You may feel hungry frequently if your diet lacks protein, fiber, or fat, all of which promote fullness and reduce appetite. Extreme hunger is also a sign of inadequate sleep and chronic stress.
Additionally, certain medications and illnesses are known to cause frequent hunger. A key feature of Prader-Willi syndrome is a constant sense of hunger that usually begins at about 2 years of age.
People with Prader-Willi syndrome want to eat constantly because they never feel full hyperphagia , and they usually have trouble controlling their weight. Kingdom fungi includes organisms that are. Kingdom protista does not include. Archaebacteria that live in marshy areas are. FAQs on Biological Classification. RBSE class 10 and 12 board exam would commence from 3rd March RBSE practical exams for Class 10 and 12 would be held from 17th Check CUET application form details, important dates and fees.
Check application process, exam details and other important details here. JEE Main Chemistry preparation tips and exam strategy. In viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane, the lipid composition of the viral envelope closely reflects that of the particular host membrane.
The outer capsid and the envelope proteins of viruses are glycosylated and important in determining the host range and antigenic composition of the virion. In addition to virus-specified envelope proteins, budding viruses carry also certain host cell proteins as integral constituents of the viral envelope. Virus envelopes can be considered an additional protective coat.
Larger viruses often have a complex architecture consisting of both helical and isometric symmetries confined to different structural components. Viruses are classified on the basis of morphology, chemical composition, and mode of replication. The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.
In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path. Such nucleocapsids form rigid, highly elongated rods or flexible filaments; in either case, details of the capsid structure are often discernible by electron microscopy. In addition to classification as flexible or rigid and as naked or enveloped, helical nucleocapsids are characterized by length, width, pitch of the helix, and number of protomers per helical turn.
The most extensively studied helical virus is tobacco mosaic virus Fig. Many important structural features of this plant virus have been detected by x-ray diffraction studies. Figure shows Sendai virus, an enveloped virus with helical nucleocapsid symmetry, a member of the paramyxovirus family see Ch.
The helical structure of the rigid tobacco mosaic virus rod. About 5 percent of the length of the virion is depicted. Individual 17,Da protein subunits protomers assemble in a helix with an axial repeat of 6.
Each more Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope. The intact nucleocapsid is about 1, nm long and 17 nm in diameter; its pitch more An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig. Lines through the centers of opposite triangular faces form axes of threefold rotational symmetry; twofold rotational symmetry axes are formed by lines through midpoints of opposite edges.
An icosaheron polyhedral or spherical with fivefold, threefold, and twofold axes of rotational symmetry Fig. Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. These axes are perpendicular to the plane of the page and pass through the centers of each figure.
Both polyhedral upper and spherical lower forms more Viruses were first found to have symmetry by x-ray diffraction studies and subsequently by electron microscopy with negative-staining techniques.
In most icosahedral viruses, the protomers, i. The arrangement of capsomeres into an icosahedral shell compare Fig. This requires the identification of the nearest pair of vertex capsomeres called penton: those through which the fivefold symmetry axes pass and the distribution of capsomeres between them. Adenovirus after negative stain electron microscopy. A The capsid reveals the typical isometric shell made up from 20 equilateral triangular faces.
The net axes are formed by lines of the closest-packed neighboring capsomeres. In adenoviruses, the h and k axes also coincide with the edges of the triangular faces. This symmetry and number of capsomeres is typical of all members of the adenovirus family. Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species.
The larger viruses contain in a core the nucleic acid genome complexed with basic protein s and protected by a single- or double layered capsid consisting of more than one species of protein or by an envelope Fig. Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins. SU stands for more Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses.
Mutation rates of 10 -4 lead to the continuous generation of virus variants which show great adaptability to new hosts. The viral RNA may be single-stranded ss or double-stranded ds , and the genome may occupy a single RNA segment or be distributed on two or more separate segments segmented genomes. In addition, the RNA strand of a single-stranded genome may be either a sense strand plus strand , which can function as messenger RNA mRNA , or an antisense strand minus strand , which is complementary to the sense strand and cannot function as mRNA protein translation see Ch.
Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins. Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components.
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