Genetics Terms Starting With K
Genetics Glossary: K
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Karyotype
/ KAIR-ee-oh-typ / · Greek: karyon (nut, nucleus) + typos (type)
Karyotype is a visual profile of an individual's complete set of chromosomes arranged by size, shape, and banding pattern, used to detect chromosomal abnormalities.
Karyotypes are prepared by arresting dividing cells in metaphase, when chromosomes are maximally condensed, and staining them with Giemsa or other dyes that produce characteristic banding patterns. Each chromosome pair is identified by its size, centromere position, and banding pattern, then arranged in a standardized order called an idiogram. Karyotyping is used clinically to diagnose numerical and structural chromosomal disorders including Down syndrome, Turner syndrome, and chromosomal translocations.
The first human karyotype showing the correct number of 46 chromosomes was published in 1956 by Joe Hin Tjio and Albert Levan. Before that publication, the human chromosome number had been incorrectly reported as 48 for nearly 30 years.
A normal karyotype does not rule out all genetic disorders. Karyotyping detects only chromosomal abnormalities large enough to be visible under a light microscope and cannot detect single-gene mutations or small copy-number variants.
Prenatal karyotyping of fetal cells obtained by chorionic villus sampling at 10 to 12 weeks of pregnancy can detect trisomy 21 and other major chromosomal abnormalities with high accuracy. Large chromosome changes are visible at this resolution, but small DNA variants below roughly 5 to 10 megabases typically go undetected.
Nondisjunction →Kinetochore
/ kih-NET-oh-kor / · Greek: kineto (movable) + chora (place)
Kinetochore is a specialized protein structure that assembles at the centromere of each chromosome and connects the chromosome to spindle microtubules during cell division.
The kinetochore consists of an inner domain that contacts centromeric DNA and an outer domain that binds microtubules and motor proteins. Proper kinetochore-microtubule attachment is monitored by the spindle assembly checkpoint, which delays cell division until all chromosomes are correctly attached to spindle fibers from opposite poles. Defective kinetochore function leads to chromosome missegregation and aneuploidy, a hallmark of many cancer cells.
In human cells, each kinetochore captures roughly 20 to 25 microtubules, forming a structure called the kinetochore fiber or k-fiber.
The kinetochore contains over 100 different proteins that collectively form an error-correction mechanism, reducing the rate of chromosome missegregation to about one per thousand cell divisions.
Cell Cycle →The kinetochore is not the same as the centromere. The centromere is a specific DNA sequence on the chromosome, while the kinetochore is the protein complex that assembles on that sequence to link the chromosome to the spindle.
In budding yeast (Saccharomyces cerevisiae), each kinetochore captures exactly one microtubule, making yeast a model system for studying kinetochore-microtubule attachment at single-filament resolution. Human kinetochores, by contrast, bind 20 to 25 microtubules per chromosome, reflecting the greater mechanical demands of a larger genome.
Knockout Gene
/ NOK-owt jeen / · English: knockout + gene
Knockout Gene is a gene that has been deliberately inactivated or deleted in an organism using molecular techniques, allowing researchers to study the effects of gene loss on phenotype.
Gene knockouts are most commonly created in mice using homologous recombination to replace the target gene with a non-functional sequence. Conditional knockouts allow gene inactivation in specific tissues or at specific developmental stages, overcoming the problem of essential genes whose complete deletion is lethal. Knockout mouse models have been central to elucidating gene function and creating animal models of human genetic diseases, with more than 10,000 mouse genes knocked out to date through international consortium efforts.
The first knockout mouse was created in 1989, work for which Mario Capecchi, Martin Evans, and Oliver Smithies shared the 2007 Nobel Prize in Physiology or Medicine.
History of Genetics →A knockout gene is not the same as a knockdown. A knockout completely eliminates gene function through permanent sequence disruption, while a knockdown reduces but does not eliminate gene expression, typically using RNA interference.
Gene Therapy Pros and Cons →Knocking out the p53 tumor suppressor gene in mice produces animals that develop tumors at a high rate within their first year of life, confirming p53 as a critical brake on uncontrolled cell division. Roughly 75 percent of these p53-null mice develop lymphomas or sarcomas before six months of age.