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In genetic engineering, the antibiotics are used
A
To keep the cultures free of infection
B
As selectable markers
C
To select healthy vectors
D
As sequences from where replication starts
Detailed Solution
A cloning vector carries a selectable marker, which helps to identify and eliminate the non-transformants and selectively permit the growth of the transformants.
Genes coding for resistance to antibiotics such as ampicillin, tetracycline, chloramphenicol and kanamycin are the usual selectable markers (e.g., $amp^R$ and $tet^R$ in pBR322).
When the cells are plated on a medium containing the antibiotic, only the cells that have taken up the plasmid survive, so the transformants are selected.
The sequence from where replication starts is the origin of replication (ori), which is a different feature of the vector.
Hence, in genetic engineering, antibiotics are used as selectable markers.
Note: the coaching key in the source PDF marks 'To select healthy vectors'; the NCERT description (and the official answer) is that antibiotic resistance acts as a selectable marker, so that is taken as correct here.
Genes coding for resistance to antibiotics such as ampicillin, tetracycline, chloramphenicol and kanamycin are the usual selectable markers (e.g., $amp^R$ and $tet^R$ in pBR322).
When the cells are plated on a medium containing the antibiotic, only the cells that have taken up the plasmid survive, so the transformants are selected.
The sequence from where replication starts is the origin of replication (ori), which is a different feature of the vector.
Hence, in genetic engineering, antibiotics are used as selectable markers.
Note: the coaching key in the source PDF marks 'To select healthy vectors'; the NCERT description (and the official answer) is that antibiotic resistance acts as a selectable marker, so that is taken as correct here.