Forms of existence of viruses. Morphology and biochemical structure of virions. Viral genome organization Structure, function and properties of virion nucleic acid, proteins, lipids and carbohydrates. Prions, role in human pathology
General Characteristics of Viruses:
 ◦ ﻿﻿molecular (non-cellular) structure;
 ◦ ﻿﻿viral genome represents only the one type of nucleic acid - DNA or RNA;
 ◦ viruses ure obligue intractuar parasites capable of propagaing sorend in the living ells
viruses are obl
- viruses ure not affected by antibiotics; viral infections are treated by special class of antiviral drugs.
Forms of existence of viruses
Viruses exist in three forms:
 1 ﻿﻿﻿virion (viral particle) - extracellular form;
 2 ﻿﻿﻿intracellular (vegetative) virus;
 3 ﻿﻿﻿a virus (provirus) integrated with the host's DNA
Virion is the extracellular form of virus existence.
The inner (or core) part of virion - the molecule of nucleic acid.
Viral nucleic acid is enwrapped with protein coat, termed a capsid.
Capsid is composed of identical protein subunits called capsomeres.,
Capsid proteins are bound tightly to nucleic acid (this complex is termed nucleocapsid).
There are 2 types of viruses:
 1 ﻿﻿﻿"Naked" (or simplex) viruses - the capsid is the only external coat.
 2 ﻿﻿﻿Enveloped viruses - have an additional outer lipid membrane (supercapsid) that is obtained from the cytoplasmic membrane of the host cell.
Nucleic acid genome
Capsid
Envelope
The outer lipid membrane (supercapsid) contains a protein called a matrix protein
On the surface of the virus there are spikes (viral glycoproteins) - these are receptors that are responsible for attaching the virus to sensitive cells
The viral structure is organized according to different types of symmetry.
 1 ﻿﻿﻿Icosahedral (or cubic) symmetry, because it is based on an icosahedral pattern - a polyhedron with 20 faces
 2 ﻿﻿﻿Helical symmetry, when the nucleocapsid is organized in a spiral. This complex is enclosed in an outer lipid membrane.
Chemical Structure of Viruses
Viral proteins are divided into structural and non-structural.
Supercapsid and capsid proteins are referred to structural proteins.
Non-structural proteins - viral enzymes (DNA- and RNA-polymerases, proteases, neuraminidase, etc.);
genomic and regulatory proteins, which control transcription and translation.
Viral lipids are obtained from the infected cells, coming into the structure of viral envelope. Overall,
lipid structures comprise up to 30% of enveloped virion mass.
Carbohydrate cover about 10-15% of virion contents.
Viral Genomic Organization
Viral genome includes a single type of nucleic acid.
Viral DNA or RNA may be single-stranded or double-stranded, segmented or non-segmented, positive
or negative, circular or linear.
Protein-encoding strand of nucleic acid is called positive, or plus-strand. This strand serves as a direct
template for subsequent transcription and translation.
Negative or minus strand means the nucleic acid chain complementary the positive one. In that case viral protein formation is impossible without preliminary synthesis of positive chain on the negative strand template.
For this purpose (-) RNA viruses contain RNA polymerase. It catalyzes the complementary RNA(+)
synthesis within infected cells. The latter serves as mRNA.
A prion is a type of protein that can trigger normal proteins in the brain to fold abnormally. Prior diseases can affect both humans and animals and are sometimes spread to humans by infected meat products.
The most common form of prion disease that affects humans is Creutzfeldt-Jakob disease (CJD).
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