What is Virus

What is Virus: Characteristic

Virology is the study of virus
Neither prokaryotic nor eukaryotic
They are acellular, that is, they contain no cytoplasm or cellular organelle
No metabolism

Using metabolic machinery of the host cell for multiplication

No ribosomes of their own
Obligatory intracellular parasites

Require living host cells in order for multiplication

Can’t grow on artificial media

Has either DNA or RNA
Depends only on host cells to replicate

Structure of a virus

Classification of What is Virus

Viruses can store their genetic information in five different types of nucleic acid

Double-stranded DNA

This includes most bacteriophage, adenoviruses, Herpes virus

Single-stranded DNA

once inside the host cell, it is converted to double stranded DNA

Phage M13, Parvoviruses

Single-stranded RNA

Yellow fever and Dengue fever

Double stranded RNA
Single stranded RNA with reverse transcriptase

The RNA is reverse transcribed into DNA.

The DNA is transcribed into viral mRNA

Retroviruses HIV

What is Virus: Replication Cycle (5 steps)

Attachment to host cell
Entry (penetration)
- Enveloped viruses:
  - By endocytosis
  - By fusion
- Non-enveloped viruses
  - By endocytosis only

Uncoating
- Uncoating is the separation of the viral nucleic acid from its protein coat once the virion is enclosed within the vesicle
Biosynthesis
- Replication of viral nucleic acid in the nucleus of the host cell by viral enzymes (may dissolve host genes to get ingredients
  - Synthesis of viral protein, such as capsid: vial genes take control of host ribosomes and direct synthesis of viral protein by host enzymes
Maturation
- Assembly of capsid and viral DNA to form a new virion
  - Newly synthesized viral proteins migrate into the nucleus and are joined with the viral DNA to form virion
Release

Enveloped viruses escape one-by-one, taking along some host cell membrane for their envelope (a budding process)

Host cells may or may not be lysed

Bacteriophages are bacteria infected by viruses

Control of Viruses

Difficult because of few vulnerabilities of viruses:

few structures, no metabolism

Can be Killed by:

Steam autoclave
Gas autoclave
Some disinfectants
Powerful radiation

Retrovirus-HIV

Retroviruses encode their own enzyme: reverse transcriptase

This enzyme utilizes viral RNA to synthesize DNA in the host cell

This newly synthesized viral DNA integrates into a host cell’s chromosome as a provirus

HIV: Human Immunodeficiency Virus

Causing disease: AIDS

AIDS: Acquired Immunodeficiency Syndrome

Has single strand RNA, two identical strands of RNA

Once in host cell, RNA reversely transcribed into DNA and is incorporated into the host chromosome

Has envelope with spikes of gp120 which allows viruses to attach to CD4 receptors of T cells
Integrated DNA

Either control the production of an active infection: new viruses bud from the host cell
Or remain latent in the host cell’s chromosome as a provirus

The transmission of HIV is through direct contact with or the transfer of infected body fluids

What are Viruses and Cancer

Several types of cancer are known to be caused by viruses

Example: HPV

Cancer:

Cancer results from the uncontrolled reproduction of cells

Carcinogens: cancer causing agents

Certain viruses may be carcinogenic

Approximately 10% of cancers are known to be virus induced
Development of cancer also involves oncogenes and immune deficiency

Oncogenes are responsible to causing cancer in cell

Oncogene Theory

Developed in the 1970’s
Oncogene: a gene that can bring about malignant transformation

They are part of the host genome and can carry normal cellular functions

Examples:

Human papillomavirus (HPV): Cervical cancers

Vaccine is available

Hepatitis B virus (HBV): liver cancer
Epstein-Barr (EB): Burkett’s lymphoma

Prions and Viroids

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