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Lecture 9 Viruses, Viroids, Prions
47

Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Dec 27, 2015

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Page 1: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Lecture 9

Viruses, Viroids, Prions

Page 2: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 3: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Virus Shapes

Page 4: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 5: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 6: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 7: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Two basic types of virions

• Naked: consist only of nucleic acid and capsid

• Enveloped virus: consists of nucleic acid, capsid, and envelope

Page 8: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 9: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 10: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 11: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 12: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 13: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 14: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Multiplication of Bacteriophages

Page 15: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Bacteriophage

• Virus that infects bacteria

• Excellent model

for other bacteria

Page 16: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 17: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 18: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 19: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 20: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 21: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 22: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 23: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 24: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 25: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 26: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Multiplication of Animal Viruses

Page 27: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 28: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Entry of Animal Viruses into their Host Cells

Page 29: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 30: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 31: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Interactions of Animal Viruses with their hosts

• Acute Infections

• Latent Viral Infections

• Persistant Viral Infections

Page 32: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 33: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 34: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 35: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Examples of Latent Infections-HSV-1

Page 36: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 37: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Persistent Viral Infections

• Occurs gradually over a long period

• Example: Measles

• Several years after contracting measles can get Subacute sclerosing panenchaphalitis

Page 38: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 39: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 40: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 41: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 42: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 43: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Viriods

• Much smaller than viruses

• Just consist of small ssRNA molecule

• No protein coat

• Infect plants

Page 44: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

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

Page 45: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.
Page 46: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.

Prions

• Cannot be killed by UV light or nucleases, can be killed by proteases and heat

• Usually cannot be transmitted across species

Page 47: Lecture 9 Viruses, Viroids, Prions. Viruses Parasites that are not cells or organisms Infect all forms of life:Bacteria, Archea, Eukarya Virus particle.