Question

Assertion: HAT medium is used for the selection of fused (hybridoma ) cells
Reason: Unfused myeloma cells cannot grow because they lack HGPRT and thus cannot replicate their DNA
A. Both assertion and reason are correct and the reason is the correct explanation for assertion
B. Both assertion and reason are correct but the reason is not the correct explanation for assertion
C. The assertion is correct but the reason is incorrect
D. Both assertion and reason are incorrect

Answer 1

Hint: HAT medium is a selective medium used for mammalian cell culture. It uses the method of inhibition mechanism by aminopterin which inhibits dihydrofolate reductase with hypoxanthine and thymidine which are essential intermediates of DNA synthesis. This mechanism is also used in monoclonal antibody production.

Complete answer: HAT(hypoxanthine–aminopterin–thymidine) medium is used for the selection of fused hybridoma cells in the process of monoclonal antibody production. After fusion of myelomas and spleen cells, HAT medium is used.
Mainly 3 types of cells are present in the fusion process :
a. Unfused spleen cells: These cells do not replicate in the culture and are selected on this basis.
b. Unfused myelomas: These cells are selected using HAT medium. This medium contains aminopterin which blocks de novo DNA nucleotide synthesis in these cells. These cells are not able to follow the other pathway known as the salvage pathway as these cells are deficient in an enzyme known as HGPRT which is an essential requirement for the salvage pathway. As a result, they are not able to replicate in the culture medium.
c. Hybridoma cells: These cells survive in the medium as they have enzyme HGPRT inherited from spleen cells. Hence they successfully alternate to salvage pathway in case de novo synthesis is blocked by aminopterin.
Hence option A is correct, i.e., Both assertion and reason are correct and the assertion is the correct explanation for reason.

Note: HAT culture method is based on the trick that aminopterin present in the culture medium blocks the de-novo synthesis of DNA but hypoxanthine and thymidine help in preventing this blockage by an alternate pathway known as salvage pathway provided cells have an enzyme named HGPRT. This method also helps in the selection of cells with working HGPRT enzymes.