Correct option is A
The correct answer is: (1)
Explanation: In the construction of a knock-in cassette for gene introduction, two markers are often used for the selection of viable clones. In this case, G418 (neomycin analog) and ganciclovir are the selection agents. The cassette structure should contain a neomycin resistance gene (neo) and a thymidine kinase gene (tk).
SoH represents the sequence of homology necessary for integration into the host genome.
Gol is the gene of interest, in this case, the β-casein gene.
neo is the neomycin resistance gene, used to select clones that have integrated the cassette.
tk is the thymidine kinase gene, which is used for negative selection (to kill cells that do not have the integrated cassette when exposed to ganciclovir).
In the correctly designed cassette, the neomycin resistance gene (neo) and the thymidine kinase gene (tk) should be arranged in a way that the viable clones can be selected using G418 for positive selection and ganciclovir for negative selection. The cassette in option (1) places neo and tk in the correct order for proper selection.
Information Booster:
neo (neomycin resistance) allows positive selection of the clones with the cassette integrated into their genome.
tk (thymidine kinase) enables negative selection, killing cells that do not have the cassette.
The sequence of homology (SoH) ensures the proper integration of the gene into the target genome.
Gol represents the gene of interest that is being introduced, in this case, the β-casein gene.
The combination of neo and tk is a common double-marker system for efficient cloning in animal cells.
The cassette ensures that only the cells with successful integration will survive and propagate.
