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 Acid2. Protein coat (capsid)
• Nucleocapsid- capsid with nucleic acid inside
• Each capsid made of identical protein subunits called capsomeres
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
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:
1. Phage must be able to attach to receptors on host cell surface
2. 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
• 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
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-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