Question

dh5α is a “cloning strain.” It is used to grow up plasmids (extra-chromosomal DNA) you want to use for other experiments. Why would you use dh5α and not K12 for this? (Note: all cloning strains are Rec- for this reason)

Answer 1

DH5alpha is a strain that is preferred for cloning genes using plasmids. It is a modified strain obtained from K12 strain of E.coli. It is preferred over K12 strain because it lacks the restriction modification system. Restriction/modification systems are known to prevent propagation of plasmids in wild type strains such as K12. This system cleaves any foreign DNA entering the cell. DH5 alpha strain also lacks endonuclease activity that can cleave plasmid DNA sequences. This strain also has mutations that allow it to take up large plasmids. Thus, large gene sequences can be inserted and cloned in this strain as compared to K12 strain. DH5 alpha is a RecA- strain and therefore cannot carry out homologous recombination. Thus, it prevents integration of plasmid DNA into chromosomal DNA. K12 is a wild type strain of E.coli that has lost its ability to grow in intestine. Thus, it is capable or recombination due to RecA expression.