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Viruses What are they? What do they do? Martin Rowland Philip Allan Publishers © 2015 Viruses vary in shape and size • Bacteriophage T4, 225 nm long • Adenovirus, 90 nm • Rhinovirus, 30 nm • Tobacco mosaic virus — rod-shaped, 250 × 18 nm • Vaccinia virus — brick-shaped or ovoid, 300 nm × 200 nm × 100 nm • Ebola virus, 970 nm • Poliovirus, 30 nm bacteriophage Ebola adenovirus Compare with Escherichia coli (a bacterium) 3000 × 1000 nm and a human red blood cell, 10 000 nm diameter, surface membrane 10 nm thick Philip Allan Publishers © 2015 What do viruses have in common? 1 Structure: ● Extremely small ● Each consists of a particle (called a virion) – outer complex of proteins (the capsid) – inner nucleic acid core (DNA or RNA) ● In addition: – some have a lipoprotein envelope outside the capsid – some contain enzymes associated with entering host cell and replication of nucleic acid Philip Allan Publishers © 2015 A simple example of viral structure ● Human papillomavirus (HPV) causes warts in humans ● The capsid contains two types of protein: – L1 (shown in yellow) – L2 (shown in red) ● The double-stranded DNA (shown in blue) is circular Philip Allan Publishers © 2015 What do viruses have in common? 2 Function: ● Viruses have no metabolism of their own, e.g. they do not use ATP and are unable to produce proteins ● Viruses can only be replicated using the metabolism of another living cell (the host) – In doing so they cause harm to the host cell ● Outside a suitable host cell, viruses are inert Philip Allan Publishers © 2015 Five different viruses ● Lambda bacteriophage (λ phage) ● Tobacco mosaic virus (TMV) ● Ebola virus ● Human immunodeficiency virus (HIV) ● Influenza (‘flu’) virus Philip Allan Publishers © 2015 Lambda bacteriophage (λ phage) ● Capsid has a head and a tail region ● Nucleic acid is double-stranded DNA, which is transcribed by the host cell into mRNA ● Infects the bacterium Escherichia coli Philip Allan Publishers © 2015 Tobacco mosaic virus (TMV) ● Capsid is a spiral of polypeptides ● Nucleic acid is single-stranded RNA, which is transcribed by host cell to form mRNA ● Infects a wide range of plants, especially tobacco and other members of the Solanaceae family Philip Allan Publishers © 2015 Ebola virus ● Capsid of protein ● Nucleic acid is single-stranded RNA, which is transcribed by the host cell to form mRNA ● Infects several types of human cell Philip Allan Publishers © 2015 Human immunodeficiency virus (HIV) ● Capsid surrounded by phospholipid and glycoprotein envelope ● Nucleic acid is single-stranded RNA, which is transcribed inside the host to form single-stranded DNA ● Enzymes include reverse transcriptase ● Infects macrophages and T helper cells of humans Philip Allan Publishers © 2015 Influenza virus ● Capsid surrounded by phospholipid and glycoprotein envelope ● Eight short, single-stranded, negative-sense RNA fragments that are transcribed by the host cell to form mRNA ● Infects epithelial cells of the nose, throat and lungs of mammals Philip Allan Publishers © 2015 How does a virus infect a cell? ● Proteins on the surface of the virus are complementary to specific proteins (or glycoproteins) on the surface membrane of a target cell ● For example, HIV attaches to a protein called CD4 found on the surface of T lymphocytes ● If a random collision is perfectly aligned, the viral protein binds to the protein or glycoprotein on the surface of the target cell ● The viral nucleic acid then enters the target cell Philip Allan Publishers © 2015 What does the viral nucleic acid do? Either: ● Becomes attached to the DNA of the host cell and remains dormant for a period of time ● This period of time is called latency ● Infection by Varicella zoster during childhood causes chickenpox. The virus can show latency in nerve cells and, in adulthood, reappear as shingles ● Herpes virus (cold sores and genital herpes) can also show latency in nerve cells Philip Allan Publishers © 2015 What does the viral nucleic acid do? or: ● Takes over control of the host cell causing it to – replicate the virus – produce new viral capsids – assemble new virions – release the new virions Philip Allan Publishers © 2015 The lytic cycle of the λ phage Philip Allan Publishers © 2015 ‘Life’ cycle of the flu virus Viral antigens attach to receptors on lung epithelial cells and trigger endocytosis New virus particles leave host cell, picking up a lipid coat in the process Viral RNA and proteins assembled by host cell to form new virus particles Philip Allan Publishers © 2015 Viral RNA enters nucleus of epithelial cell where it is transcribed and replicated Copies of viral RNA leave nucleus and are translated by host cell’s ribosomes How can viral infections be treated? ● Since viruses have no metabolism, they cannot be ‘killed’ ● Vaccines against one or more viral antigens are effective in reducing the risk of infection ● Antivirals prevent one or more stages of the viral life cycle. For example: – prevent binding of the virus to the target cell – inhibit virus-specific enzymes, e.g. those that allow the virus to enter the target cell, allow virus nucleic acid to become incorporated into host DNA, or allow replication of viral nucleic acid ● Prevention of spread is a key strategy Philip Allan Publishers © 2015