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Imprinting and Uniparental Disomy

Posted by Surgery on Oct 6, 2008
Genomic imprinting is a phenomenon, believed to be set in meiosis, that labels a gene as originating from the father or the mother. This label, which defines parent of origin, is temporary and is reset in each generation. When the parental origin of a gene affects its expression, the gene is said to be imprinted (45:).
An illustration of genomic imprinting involves two conditions with mental retardation as a major feature, Prader-Willi syndrome and Angelman syndrome. Prader-Willi syndrome is a condition characterized by hypotonia, hypogonadism, failure to thrive in infancy, and later, hyperphagia with resulting obesity. Angelman syndrome is characterized by absent speech, ataxic movements, seizures, and paroxysms of laughter. Patients with these conditions often (seen in about 70% of cases) have an interstitial deletion of the long arm at chromosome 15q11–13, detectable by FISH (46,47:). However, the parental origin of this deletion differs between these two conditions: when this deletion occurs on the father’s chromosome 15, patients have Prader-Willi syndrome, but when it occurs on the mother’s chromosome 15, the patients have Angelman syndrome.

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Principles of Genetics, Scientific Principles | No Comments

ENDOPLASMIC RETICULUM: SYNTHESIS AND PROCESSING OF EXPORTED AND MEMBRANE PROTEINS : PLAVIX

Posted by Surgery on Jul 8, 2008

In eukaryotic cells, protein synthesis is carried out by two separate populations of ribosomes: free and membrane bound. Membrane-bound ribosomes are attached to the cytosolic side of the ER membrane and are engaged in the synthesis of export and membrane proteins ( ). The ER is a membranous labyrinthine compartment formed by branching tubules and flattened sacs called cisternae. It extends throughout the cytosol in a netlike shape (Fig. 1-3 ). endoplasmicThe ER membrane forms a continuous sheet enclosing a single internal space called the lumen or the cisternal space, and it is continuous with the nuclear envelope. The ER membrane may constitute more than one-half of the total membrane in an average eukaryotic cell, and the ER cisternal space occupies more than 10% of the total cell volume. The size of the ER could change, depending on the metabolic state of the cell. Thus, B cells after activation (plasma cells: ) increase the ER size substantially due to the large amount of proteins that are secreted (immunoglobulins : ). In its extension, three specific regions of the ER can be differentiated based on their structure and function. The rough ER (RER) contains attached ribosomes on the cytosolic side of the membrane, which gives a bumpy appearance as observed by electron microscopy.

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NEWS SURGERY, Scientific Principles | No Comments