Chapter 19 Viruses

Overview: A Borrowed Life Viruses lead “a kind of borrowed life” between life-forms and chemicals. The origins of molecular biology lie in early studies of viruses that infect bacteria. A virus consists of a nucleic acid surrounded by a protein coat.

Tobacco Mosaic Virus Causes Disease RESULTS Tobacco Mosaic Virus Causes Disease 1 Extracted sap from tobacco plant with tobacco mosaic disease 2 Passed sap through a porcelain filter known to trap bacteria 3 Rubbed filtered sap on healthy tobacco plants Figure 19.2 What causes tobacco mosaic disease? 4 Healthy plants became infected

Structure of Viruses : Viruses are not cells. Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and often a membranous envelope. A capsid is the protein shell that encloses the viral genome. Viral envelopes surround the capsids of viruses and enable the viruses to evade detection by the host likely because viral envelopes are derived from the host cell’s membrane.

Viruses / Structures RNA DNA Membranous envelope Head RNA Capsomere Capsid Tail sheath Capsomere of capsid Tail fiber Glycoprotein Glycoproteins 18  250 nm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm Figure 19.3 Viral structure 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Bacteriophages, also called phages, are viruses that infect bacteria. Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell. Each virus has a host range, a limited number of host cells that it can infect.

Viral Reproductive Cycle VIRUS Entry and uncoating 1 DNA Capsid Transcription and manufacture of capsid proteins 3 2 Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Figure 19.4 A simplified viral reproductive cycle Self-assembly of new virus particles and their exit from the cell 4

Phages have two reproductive cycles: the lytic cycle and the lysogenic cycle The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell. The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses. Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA.

Lytic Cycle Attachment Entry of phage DNA and degradation of host DNA 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host. The viral DNA molecule is incorporated into the host cell’s chromosome. This integrated viral DNA in the host genome is known as a prophage = dormant. Every time the host divides, it copies the dormant phage DNA and passes the copies to new cells. Stress or immunosuppression triggers the virus genome to exit the bacterial chromosome and switch to the lytic mode.(Cold sores)

Viral Reproductive Cycles: Lytic and Lysogenic Daughter cell with prophage Phage DNA The phage injects its DNA. Binary Fission Cell divisions produce population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle The bacterium reproduces, copying the dormant prophage & transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage Prophage New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a dormant prophage.

Viruses Animal Viruses: There are two key variables used to classify viruses that infect animals: DNA or RNA? Single-stranded or double-stranded?

Animal Viruses Table 1

Viral Envelopes - Help Viruses Attach to Host and Evade Host Detection Viral glycoproteins on the membranous envelope bind to specific receptor molecules on the surface of a host cell. Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit.

Virus and Host Cell Capsid and viral genome enter the cell Capsid RNA Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins Figure 19.7 The reproductive cycle of an enveloped RNA virus New virus

RNA as Viral Genetic Material Retroviruses use reverse transcriptase to copy their RNA genome into DNA. (reverse of transcription) HIV (human immunodeficiency virus) is a retrovirus.

RetroVirus: Reverse Transcriptase Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse transcriptase HIV Membrane of white blood cell HIV HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS RNA genome for the next viral generation mRNA New virus New HIV leaving a cell

The viral DNA that is integrated into the host genome is called a provirus. Unlike a prophage, a provirus remains a permanent resident of the host cell. The host’s RNA polymerase transcribes the proviral DNA into RNA molecules. The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell.

Evolution of Viruses Viruses do not fit our definition of living organisms - exceptions to Cell Theory. Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid. Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments. Plasmids, transposons, and viruses are all mobile genetic elements.

Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen. Vaccines can prevent certain viral illnesses. Viral infections cannot be treated by antibiotics. Antiviral drugs can help to treat, though not cure, viral infections. Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory.

Flu epidemics are caused by new strains of influenza virus to which people have little immunity. Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity. These strains can cause pandemics, global epidemics. The “avian flu” is a virus that recently appeared in humans and originated in wild birds.

Viroids & Prions: The Simplest Infectious Agents Viroids are circular RNA molecules that infect plants and disrupt their growth. (No protein) Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals. Prions propagate by converting normal proteins into the prion version. Scrapie in sheep, mad cow disease, CWD and Creutzfeldt-Jakob disease in humans are all caused by prions.

Prions = Infectious Proteins Original prion Prion Aggregates of prions New prion Normal protein Figure 19.11 Model for how prions propagate

Prions Prions are scary because they can be infectious between species and are difficult to sanitize against Can be the result of natural-occurring genetic mutation or infection Eat your brains

You should now be able to: Explain how capsids and envelopes are formed. Distinguish between the lytic and lysogenic reproductive cycles. Explain why viruses are obligate intracellular parasites. Describe the reproductive cycle of an HIV retrovirus. Describe three processes that lead to the emergence of new diseases. Describe viroids and prions.