From previous page posted, we introduced the meaning of viruses, their sizes and design http://drjiyanemedicalhelp.com/what-makes-viruses-difficult-to-respond-on-treatment/ . Now on the next topic we will be looking at genome structure, of which is the one that make viruses very difficult to respond very well on treatment or get cleared from our system.
Viral genomics structure
Structural diversity among the viruses is the most obvious, when the make-up of the viral genomes is considered. Genomes can be made of ribonucleic acid (RNA) or deoxyrionucleic acids (DNA) and be either double stranded or single stranded.
For viruses with single-stranded genomics, the nucleic acid can be either of the same polarity (indicated by a + or – for a different polarity) from that of the viral messenger-RNA (mRNA) produced during the viruses causing infection and diseases.
Two types of genomics structure are known, the linear and the circular. Most viruses have a single nucleic acid molecules for their genomics structures, in some cases several pieces of nucleic acid constitute the complete genome.
Viruses as such have segmented genomes. One virus class which is retrovirus (e.g.HIV) carries two identical copies of its genomics structures and is therefore diploid.
This is what is rarely and complicated about HIV. A few viral genomes such as picornaviruses, hepatitis B virus, and adenoviruses contain covalently attached protein on the ends of the polynucleotide chains.
Viral genomics subunit structure of Capsids
The capsid of all viruses are composed of many copies of one or at most several kinds of protein subunits.This fact follows from:
1. All viruses code for their own capsid proteins, and it turns out that even if the entire coding capacity of the genomics structures were to be used to specify a single giant capsid protein, the protein would not be large enough to enclose the nucleic acid genomes.
This lead to multiple protein copies required. The simplest spherical virus contains 60 identical protein subunits.
2. Viruses are such highly symmetric structures that it is not uncommon to visualize naked capsid viruses in the electron microscope as a crystalline arrays.
The presence of many identical protein subunit in viral capsids or the existence of many identical spikes in the membrane of enveloped viruses has important implications for adsorption, hemagglutination, and recognition of viruses by neutralizing antibodies http://drjiyanemedicalhelp.com/microorganism-the-cause-of-infection-leading-to-a-disease/
Viruses genomics cylindrical shape
A cylindrical shape is the simplest structure for a capsid. The first virus to be crystallized and studied was a plant virus, which is tobacco mosaic virus (TMV).
The capsid of a TMV is shaped like a rod or, a cylinder, with the RNA genomics structure wound in a helix inside it. The capsid is composed of many copies of a single kind of protein subunit arranged in a closed packed helix, which places every subunit in the same microenvironment.
Because of the helical arrangement of the subunits, viruses that have this type of design are often said to have helical symmetry. Thus, the nucleocapsids of influenza, measles, mumps, rabies, and poxviruses are probably constructed with a helical arrangement of protein subunit in close association with the nucleic acid genomics structures.
The viruses genomics spherical shape
Construction of a spherical shaped virus similarly involves the packing together of many identical subunits, but in case the subunits are placed on the surface of a geometric solid called an icosahedron.
Because the icosahedron belong to the symmetric structural group, the spherically shaped viruses are said to have cubic symmetry (note that the term ‘cubic’, used in these contexts, has nothing to do with the more familiar shape called cube).
When viewed in the electron microscope, many naked capsid viruses and some nucleocapsids appear as spherical particles with a surface topology that makes it appear that they are constructed of identical ball-shped subunits.
These visible structures are referred to as morphological subunits. Morphological subunits are composed of either five or six individual protein molecules, each one referred to as a structural subunit, or promoter.
In the simplest case of a virus with a cubic symmetry, I’ve promoters are placed on each 12 vertices of the icosahedron forming a pentamer. While, the capsid is composed of a total of 60 promoters.
This arrangement places every promoter in the same microenvironment as every other promoter in the case of helical symmetry.To accommodate the larger cavity required by viruses with large genomics structures, the capsid’s contain many more promoters.
These viruses are based on a variation of the basic icosahedron in which the construction involves a mixture of pentamers and hexamers instead of only pentamers. A detailed description of is higher level of virus structure is beyond the scope of this text.
Special surface genomics structures
Many viruses have structures that protrude from the surface of the virion. These genomics structures are important for the two earliest steps of infection, adsorption, and penetration.
The examples of the surface structure include the spikes of adenovirus and the glycoprotein spikes found in the membrane of enveloped viruses.
Even viruses without obvious surface extensions probably contain short projections, which is more of the obvious spikes, are involved in the specific binding of the virus to the cell surface.
Classification of viruses causing infection and diseases in human
Table 1 Represent classification of RNA and DNA human viruses causing diseases
Table 1 present a classification scheme for human viruses that is based solely on their genomics structure. The viruses are arranged in order of increasing genomics structural size.
It is important to bear in mind that phylogenetic relationships cannot be inferred from this taxonomic scheme. The tables should not be memorized, but instead used as a reference guide to viruses structures.
The terminal protein molecules as well as other special genomics structures found in other viruses play key roles in the replication process.
In general, viruses with similar genomics structures exhibit similar replication strategies.
See the next article, where we will be discussing viruses multiplication, viral infection of cells may be productive or none productive, animal viruses may cause oncogenic transformation.