19/06/2013. Viruses are not organisms (do not belong to any kingdom). Viruses are not made of cells, have no cytoplasm, and no membranes.

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1 VIRUSES Many diseases of plants and animals are caused by bacteria or viruses that invade the body. Bacteria and viruses are NOT similar kinds of micro-organisms. Bacteria are classified as living organisms, while viruses are not. Viruses are not organisms (do not belong to any kingdom). Viruses are not made of cells, have no cytoplasm, and no membranes. Viruses are parasites: they are only able to reproduce within a host organism. They are nucleic acid (DNA or RNA) packaged in protein. Viruses are host specific and tissue specific. These characteristics are determined by proteins on the virus. The proteins must lock onto specific host cell membrane receptor. Virus particles (VIRIONS) consist of two or three parts: - the genetic material (DNA or RNA); - a protein coat that protects these genes, called a CAPSID - in some cases an envelope of lipids (from the host!) that surrounds the protein coat when they are outside a cell. (VIRAL ENVELOPE) 1

2 Viral Envelope The shapes and sizes of viruses range from simple helical and icosahedral forms to more complex structures. Envelope Glycoprotein Genetic Material Capsid Viral Genome May consist of : Double or single stranded DNA Double or single stranded RNA Viruses are known as DNA viruses or RNA viruses 2

3 Classification of Viruses Viral Replication Virologists classify viruses by: 1. Host specificity 2. Type of nucleic acid 3. Geometry of the capsid 4. Envelope (presence/absence) 5. Tissue specificity 6. Type of replication One characteristic viruses do share with living things is the ability to multiply. However, a virus cannot do this on its own. Viruses use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. There are six basic stages in the life cycle of viruses: 1. Attachment is a specific binding between viral capsid proteins (GLYCOPROTEINS) and specific receptors on the host cellular surface. 2. Penetration follows attachment; viruses enter the host cell through endocytosis or membrane fusion. Some viruses inject their genome into the bacterial cell while the viral capsid remains outside. 3. Uncoating is a process in which the viral capsid is degraded. 4. Replication involves synthesis of new viral genetic material 5. Assembly of the virus particles. 6. Viruses are released from the host cell by lysis-a process that kills the cell by bursting its membrane. 3

4 This cycle is called the LYTIC CYCLE 30 min. 200 new viruses. Lysogenic Cycle The second method of viral replication involves virus attachment, followed by membrane fusion or injection. This genetic material travels through the cell and the DNA or RNA ends up being incorporated into the host's own DNA. Forms a Prophage/Provirus - viral genome within host genome 4

5 Diptheria, botulism and scarlet fever would be harmless to humans, if not for the prophages that induce the host bacteria to make toxins So now when the host replicates it's cells, the viral genome is also replicated by the host! At some point (environmental cue) the viral genetic material can be triggered to enter the LYTIC CYCLE. Another outcome may be that the viral DNA causes a slight change in the normal replication of the hosts cells causing uncontrolled cell division or CANCER. Ex. Lymphomas and nasopharyngeal cancer - Epstein- Barr virus Cancer of the liver - hepatitis B virus, hepatitis C virus Cancer of the cervix - human papillomaviruses T-cell leukemia and hairy-cell leukemia - Human T lymphotropic virus type 1 (HTLV- 1) and type 2 (HTLV-2) Kaposi sarcoma - human herpesvirus 8 (HHV-8), associated with 5

6 RNA Viruses The host reproduces the viral genome and capsids HIV is a retrovirus Multiple types infect animals. Some are retroviruses and are packaged with the enzyme reverse transcriptase. These viruses carry an RNA template, and from it synthesize DNA The DNA then integrates into host cell genome Viral Diseases Influenza - respiratory Hepatitis - liver Chicken Pox - skin Rabies - spinal column / brain Encephalitis - brain 6

7 Viroids Viroids are pure RNA that infect plants ; do not encode any proteins Cause errors in plant growth regulation and metabolism Prions Pure tiny protein molecules, no nucleic acids! Mad Cow disease - Bovine Spongiform Encephalopathy (BSE) Scrapie in sheep Creutzfeldt Jakob disease in humans Believed to be a misfolded protein normally found in brain cells When prion enters host cell normal proteins are induced to deform into the prion Immune Response There are 2 types of immune response: Non-specific - mechanical barriers, fever, inflammation etc. Immunity - The ability to defend against antigens. This is the one we will focus on. Immunity is based on the body recognizing foreign substances called ANTIGENS and responding to them. In your body B cells and T cells are types of lymphocytes (White blood cells) that can respond to antigens 7

8 B cells recognize antigens and with the help of T cells (helper), turn into PLASMA cells. Plasma cells synthesis and secrete proteins called ANTIBODIES. Antibodies are antigen specific (think lock and key). Antibodies bind to antigens and inactivate them. Vaccination Attenuated or weakened viruses are introduced into your body. This causes a mild infection and stimulates your body to produce ANTIBODIES against the virus. Antibodies "tag" the virus for destruction by the immune system 8