Lecture 9 Viruses, Viroids, Prions

Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle called virion Comprised of two parts: 1. Nucleic Acid 2. Protein coat (capsid) Nucleocapsid- capsid with nucleic acid inside Each capsid made of identical protein subunits called capsomeres

Virus Shapes

Two basic types of virions Naked: consist only of nucleic acid and capsid Enveloped virus: consists of nucleic acid, capsid, and envelope

Virus attachment to host cells All viruses must be able to attach to specific receptors on host cells Attachment spikes project from the capsid or envelope In viruses with tails- tail fibers attach the virus to the host cell

Virus size Approximately 100 to 1000 fold smaller than the cells they infect Size generally from 10 nm to 500 nm The smallest viruses contain very little nucleic acid, perhaps as little as 10 genes

Viral Genome Structure of viral genome is unusual Contain only single type of nucleic acid- DNA or RNA DNA may be linear or circular, either double-stranded or single-stranded RNA is usually single-stranded

Overview- Replication Cycle of Viruses Can only multiply within living cells that are actively metabolizing Viruses lack cellular components necessary to harvest energy and synthesize proteins Viruses must use structures and enzymes of cells they infect to support their own reproduction- considered parasites

Multiplication of Bacteriophages

Bacteriophage Virus that infects bacteria Excellent model for other bacteria

Two possible outcomes following viral infection of a bacterial cell Lytic Infection: viruses multiply inside the cells they invade Lysogenic Infection: integrate viral DNA into host cell chromosome; the virus DNA replicates as the bacterial chromosome replicates

Replication of lytic phages Phage nucleic acid enters the bacterium and capsid remains outside Nucleic acid replicated along with phage proteins Many virions are formed Phages exit by bursting the cell Phages that go through this life cycle are called virulent Virulent: has the ability to overcome host defenses and cause disease

Lysogenic Infection The viral DNA is incorporated into the host genome Each time the host chromosomes replicated and split into new cells, so is the viral DNA Phage may excise itself from the chromosome and later and revert to lytic growth

Host range of phages Host range: number of different bacteria that a particular phage can infect Two factors determine the host range of a phage: Phage must be able to attach to receptors on host cell surface The restriction modification system of the host cell

Receptors on bacterial surface Receptors vary in chemical structure and location Receptors are usually on cell wall, although a few phages attach to pili or flagella Receptor sites can be modified, thereby creating a resistant cell Some temperate phages can alter the cell surface, an example of lysogenic conversion As a result receptor no longer available Thus, prophage protects it’s host and, in turn, is able to keep replicating inside of it

Restriction Modification System In some bacteria to protect themselves from viral infection Bacterial cell makes restriction enzyme and methylating enzyme Methylating enzyme adds methyl group to bacterial DNA Bacteria now knows this is it’s own DNA Uses restriction enzyme to cut any DNA that is not methylated Cuts viral DNA- inactivating it

Multiplication of Animal Viruses

Multiplication of Animal Viruses Similar to bacteriophage replication Animal viruses attach to host plasma membrane via spikes on the capsid or envelope Animal viruses are usually taken into the cytoplasm as intact nucleocapsids Uncoating is the separation of the capsid from the genome

Entry of Animal Viruses into their Host Cells

Interactions of Animal Viruses with their hosts Acute Infections Latent Viral Infections Persistant Viral Infections

Latent Viral Infections When acute infection followed by symptomless period and then reactivation of disease Symptoms of the initial and reactivated diseases may differ Provirus: latent form of virus in which viral DNA incorporated into host DNA

Examples of Latent Infections- HSV-1

Examples of Latent Infections- varicella zoster virus Initial infection of children- chicken pox Then can remain latent for years with no disease symptoms Can be reactivated and cause shingles Chicken pox and shingles- different diseases caused by same virus

Persistent Viral Infections Occurs gradually over a long period Example: Measles Several years after contracting measles can get Subacute sclerosing panenchaphalitis

Retroviridae Family of viruses that carry their genetic information as ssRNA Have enzyme reverse transcriptase which forms a DNA copy that is then integrated into the host genome

Viral-Induced Tumors Tumor- results from abnormal growth of cells Benign tumor- growth remains within defined region, not carried to other areas Malignant tumor- when abnormal growth spreads to other parts of body Tumor-causing viruses- Oncogenic Viruses

Viral-Induced Tumors Proto-oncogenes- normally occurring genes They can be converted to oncogenes by: radiation chemical carcinogens DNA damage Viruses Oncogenes then cause tumors

Viriods Much smaller than viruses Just consist of small ssRNA molecule No protein coat Infect plants

Prions Proteinaceous infectious agents Contain only protein, no nucleic acid Linked to number of fatal diseases in humans and animals Obligate intracellular parasite How does it replicate if no nucleic acid? Prion protein converts host protein to prion protein

Prions Cannot be killed by UV light or nucleases, can be killed by proteases and heat Usually cannot be transmitted across species