knockout
Summary
A knockout is an organism or cell line in which a chosen gene has been deliberately disabled or deleted, so it makes no functional product. Comparing it with the wild type shows what the gene normally does. The key inference is loss of function.
Detail
Meaning: A knockout has a target gene inactivated in its DNA, either by homologous recombination, which swaps the normal gene for a nonfunctional copy, or by CRISPR-Cas9, which cuts the gene and leaves disabling mutations. A germline knockout, such as a knockout mouse, passes the change to offspring. The logic is loss of function: if the knockout differs from the wild type, the difference points to the missing gene's normal role.
On the MCAT: A passage describes mice lacking an enzyme, receptor or transcription factor. Predict the outcome: the substrate builds up, the product falls, and steps that depend on the protein fail. The wild type is the control, and a rescue experiment, in which restoring the gene reverses the phenotype, strengthens the conclusion. For a recessive gene, a phenotype is expected in homozygous knockouts, while heterozygotes often resemble the wild type.
Common confusion: A knockout is not a knockdown. A knockdown, such as RNA interference with siRNA, lowers mRNA or protein levels while the gene stays intact, and the effect is partial and often temporary. A knockout changes the DNA. Also, a knockout with no visible phenotype does not prove the gene is useless, because a related gene can sometimes compensate.
Sources
- Lehninger Principles of Biochemistry
- Molecular Biology of the Cell
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.