Transfection is a process by which a virus introduces foreign genetic material into a cell. Viruses naturally reproduce by injecting their genetic material into a host cell, where those genes are incorporated into the host's genes. When the host genes are expressed (transcribed into RNA), the virus's genes are expressed along with it. Scientists have taken advantage of this mechanism of viral replication by introducing a gene of interest into a virus, which then injects it into a host cell.
Cationic liposome transfection is a nonviral alternative to viral transfection. In cationic liposome transfection, molecules that are partly hydrophobic and partly hydrophilic spontaneously form micelles (similar to bubbles) when introduced into an aqueous solution such as blood or the inside of a cell. When target DNA is added to these micelles, the resulting complex almost completely shields the DNA. As the cationic liposome enters a cell, it carries the DNA with it and shields it from enzymes and chemicals that might otherwise damage it.
Genes can be silenced by introducing a stop codon into the genes. Most codons, or sets of three adjacent nucleotides in the exons of mRNA, encode one amino acid each. Amino acids link together to form proteins. Three specific codons, the stop codons, cause translation to stop so that no more amino acids will be added to the growing protein. By using restriction enzymes to alter sequences of DNA, geneticists can insert a stop codon into a gene. When this gene is expressed, the stop codon stops protein synthesis as soon as the cell's translation machinery reaches the stop codon.
By using restriction enzymes to cut specific sequences of DNA, geneticists can insert new genetic material into an organism's genome. Restriction enzymes work like sequence specific scissors in that they only cut DNA at the sites of specific sequences. After the foreign gene is introduced into the cell, another enzyme called ligase connects the fragments of DNA, thus incorporating the foreign gene into the organism's genome.