During gamete formation, the homologous pair of autosomes gets separated from each other and moves to different gametes, so that each gamete receives haploid set of chromosomes.

The most commonly used bioreactors are of stirring type. A stirred - tank reactors is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents. The stirrer facilitates even mixing and oxygen availability throughout the bioreactor. The bioreactor has an agitator system, an oxygen delivery system and a foam control system, a temperature control system. pH control system and sampling ports so that small volumes of the culture can be withdrawn periodically.

A nematode Meloidogyne incognita infects the roots of tobacco plants and causes a great reduction in yield. A novel strategy was adopted to prevent this infestation which was based on the process of RNA interference (RNAi).

Using Agrobacterium vectors, nematode-specific genes were introduced into the host plants. The introduction of DNA was such that it produced both sense and antisense RNA in the host cells. These two RNA’s being complementary to each other formed a double-stranded (dsRNA) that initiated RNAi and thus, silenced specific mRNA of the nematode.

The consequence was that the parasite could not survive in a transgenic host expressing specific interfering RNA. The transgenic plant, therefore, got itself protected from the parasite.

Small volume cultures cannot yield appreciable quantities of products. To produce in large quantities, the development of bioreactors, where large volumes (100-100 litres) of culture can be processed, was required. Thus, bioreactors can be thought of as vessels in which raw material are biologically converted into specific products, individual enzymes, etc., using the microbial plant, animal or human cells.

(a) Restriction enzymes :

1. Restriction enzymes belong to the class of enzymes nucleases which breaks nucleic acids by cleaving their phosphodiester bonds.
2. Since Restriction endonucleases cut DNA at specific recognition site, they are used to cut the donor DNA to isolate the desired gene.
3. The desired gene has sticky ends which can be easily ligated to cloning vector cut by same restriction enzymes having complementary sticky ends to form recombinant DNA
4. An example is EcoR1 which is obtained from E.coli bacteria “R” strain which cuts DNA at specific palindromic Recognition site.
   5‘ GAATTC 3‘
   3‘ CTTAAG 5‘

(b) Plasmids :

1. Plasmids are autonomous, extrachromosomal circular double-stranded DNA of bacteria
2. Since they are small and self replicating,they are used as cloning vectors in genetic engineering.
3. Some plasmids have antibiotic resistance genes which can be used as marker genes to identify recombinant plasmids from non-recombinant ones.
4. The plasmids are cut and ligated with desired genes and transformed into a host cell for amplification to obtain the desired products.
5. An example of artificially modified plasmids is pBR322 ( constructed by Bolivar and Rodriguez) or pUC (constructed at university at California).