DNA cloning or molecular cloning is the biological or laboratory process of creating genetically identical organisms, or exact copies of DNA fragments. The process of cloning a DNA plasmid involves fragmentation of the DNA vector, ligation of the new nucleotide fragment onto an appropriate cloning vector, transfection of the product into a cell, then selection of the successfully transfected cells and expression of the gene. Nucleotide sequences, or genes, can also be replicated by the polymerase chain reaction (PCR). Recombinant DNA, or chimeric DNA, is that which has been made from two different species. Ligases are used to catalyze the joining of two complementary DNA probes or single-strands of DNA. Genetic engineering involves direct manipulation of an organism′s genome, including growing cells in microbial media, examining alleles, and using expression vectors or constructs to introduce a specific gene into a target cell.