Homozygous transgenic cells are often referred to as "knock-outs". These knock-outs can then be examined to determine what, if any, phenotype is associated with mutations in the targeted gene.

If a knock-out phenotype is found, then the locus responsible for the mutation can be placed on a genetic map by standard crossing and segregation analysis.

The knock-out strategy can be used to test whether an isolated gene functions as thought. Also, panels of knock-out mutants can be created by random insertion of transposable elements.

An important mechanism involved in the regulation of bacterial gene expression in response to environmental signals is the use of alternative sigma fact

In order to generate a mutation in the rpoS gene allelic exchange may be carried out by the use of a suicide vector. Disruption of chromosomal rpoS DNA by a fragment conferring some sort of antibiotic resistance can be achieved by homologous recombination by a plasmid. To construct this plasmid two separate cloning steps can be performed. The insertion mutation can be obtained by ligating a DNA fragment from a vector encoding for antibiotic resistance into the rpoS. The ligation product can then be introduced in to the suicide plasmid (mentioned in the beginning) at certain restriction sites that would be present within the multiple cloning site of the plasmid. The resulting plasmid confers antibiotic resistance that cannot replicate within E. coli. This plasmid can then be integrated into the chromosomal rpoS region by selecting for the particular antibiotic resistance. A second crossover resulting in the loss of the plasmid sequence and retention of the insertion mu

Some common words found in the essay are:
, DNA PCR, Journal Bacteriology, selectable marker, Response Borrelia, antibiotic resistance, dna fragment, Patricia Rosa, interrupted gene, selectable marker gene, cloned plasmid, marker gene, resulting plasmid, allelic exchange, chromosomal rpos, homologous recombination,
Approximate Word count = 660
Approximate Pages = 3 (250 words per page double spaced)