Biotechnology: Principles and Processes

A stirred-tank reactor is usually cylindrical or has a curved base to facilitate the mixing of the reactor contents.
The stirrer facilitates even mixing and oxygen availability throughout the bioreactor; alternatively, air can be bubbled through the reactor (sparged stirred-tank).
The bioreactor also has an oxygen delivery system, a foam control system, a temperature control system, a pH control system and sampling ports.
Purification and addition of preservatives are parts of downstream processing, done after the bioreactor stage.
Hence stirred-tank bioreactors are designed for availability of oxygen throughout the process.

Restriction enzymes belong to the class of enzymes called nucleases, which are of two kinds: exonucleases and endonucleases.
Exonucleases remove nucleotides from the ends of DNA.
Endonucleases make cuts at specific positions within the DNA.
Each restriction endonuclease inspects the length of the DNA, recognises a specific palindromic nucleotide sequence, binds to it and cuts each strand of the double helix at specific points in the sugar-phosphate backbone.
Hence restriction endonucleases make cuts at specific positions within the DNA molecule.

A palindromic sequence in DNA is a sequence of base pairs that reads the same on the two strands when the orientation of reading is kept the same (5′ → 3′ on both).
5′ GAATTC 3′ / 3′ CTTAAG 5′: the upper strand read 5′ → 3′ is GAATTC, and the lower strand read 5′ → 3′ is also GAATTC. The two strands are complementary and the sequence is a palindrome.
This is the recognition sequence of EcoRI, which cuts between G and A on each strand, a little away from the centre of the palindrome, producing sticky ends.
In the other three pairs the two strands are not even complementary to each other, so they are not palindromes.
Hence the sequence that can be cut is 5′ GAATTC 3′; 3′ CTTAAG 5′.

Retroviruses in animals have the natural ability to transform normal cells into cancerous cells by inserting their genetic material into the host genome.
This ability has been put to use: retroviruses have been disarmed (their disease-causing genes removed) and are now used to deliver desirable genes into animal cells.
In the same way, the Ti plasmid of $Agrobacterium\ tumefaciens$ is used for plants.
Baculovirus is an insect pathogen used as a biocontrol agent, $Salmonella\ typhimurium$ is a bacterium and $Rhizopus\ nigricans$ is a fungus; they are not used as cloning vectors for higher organisms.
Hence the vector is the retrovirus.