the normal gene sequence. Consequently, the marker gene may no longer function; this is called insertional inactivation. For example, insertion into the lacZ gene can help distinguish recombinant colonies in colour-based screening. Exam tip: if an insert enters the middle of a marker gene, think of gene disruption or inactivation.

In blue-white selection, insertion of foreign DNA within lacZ can disrupt lacZ function. As a result, β-galactosidase is not produced and X-gal is not converted into a blue product, so the colony appears white. In contrast, an active lacZ gene generally produces a blue colony. Exam tip: treat a white colony as a potential recombinant and remember that further screening or analysis may be needed for confirmation.

In blue-white selection, an intact lacZα region allows production of functional β-galactosidase, which cleaves X-gal and produces a blue colony. Thus, a blue colony generally indicates that the target DNA insert has not been inserted into the lacZ region, so the vector is non-recombinant. In contrast, an insert disrupts lacZα and usually gives a white colony. Exam tip: blue colony = active lacZ = usually no insert.

Antibiotic growth shows vector uptake. Screening or analysis may be needed to confirm the insert.

ampR gives ampicillin resistance. It allows selection of transformed cells on ampicillin medium.

tetR is a selectable marker linked with tetracycline resistance. Remember it among pBR322 marker genes.

The governing concept is plasmid copy number, which describes how many copies of a plasmid are maintained in each host cell. pUC vectors generally have a high-copy-number origin of replication, so one bacterial cell can contain many copies of the vector rather than only one or a few. When a culture of such cells is grown and the plasmid is isolated, the total yield of vector DNA is usually higher, provided the insert and host remain stable. Thus option A is correct: high copy number can provide more plasmid DNA for analysis, restriction digestion, sequencing, or further cloning. It does not mean that the host loses all DNA, digests protein, or freezes water.