Homologous recombination occurs between two homologous DNA molecules. It is also called DNA crossover. During meiosis, two homologous pairs of sister chromatids align side by side. The DNA crossover is very likely to occur. It could be as often as several times per meiosis.
Note: DNA crossover. (a) Two homologous pairs of sister chromatids align side by side. (b) The two homologs are connected at a certain point called chiasma. (c) The two homologs exchange the DNA segment from the chiasma to the end of chromosomes.
Note: crossing over does sometimes happen in mitosis too. It takes place in emryogenesis and is fairly rare, but not absent. If you ever saw a person with black hair and some white locks it is the result of this mitotic crossing-over
Homologous Recombination is a in in vivo method. This method is used to achieve desired mutations (e.g. deletion or insertion of any gene fragment in bacterial chromosome) in bacterial (chromosomal DNA). As explain schematically above. There are homolog sequences both bacterial ( genomic-chromosomal DNA) and target DNA which is created according to desired mutation. These homolog sequences cross- over by the help of bacterial enzymes. For example, In E.coli, Rec proteins activate this mechanism, There are Rec ABCDE and All of them has a different mission. Also, for this purpose, for crossing over of these homolog sequences there are several method.
If you want to detailed information about these strategies and mutations, I can send some articles.
With my love ( good luck)