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According to Eichler and Sankoff the two mean processes in eukaryotic chromosome evolution are 1) rearrangements of chromosomal segments and 2) localized duplication of genes. Clustering could be explained by reasoning that all genes in a cluster are originated from tandem duplicates of a common ancestor.
The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm covering.
An arête is a thin ridge of rock that is formed by glacial erosion. [2] Pressure ridge (ice) An ice pressure ridge is a ridge of deformed ice along the boundaries of individual ice floes when the ice floes on a lake or ocean collide and compress their edges. The average height of a sea ice pressure ridge is between 5 and 30 meters. [8]
The apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AER and limb mesenchyme —including the zone of polarizing activity (ZPA)—continue to communicate ...
The genital ridge appears at approximately five weeks, and gives rise to the sex cords. A.—Diagram of the primitive urogenital organs in the embryo previous to sexual distinction. * 3. Ureter. * 4. Urinary bladder. * 5. Urachus. * cl. Cloaca. * cp. Elevation which becomes clitoris or penis. * i. Lower part of the intestine. * ls.
In medicine, the term arcus superciliaris or the English translation superciliary arch. This feature is different from the supraorbital margin and the margin of the orbit. Some paleoanthropologists distinguish between frontal torus and supraorbital ridge. [2] In anatomy, a torus is a projecting shelf of bone that unlike a ridge is rectilinear ...
The carina is a cartilaginous ridge separating the left and right main bronchi that is formed by the inferior-ward and posterior-ward prolongation of the inferior-most tracheal cartilage. [ 2 ] The carina occurs at the lower end of the trachea - usually at the level of the 4th to 5th thoracic vertebra .
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms.It is intended as introductory material for novices; for more specific and technical definitions from sub-disciplines and related fields, see Glossary of cell biology, Glossary of genetics, Glossary of evolutionary biology, Glossary of ecology ...