A vector that can clone only a small DNA fragment is

The correct answer is plasmid.

In recombinant DNA technology, cloning vectors are vehicles used to carry foreign genetic material into another cell, where it can be replicated and/or expressed. Different vectors have different carrying capacities for foreign DNA inserts. Let us analyze the cloning capacities of the given options to determine which one can clone only a small DNA fragment:

1. Plasmids: Plasmids are small, circular, double-stranded extra-chromosomal DNA molecules found naturally in bacteria. They are widely used as vectors for introducing foreign DNA into recipient cells. However, they have a limited cloning capacity and can generally host inserts ranging in size from only 0.1 to about 10 kilobase pairs (kb).

2. Cosmids: Cosmids are hybrid vectors constructed by combining features of plasmids (such as the origin of replication and antibiotic resistance marker) and phage lambda (specifically the cohesive end site or cos site). They can accommodate larger DNA fragments, typically ranging from 30 to 45 kb.

3. Bacterial Artificial Chromosomes (BACs): BACs are DNA constructs based on a functional fertility plasmid (F-plasmid) of Escherichia coli. They are designed to clone large segments of DNA, typically with insert sizes ranging from 100 to 300 kb, and are crucial for genome sequencing projects.

4. Yeast Artificial Chromosomes (YACs): YACs are engineered chromosomes containing yeast centromeric, telomeric, and autonomously replicating sequences. They have the largest cloning capacity among the options listed, capable of carrying inserts from 100 kb to over 1000 kb (1 Mb).

Comparing the cloning capacities, plasmids have the smallest insert size limit (up to 10 kb). Therefore, plasmids are the vectors that can clone only a small DNA fragment.