Question

Why is the host cell grown first and foreign protein gets expressed later?

Answer 1

Hint: Host cell is defined to be a living cell which can be invaded by or capable of being invaded by a foreign pathogen. A foreign protein on the other hand is produced externally and if introduced can cause harm to the host cell.

Complete answer:
Host cells that are utilised for carrying out experiments for characterization of pathogens or determining expression of proteins are grown first. The growth of the host cells takes place in a suitable medium that allows the host cell to grow without any limitation. After the growth has taken place the foreign protein, whose expression needs to be determined, is introduced into the host cell. The introduction of the foreign protein takes place after the host cell has been grown because the foreign protein can be harmful to the host cell and can cause its death or improper functioning. So the foreign protein is placed on plasmids known as expression vectors which have promoters that can be turned off or on according to the requirement. This is to make sure that the host is not killed because if the host dies we will not be able to determine and characterize the foreign protein. The foreign protein is then monitored and important characterization, classification and determination of their function then takes place.

Note:
The most common host taken for foreign protein expression is Escherichia coli because it grows rapidly which causes high levels of protein expression and also because introduction of DNA into E.coli is easier compared to other microorganisms.