What is the major use of BAC vector?

Correct answer: Large DNA insert cloning

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.”