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p arm of a chromosome
The short arm of a chromosome. The "p" comes from the French "petit" meaning small. All human chromosomes have 2 arms - the p (short) arm and the q (long) arm - that are separated from each other only by a primary constriction, the centromere, the point at which the chromosome is attached to the spindle during cell division.The symbol "q" was chosen for the long arm simply because it followed "p" in the alphabet
p in biochemistry
The abbreviation for protein. For example, p53 is a protein (53 kilodaltons in size).
p in population genetics
The frequency of the more common of two different alternative (allelic) versions of a gene. (The frequency of less common allele is q).
p53
A tumor suppressor. The designation p53 stands for protein 53 kilodaltons in size.
p53 is produced by a gene that functions to suppress the growth of tumors. The p53 gene is the most commonly mutated gene known in human cancer. Like other tumor-suppressor genes, p53 normally controls cell growth. If p53 is physically lost or nonfunctioning (inactivated), this tends to allow the cell to divide without restraint.
The level of p53 has prognostic (predictive) value. For example, breast cancer patients after mastectomy who have high levels of p53 are at higher risk for cancer recurrence than women with low levels of p53. The buildup of p53 within a cancer cell is a sign that p53 is not working properly to suppress the growth of the tumor.
PA (posteroanterior)
In anatomy, PA stands for posteroanterior: from back-to-front. For example, a PA X-ray of the chest is taken from back-to-front.
PA in this respect is the opposite of AP which stands for anteroposterior: from front-to-back.
PA X-ray
An X-ray picture in which the beams pass from back-to-front (posteroanterior). As opposed to an AP (anteroposterior) film in which the rays pass through the body from front-to-back.
Pacemaker
Usually, an artificial device that sends electrical impulses to the heart in order to set the heart rhythm. Although there are different types of pacemakers, all are designed to treat bradycardia, a heart rate that is too slow. Some pacemakers function continuously and stimulate the heart at a fixed rate or at an increased rate during exercise. A pacemaker can also be programmed to detect an overly long pause between heartbeats, and then stimulate the heart.
See also pacemaker, internal; pacemaker, natural.
Pacemaker, internal
A pacemaker sets the heart rate. An artificial pacemaker is a device that does this. It uses electrical impulses to regulate the heart rhythm or reproduce that rhythm.
An internal pacemaker is one in which the electrodes into the heart, the electronic circuitry and the power supply are all implanted (internally) within the body.
Although there are different types of pacemakers, all are designed to treat bradycardia, a heart rate that is too slow. Some pacemakers function continuously and stimulate the heart at a fixed rate or at an increased rate during exercise. A pacemaker can also be programmed to detect too long a pause between heartbeats and then stimulate the heart.
The implantable artificial pacemaker was invented by Wilson Greatbatch in 1958.
Pachyonychia congenita of the Jadassohn-Lewandowski type
This is a type of pachyonychia congenita (elephant nails from birth). The characteristic features include:
Abnormally thick curved nails (onychogryposis)
Thickening of the skin (hyperkeratosis) of the palms, soles, knees and elbows
White plaques (leukoplakia) in the mouth
Excess sweating (hyperhidrosis) of the hands and feet
Teeth are already erupted at birth (natal teeth)
Generation after generation in a family may show the syndrome. It is an autosomal dominant trait. The gene responsible for the syndrome is on chromosome 12 (in band 12q13) and a single copy of the gene (named PD1) is capable of causing the disease. The basic abnormality is a mutation (change) in a gene for keratin, a primary constituent of nails, hair, and skin. Alternate names for the syndrome include pachyonychia congenita with natal teeth and type 1 pachyonychia congenita. The syndrome is named for the professor of dermatology at the University of Bern in Switzerland, Josef Jadassohn (1860-1936), and his colleague, Felix Lewandowski (1879-1921), who first described the syndrome in 1906. About their patient, a 15-year-old girl, they wrote: "The nail plates of all the fingers and toes are extremely thickened, and so hard that they cannot be cut with a scissors; the father has to trim them with a hammer and chisel."
Pachyonychia congenita with natal teeth
This is a type of pachyonychia congenita (elephant nails from birth) in which teeth are evident at birth. It is called the Jadassohn-Lewandowski syndrome. The characteristic features include:
Abnormally thick curved nails (onychogryposis)
Thickening of the skin (hyperkeratosis) of the palms, soles, knees and elbows
White plaques (leukoplakia) in the mouth
Excess sweating (hyperhidrosis) of the hands and feet
Teeth are already erupted at birth (natal teeth)
Generation after generation in a family may show the syndrome. It is an autosomal dominant trait. The gene responsible for the syndrome is on chromosome 12 (in band 12q13) and a single copy of the gene (named PD1) is capable of causing the disease. The basic abnormality is a mutation (change) in a gene for keratin, a primary constituent of nails, hair, and skin. Alternate names for the syndrome include pachyonychia congenita of the Jadassohn-Lewandowski type and type 1 pachyonychia congenita. The syndrome is named for the professor of dermatology at the University of Bern in Switzerland, Josef Jadassohn (1860-1936), and his colleague, Felix Lewandowski (1879-1921), who first described the syndrome in 1906. About their patient, a 15-year-old girl, they wrote: "The nail plates of all the fingers and toes are extremely thickened, and so hard that they cannot be cut with a scissors; the father has to trim them with a hammer and chisel."
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