Cloning Vectors

pUC vectors are linked with lacZ-based blue-white screening. This is a simple example of recombinant screening.

T-DNA can be transferred into plant cells. Modified Ti plasmid is useful in plant biotechnology.

The direct answer is option A: cloning large DNA inserts. BAC means bacterial artificial chromosome. It is an engineered DNA vector based on bacterial chromosome elements and is maintained in a bacterial host. BACs are designed to carry much larger DNA fragments than ordinary small plasmids, often hundreds of thousands of base pairs. They are useful when a researcher needs to clone a long genomic region while maintaining it relatively stably. Option A is correct because large-insert cloning is the major purpose of a BAC vector. Option B is wrong because protein digestion is an enzymatic biochemical activity, not the main function of a DNA vector. Option C is wrong because leaf coloring is unrelated to BAC construction and cloning. Option D is wrong because water purification is not the role of a cloning vector. A BAC does not itself automatically produce the desired protein; it mainly carries and maintains a large DNA segment in a host. In an experiment, the cloned fragment may later be analyzed, sequenced, or transferred for further work. Memory cue: BAC = Bacterial Artificial Chromosome, so think “big DNA carrier.”

The governing concept is the carrying capacity of cloning vectors. YAC stands for yeast artificial chromosome and is designed to behave in yeast like a chromosome, containing elements such as a centromere, telomeres, and an origin needed for maintenance and replication. Because of this chromosome-like organization, a YAC can accommodate DNA fragments that are much larger than those usually carried by ordinary plasmids. Therefore option A is correct: YAC vectors are useful for cloning very large DNA inserts, including extensive genomic regions. The remaining options are not meaningful classes of DNA inserts; water, fat, and protein pigments are not cloned as DNA fragments in a YAC.

The direct answer is A: cos sites in a cosmid come from bacteriophage lambda. A cosmid is a plasmid-like cloning vector that carries the cos, or cohesive-end, sequences of lambda phage. These sequences are recognized during lambda-like packaging, which helps cosmids handle relatively large DNA inserts; after delivery, the cosmid behaves as a plasmid in a suitable host. Option A is correct because lambda phage supplies the cos sequences. Option B, chlorophyll, is a photosynthetic pigment and has no role in cosmid construction. Option C, starch, is a carbohydrate reserve and is not a vector sequence. Option D, hemoglobin, is an oxygen-carrying protein and is also unrelated. Do not confuse the cos site with the whole lambda genome: a cosmid usually combines lambda cos sequences with plasmid features. Memory cue: cosmid = cos from lambda plus plasmid.

A bacteriophage is a virus that infects bacteria. Modified phages can be used as cloning vectors.

BAC is useful for carrying large inserts. Simple plasmids are usually convenient for smaller inserts.

A non-transformed cell lacks the resistance marker. Therefore it cannot grow on an antibiotic plate.

Competent cells are prepared to take up external DNA. This is an important step in transformation.

Heat shock can be used to increase plasmid uptake in competent bacteria. It is part of a transformation method.

In electroporation an electric pulse can make the cell membrane temporarily permeable. This can increase DNA uptake.

An expected band can indicate insert presence and size. It is checked after restriction analysis.

Sequencing can confirm the correct sequence and direction of the insert. It is useful for final confirmation.

An empty vector can grow on antibiotic due to the marker. But screening is needed because it lacks the desired insert.

In self-ligation the vector joins its own ends. This does not form the desired recombinant clone.

Removing phosphate from vector ends can reduce self-ligation. This helps reduce background colonies.

Two different restriction ends can reduce vector self-ligation. This can improve desired insert cloning.

Insert capacity is related to the size of DNA fragment a vector can carry. Different vectors can have different capacities.

A plasmid can be convenient for small inserts and BAC can be useful for large inserts. Vector choice can depend on insert size.

In a gene library many DNA fragments can be cloned in different vectors. This is useful in genome study.

In a genomic library DNA fragments of the genome are kept in vectors. This allows study and storage.

In a cDNA library complementary DNA fragments made from expressed genes are cloned. The vector carries these fragments.

The direct answer is option A: a screening marker. A vector feature used in screening helps a researcher distinguish colonies that contain a recombinant construct from colonies that do not. For example, in lacZ-based blue-white screening, insertion of foreign DNA can disrupt lacZ activity. Under the proper indicator conditions, colonies with an intact lacZ region may appear blue, whereas colonies with the inserted fragment may appear white. The exact color result depends on the host and medium, but the principle is differential identification. Option A is correct because a screening marker gives a visible or testable difference between colony types. Option B is wrong because ori, the origin of replication, helps the vector replicate; it does not by itself distinguish recombinant from non-recombinant colonies. Option C is wrong because the host cell wall is not a vector screening feature. Option D is wrong because a water molecule has no role in identifying recombinant colonies. Do not confuse a selectable marker with a screening marker: selection allows certain cells to grow, while screening helps distinguish among survivors. Memory cue: selection answers “who grows?”; screening answers “which survivor is recombinant?”

The origin of replication (ori) is the specific DNA sequence from which replication of vector DNA begins. Therefore, it is directly associated with formation of vector copies and can influence copy number. A selectable marker helps identify transformed cells; it does not initiate replication. Exam tip: Link ori with replication, selectable markers with selection, and restriction sites with insertion of foreign DNA.

A selectable marker directly helps select transformed cells. Antibiotic resistance marker is a common example.