Describe briefly the following:
$(a)$ Origin of replication
$(b)$ Bioreactors
$(c)$ Downstream processing

(N/A) Origin of Replication: This is a specific sequence in the genome from where replication starts,and any piece of $DNA$ when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked $DNA$. Thus,if one wants to recover many copies of the target $DNA$,it should be cloned in a vector whose origin supports a high copy number.
$(b)$ Bioreactor: Bioreactors are vessels in which raw materials are biologically converted into specific products by microbes,plant cells,animal cells,and/or their enzymes. The bioreactor provides optimum growth conditions (temperature,$pH$,substrate,salts,vitamins,oxygen) and facilitates the production of desired products. The most commonly used bioreactor is the stirred-tank type. $A$ stirred-tank bioreactor is usually a cylindrical vessel with a curved base to facilitate the mixing of contents. In a sparged stirred-tank bioreactor,sterile air bubbles are sparged. The stirrer facilitates mixing and oxygen availability throughout the bioreactor. It includes an agitator system,oxygen delivery system,foam control system,temperature control system,$pH$ control system,and sampling ports.
$(c)$ Downstream Processing: After the biosynthesis of the product,it is subjected to a series of processes collectively called downstream processing before it is marketed as a finished product. The two main processes are separation and purification. The product is then formulated with suitable preservatives. Such formulations must undergo clinical trials in the case of drugs.