34.Unconventional viruses. Defective viruses.

Unconventional viruses also known as “atypical viruses”.

So, atypical viruses include two agents – there’re viroids and prions.

Viroids – small supercoiled circular RNA molecules. There’re don’t have a capsid.

Contains RNA only.

Viroids - infectious agents are low molecular weight, highly complementary, circular, single-stranded RNA (a few hundred nucleotides).

Viroids unlike viruses do not have protein shells, and consist only of the infectious RNA molecule. They have no antigenic properties

Viroids have a very small size - length of the RNA molecule is equal to 1 * 106 mm.

Molecules viroids are single-stranded RNA ringed.

RNA molecules of viroids, do not encode own proteins because their multiplication can occur either autocatalytically or with participation the host cell

All known viroids are built on the same plan - 300-400 nucleoids form a ring, which is held the base pairs to form a double-stranded rod-shaped structure with alternating short single-and double-stranded regions

Prions – proteinlike infectious particle. There’re devoid of DNA or RNA.

Prions do not contain nucleic acids and consist only of the modified protein molecules of the host.

They're small, proteinaceous particles and there's controversy as to whether they contain any nucleic acid, but if there's any, there's very little, and almost certainly not enough to code for protein

Conditions for the occurrence of prion diseases are unique. They can be formed in three scenarios: as infectious, sporadic and hereditary lesions. In the latter case the main role played by genetic predisposition [

Protein secreted by many human and animal diseases damaging the nervous system: Alzheimer's disease and Down's syndrome

35. Diversity of viruses. Classification criteria. Nomenclature of viruses.

Classification criteria type of nucleic acid the size and morphology of virions types of symmetry having supercapsid type of host cells the antigenic properties type of infection transmission.

Nomenclature

Kingdom –Vira

Family-Viridae

Subfamily – Virinae

Genus – Virus

Species – a special name (Influenza, Virus H₅N₁ rubella virus, herpes and etc.)

Viruses are divided into 6 classes on the structure of nucleic

acid:

Class I - viruses containing two stranded DNA.

Class II - virus containing single stranded DNA.

Class III - viruses containing two stranded RNA.

Class IV - viruses containing "Plus" chain RNA.

Class V - viruses, containing a "minus" chain RNA.

Class VI - viruses containing reverse transcriptase (retroviruses)

Class I: viruses containing double-stranded DNA

Viruses containing double-stranded DNA replication enter the cell nucleus, as it requires cellular DNA polymerase. Also, DNA replication of these viruses is strongly dependent on the stage of the cell cycle. In some instances, the virus may induce cell division, which may result in cancerous degeneration. Examples of such viruses are Herpesviridae,

The representatives of the family Poxvirus genomic DNA is not replicated in the nucleus.

Class II: viruses containing single-stranded DNA

viruses of the family Circoviridae And Parvoviridae replicate genomic DNA in the nucleus and in the replication intermediate form - double stranded DNA.

Class III: viruses containing double-stranded RNA

Like many RNA viruses, the representatives of class III to replicate the genomic RNA in the cytoplasm and use the host polymerase to a lesser extent than DNA viruses. Class III includes two large family Reoviridae and Birnaviridae. Monocistronic replication, the genome is segmented, each gene encodes a protein

Classes IV and V: viruses containing single-stranded RNA

Classes IV and V include two types of viruses, the replication of which is independent of the cell cycle stage. Along with viruses containing double-stranded DNA viruses, those most studied.

Class IV: viruses containing single-stranded (+) RNA

Directly to the (+) genomic RNA viruses can be a class IV protein synthesis on ribosomes of the host cell. Viruses are classified into two groups, depending on the characteristics of RNA:

• a virus polycistronic mRNA translation leads to the formation of a polyprotein which is then cut into a mature protein. On one chain RNA can be synthesized by several different proteins, which reduces the length of the gene.

• a complex viruses contain subgenomic mRNA transcription, protein synthesis is shifted reading frame is also used proteolytic processing of polyproteins. These mechanisms provide a synthesis of different proteins with the same chain of RNA.

Classes IV and V: viruses containing single-stranded RNA.

Classes IV and V include two types of viruses, the replication of which is independent of the cell cycle stage. Along with viruses containing double-stranded DNA viruses, those most studied.

Class IV: viruses containing single-stranded (+) RNA

Directly to the (+) genomic RNA viruses can be a class IV protein synthesis on ribosomes of the host cell. Viruses are classified into two groups, depending on the characteristics of RNA:

• a virus polycistronic mRNA translation leads to the formation of a polyprotein which is then cut into a mature protein. On one chain RNA can be synthesized by several different proteins, which reduces the length of the gene.

• a complex viruses contain subgenomic mRNA transcription, protein synthesis is shifted reading frame is also used proteolytic processing of polyproteins. These mechanisms provide a synthesis of different proteins with the same chain of RNA.

Class VI: viruses containing single-stranded (+) RNA replicating by step DNA

The most well-studied family of this class of viruses are retroviruses. Class VI viruses using the enzyme reverse transcriptase for the conversion of (+) RNA into DNA. Instead of using RNA as a template for the synthesis of proteins, viruses used in this class of DNA matrix which integrates into the host genome integrase enzyme. Further replication occurs using polymerase of the host cell. The most well-studied member of this group of viruses is HIV.

Class VII: viruses containing double-stranded DNA is replicated through a stage of single-stranded RNA

A small group of viruses which includes hepatitis B virus of the family Hepadnaviridae, are double-stranded genomic DNA is covalently closed in a ring shape and is a template for the synthesis of viral mRNAs and subgenomic RNAs. Subgenomic RNA serves as a template for synthesis of the DNA genome of the virus enzyme reverse transcriptase.