BAC ( Bacterial artificial chromosome)

BAC are simply plasmids desigened for the cloing of very segments of DNA. They generally includes selecatable markers such as chloramphenicol resistance (MR), as well as a very replication origin (ori) that maintains the plasmid at one or two copies per cell. DNA framents several hundred base pairs in the lenght are cloned into the BAC vectors. The bacterial used as host for recombinnat BAC have mutation that comprises the structure of the bacteria cell wall, facilitating the uptake of these large DNA molecules.

COSMID

The way in which DNA is replicated is of particular interest in the development of larger insert cloning vectors termed cosmid. Cosmids are plasmids that contain minimum of 250 bp of DNA consisting of the cos site (sequnence yielding cohesive ends.) these are especially useful for the analysis of highly complex genome mapping projects.
Cosmid vectors have been constructed that incorporate the cos .sites from bacteriophage lemda and also the essential feature of a plasmid , such as the plasmid origin of replication , a gen for drug resistance and several unique restriction sites for insertion of the DNA to be cloned when a cosmid , the produducts will include cocatamers of alternating cosmid vector and inserts. The only requirement for a length of DNA to be packed into viral heads, therefore, is that it should contain cos site spaced the correct distance apart in practice tis spacing can range between 37 to 52 kb. Such DNA can be packed in vitro if is very small, inserts of about 40kb in lenght will be most readily packaged. Once inside the cell, the DNA recircularrises through its , and from, then own wards behave exactly like a plasmid.

Problems in large scale culture of plnt cells

Large acale culture of plant cells for commercial applications like biochemical, artificail seed etc. Producation presents serveral problems which are summarised below.
1. Plant cells have much slower growth rates than bacteria anf fungi, therfore, larger reactores and longer fermentation times are necessary.
2. The long fermentation time increases the risk contamination.
3. Plant cells are rather sensitive to shear. Therefor, fementers with conventional mechanical stirring are not suitable for their culture. Bioreactors having specially designed mechanical stirrers airlift fermenters are far more suitable.