BSC 2011 Chapter 21 Viruses Viruses Viruses are
BSC 2011 Chapter 21: Viruses
Viruses • Viruses are diverse • Vary in structure, replication method and target hosts • Nearly all life forms have viruses that infect them • Virion – single virus particles (very small – 20 -250 nm) • Virions are the infectious form of a virus outside the host cell • Classified based on their type of genome
Evolution of viruses 3 main hypotheses: 1. Devolution 2. Escapist 3. Alongside
Viral Morphology Viruses are acellular Lack most components of cells Virion consists of nucleic acid core, outer protein coat (capsid) and sometimes an outer envelope Envelope – made of protein and phospholipid membranes derived from host cell Virion may also contain additional proteins such as enzymes
4 main groups of viral shapes Filamentous – long and cylindrical (many plant viruses) Isometric – roughly spherical (poliovirus and herpesvirus) Enveloped – membrane surrounding capsid (animal viruses such as HIV) Head and Tail – infect bacteria and have a head (isometric) and tail (filamentous)
• Many viruses use glycoproteins to attach to host cells via viral receptors • Envelope viruses have matrix proteins that stabilize the envelope and play a role in assembly of progeny virions
Viral Genomes • Viruses can use either DNA or RNA as genetic material • Virus core contains genome • Viral genomes are small, contain genes that encode proteins the virus cannot get from the host cell • Genome can be sing/double stranded, linear/circular, DNA/RNA
DNA viruses Viral DNA directs host cell’s replication proteins to synthesize new copies of the viral genome and transcribe and translate that genome into viral proteins Ex. Chickenpox, Hep B, Herpes
RNA Viruses • Contain only RNA as genetic material • RNA viruses encode enzymes that can replicate RNA into DNA • These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases • Therefore: mutations in RNA viruses occur more frequently than in DNA viruses. • This causes them to change and adapt more rapidly to their host Ex. Hep C, measles, rabies
Baltimore Classification • Groups organisms according to how the m. RNA is produced during the replicative cycle of the virus (7 groups) Group I: ds. DNA (double stranded DNA); m. RNA is produced by transcription in much the same way as with cellular DNA Group II: ss. DNA (single stranded DNA); converts SS into DS before transcription to m. RNA can occur
Group III – ds. RNA genome; strands separate and one is used as a template for generation of m. RNA using RNA dependent RNA polymerase encoded by the virus Group IV – (+)ss. RNA genome positive polarity; Positive polarity allows genomic RNA to serve directly as m. RNA; ds. RNA intermediates are used to make copies of the genome and produce m. RNA
• Group V – (-)ss. RNA genome negative polarity; Negative sequence is complementary to m. RNA; ds. RNA intermediates are used to make copies of the genome and produce m. RNA • Group VI – diploid ss. RNA genomes – must be converted using enzyme reverse transcriptase to ds. DNA, the ds. DNA is transported to the nucleus of host cell and inserted into host genome. Then m. RNA is produced by transcription of viral DNA that was integrated into the host genome
Group VII – partial ds. DNA genome; make ss. RNA intermediates that act as m. RNA, but are also converted back into ds. DNA genomes by reverse transcriptase
• Viruses are obligate intracellular parasites • Must attach to a living cell, be taken inside, manufacture its proteins, copy its genome and escape in order to infect other cells • Viruses can infect only certain species of hosts and only certain cells within that host • Cells that a virus may use to replicate are called permissive
• A virus must use cell processes to replicate • Cytopathic (causing cell damage) effects can change cell functions or even destroy the cell • Some infected cells (ex. Rhinovirus) die through lysis (bursting) or apoptosis releasing all progeny virions at once
• The symptoms of viral diseases result from immune response to the virus which attempts to control and eliminate the virus from the body • Many animal viruses such as HIV leave infected host cells through budding • Budding – where virions leave the cell individually; in this case, cells do not immediately lyse and die • Most viral infections follow similar steps: attachment, penetration, uncoating, replication, assembly and release
Steps of viral infection 1. Attachment – virus attaches to special receptors on host cell surface 2. Entry (penetration) – various methods – can fuse with membrane, be taken in through endocytosis or injected into the cell 3. Uncoating – once inside, viral capsid is degraded and viral genome released 4. Replication and Assembly – many different mechanisms which depend on viral genome (see Baltimore classification) 5. Egress (escape/release) – release new virions produced in host organism allowing them to infect adjacent cells and repeat (lyse vs budding)
Different Host and their viruses 1. Bacteriophages – viruses that infect bacteria Productive – when infection results in production of new virions Lytic cycle – bursts out ex. T 4 (infect E coli) Lysogenic cycle – when virus remains in cell without being released; viral genome is incorporated into host cell genome (usually refers to bacteriophages) Prophage – when phage DNA is incorporated into host cell genome ex. Lambda virus (infect E. coli) Latency – virus can infect cells but not produce virions for long period of time ex. Herpes viruses (infect humans)
2. Animal Viruses • Do not have to penetrate a cell wall Enter through: • Endocytosis • Injection • Fusion (enveloped) • Associated with a variety of human diseases
Animal viruses continued… • Acute disease – symptoms get increasingly worse for short time followed by elimination of virus from body (ex. cold and flu) • Chronic infections – caused by certain viruses (ex. Hep C) • Intermittent symptoms – ex. Herpes simplex • Asymptomatic
3. Plant viruses • Core is DNA or RNA • Ex. Tobacco mosaic virus • Damage to cell walls allows virus to enter • Damage: weather, insects, animals, fire, humans etc
Plant viruses continued • Plant offspring may inherit viral disease from parent plants • Horizontal transfer – when plant viruses are transferred between different plant • Vertical transfer – when plant viruses are inherited from a parent
Symptoms of viral disease in plants: • Hyperplasia – abnormal proliferation of cells that causes plant tumors (galls) • Hypoplasia – decreased cell growth • Cell necrosis – kills plant cells
Prevention and Treatment of Viral Infections Vaccines – • Primary method of controlling viral diseases • Vaccines prepared using live viruses, killed viruses or molecular subunits of the virus • Killed and subunit vaccines are incapable of causing disease • Live vaccines are usually most effective
• Live vaccines are made by attenuating (weakening) the virus • Danger of live vaccines – small change in virus could revert it back to its disease causing form by back mutations • Some vaccines are in continuous development because certain viruses (HIV and Influenza) have high mutation rates
Vaccines and antiviral drugs for treatment • In some cases, vaccines can be used to treat an active viral infection • Ex. Rabies and Ebola • Antiviral drugs 0 • Used to control and reduce symptoms • Inhibit virus by blocking action of 1 or more proteins
• Antivirals have been developed for: • Genital herpes • Influenza • HIV • Ribavirin – broad antiviral
Other a cellular entities Prions – infectious protein particles (no genetic material) • Cause Kuru and Mad Cow Disease Viroids – Plant pathogens, small ss circular RNA genome Much simpler than viruses Don’t manufacture proteins Known to infect plants and responsible for massive crop failures
Are Viruses Living or Nonliving? READ THE FOLLOWING ARTICLE: • https: //microbiologysociety. org/publication/past-issues/what-islife/article/are-viruses-alive-what-is-life. html
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