Practice question

Homologous recombination guided gene targeting requires extensive homology arms matching endogenous locus to direct strand exchange, and selectable marker like neo to identify integration events. Endogenous recombination machinery of embryonic stem cells including Rad51 mediated repair executes crossover. Origin of replication is plasmid bacterial element needed for propagation in E coli during vector construction and preparation but dispensable inside mammalian genome since integration occurs via chromosomal recombination, not autonomous replication. Therefore Ori is not required for actual gene targeting event in eukaryotic nucleus.