high-capacity vectors (cosmid, BAC, YAC)
/ BAC = 'back'; YAC = 'yack' /
An ordinary plasmid is like a small envelope — fine for a single letter, but useless for shipping a thick manuscript. When biologists needed to clone DNA pieces far larger than one gene — to assemble whole genomes — they built bigger 'envelopes'. High-capacity vectors are vectors engineered to carry inserts much larger than a plasmid can hold.
There is a ladder of capacity. A plasmid holds up to roughly 10 thousand base pairs. A bacteriophage (lambda) vector replaces part of the virus's own genome with your DNA and can package about 20 thousand. A cosmid combines a plasmid with the phage's DNA-packaging signal (the cos site) and reaches roughly 45 thousand. A BAC (bacterial artificial chromosome) is built from the bacterial F-plasmid and carries up to about 300 thousand base pairs, copied stably in E. coli. A YAC (yeast artificial chromosome) goes furthest — it is built from the essential parts of a real yeast chromosome (a centromere, two telomeres, and an origin) and can hold a million base pairs or more, replicating inside yeast cells like a genuine chromosome.
Capacity mattered enormously for the Human Genome Project. To sequence a 3-billion-base genome you must first break it into manageable, overlapping chunks and clone each in a vector; the bigger each chunk, the fewer clones you need and the easier it is to piece the genome back together. BACs in particular became the backbone of genome libraries. The trade-off is that larger inserts are harder to handle and YACs especially are prone to instability and rearrangement, which is why BACs, being more stable, became the genome-mapping favorite.
To build a human genomic library, scientists shear the genome into fragments of roughly 150 thousand base pairs and clone each into a BAC. A few tens of thousands of BAC clones then cover the whole 3-billion-base genome, versus the millions of plasmid clones it would otherwise take.
Bigger inserts mean fewer clones to cover a whole genome.
Bigger is not always better: YACs can carry the most DNA but tend to recombine and lose pieces, so for reliable, stable cloning of large fragments BACs are usually preferred.