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Spread the knowledge. Cell, virus, microbe. Post navigation Gastroesophageal Reflux gerd. Geographic Tongue. Leave a Reply Cancel reply Your email address will not be published. Anatomy Note. However, most virus particles must be viewed with electron microscopes. The simplest virions consist only of a nucleocapsid, which is composed of a nucleic acid, either DNA or RNA, and a protein coat called a capsid.
The capsid surrounds the viral nucleic acid, protects the viral genome, and often aids in its transfer between host cells. Some virions are covered by a lipid membrane, and these are termed enveloped viruses, whereas those lacking a membrane are called non-enveloped or naked viruses.
Notice what is missing from viruses: Ribosomes for protein synthesis and a mechanism for generating ATP. Cytoplasm is absent, and while a few enzymes may be found, there are not enough to sustain cellular processes. Non-enveloped viruses construct a capsid from many copies of one protein and a few minor proteins. Each subunit is termed a protomer, and thousands of protomers self-assemble to form the capsid. In contrast, enveloped viruses require both nucleocapsid proteins and additional proteins to anchor the membrane.
Some viruses use noncapsid proteins as scaffolding upon which the capsids are assembled. Probably the most important advantage of this design strategy is that the viral genome is used with maximum efficiency. For example , the tobacco mosaic virus TMV capsid is constructed using a single type of protomer. Recall that the building blocks of proteins are amino acids and that each amino acid is encoded by three nucleotides.
The TMV protomer is amino acids in length. Therefore only about nucleotides are required to code for the coat protein. The entire TMV genome consists of only 6, nucleotides. Thus only a small fraction of the genome is used to code for the capsid. Suppose, however, that the TMV capsid were composed of six different protomers, all about amino acids in length. If this were the case, about 2, nucleotides in the TMV genome would be required just for capsid construction, and much less genetic material would be available for other purposes.
Helical capsids are shaped like hollow tubes with protein walls. Tobacco mosaic virus is a well-studied example of helical capsid structurev. The self-assembly of TMV protomers into a helical arrangement produces a rigid tube. This sheath composed of innumerable protein molecules.
Each of the protein molecules of the capsid sheath is known as capsomere. The capsomere molecule type is specific for the specific virus. Generally, the capsid is biologically inert, but in addition to protection to protection of the nucleic acid, it helps in infecting the host. Since capsid is a protein molecule, it acts as the antigen. It causes sneezing in common cold.
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