Final answer:
Bacterial Artificial Chromosomes (BACs) are the most suitable vectors for cloning 40-45-kb DNA fragments due to their capacity to carry large DNA inserts and their use in creating genomic libraries.
Step-by-step explanation:
The type of bacterial vector best suited for cloning 40-45-kb DNA fragments is Bacterial Artificial Chromosomes (BACs). BACs are large plasmid-based vectors that can carry large DNA fragments of up to 300 kb, which means they are well-suited to carrying DNA fragments in the 40-45 kb range. On the other hand, cosmids can clone up to 40 Kb, phage vectors typically house less than cosmid capacities, and regular plasmid vectors are suitable for much smaller DNA fragments and typically accommodate up to 15 Kb.
For cloning large DNA fragments, such as those 40-45 kb in size, the use of vectors like BACs allows for the maintenance and manipulation of larger genomic segments, making them an essential tool in the construction of genomic libraries and the study of complex eukaryotic genomes.