Practice question

pUC18 is a high-copy-number derivative of pBR322 engineered with modified pMB1 origin, lacZ alpha fragment for blue-white screening and ampicillin resistance. Total plasmid size approximates 2686 base pairs, commonly rounded to 2.7 kb. Due to small backbone, insert size is limited, typically 5-10 kb, before stability declines. The 2.7 kb refers to vector backbone length itself, not maximum insert. Knowledge of precise vector size is important for restriction analysis, estimating recombinant size on agarose gels and calculating molar ratios for ligation reactions in molecular cloning experiments.