Chapter 19 Viruses

Microbial Model Systems Are viruses living organisms? –Maybe The origins of molecular biology lie in early studies of viruses that infect bacteria T4 bacteriophage attacking E. coli

Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes Viruses are smaller and simpler than bacteria Virus Bacterium Animal cell Animal cell nucleus 0.25 µm

Viruses Viruses were detected indirectly long before they were actually seen Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease In 1935, this hypothesis was confirmed by crystallizing the infectious particle, tobacco mosaic virus (TMV)

Structure of Viruses Viruses are not cells Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viral genomes may consist of: 1.Double- or single-stranded DNA 2.Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

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

(a) Tobacco mosaic virus 20 nm 18 x 250 nm Capsomere of capsid RNA DNA Capsomere Glycoprotein 70–90 nm (diameter) 50 nm (b) Adenoviruses capsid- is the protein shell that encloses the viral genome; each protein subunit is a capsomere

Some viruses have membranous envelopes that help them infect hosts –surround the capsids of influenza viruses and many other viruses found in animals –are derived from the host cell’s membrane, contain a combination of viral and host cell molecules Membranous envelope RNA Capsid Glycoproteins 80–200 nm (diameter) 50 nm (c) Influenza viruses

Bacteriophages, also called phages, are viruses that infect bacteria They have: –the most complex capsids found among viruses –an elongated capsid head that encloses their DNA –a protein tailpiece that attaches the phage to the host and injects the phage DNA inside Head DNA Tail sheath Tail fiber 80 x 225 nm 50 nm (d) Bacteriophage T4

DNA VIRUS Capsid HOST CELL Viral DNA Replication Entry into cell and uncoating of DNA Transcription Viral DNA mRNA Capsid proteins Self-assembly of new virus particles and their exit from cell 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 Once a viral genome has entered a cell, the cell begins to manufacture viral proteins The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules Viral Replicative Cycle host enzymes replicate the viral genome

Reproductive Cycles of Phages Phages are the best understood of all viruses Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Phages have two reproductive mechanisms: 1. the lytic cycle 2. the lysogenic cycle

Attachment 1 Entry of phage DNA and degradation of host DNA Attachment 1 2 Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment Phage assembly Assembly Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment HeadTailTail fibers 3 Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment A phage that reproduces only by the lytic cycle is called a virulent phage The Lytic Cycle produces new phages digests the host’s cell wall culminates in the death of the host cell

The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome; this integrated viral DNA is known as a prophage Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Certain factors determine whether Lysogenic cycle Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage Many cell divisions produce a large population of bacteria infected with the prophage. Daughter cell with prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. temperate phage

Phage DNA The phage attaches to a host cell and injects its DNA. Bacterial chromosome Prophage Lytic cycle Lysogenic cycle Temperate phage only Virulent or temperate phage Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromo- some and initiate a lytic cycle Lysis of host cell causes release of progeny phages Destruction of host DNA Production of new phages

Two key variables in classifying viruses that infect animals: –DNA or RNA –single-stranded (ss) double-stranded (ds)

DNA RNA HPV

Retroviruses The broadest variety of RNA genomes is found in viruses that infect animals Retroviruses- use reverse transcriptase to copy their RNA genome into DNA Human immunodeficiency virus (HIV) is the retrovirus that causes acquired immunodeficiency syndrome (AIDS) Class VI. ssRNA; template for DNA synthesis

HOST CELL reverse transcriptase- catalyzes the synthesis of a DNA strand complementary to the viral RNA and a second DNA strand complementary to the 1st Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA Provirus RNA genome for the next viral generation mRNA New HIV leaving a cell HIV entering a cell 0.25 µm HIV Membrane of white blood 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

Viral Disease Diseases caused by viral infections affect humans, agricultural crops, & livestock worldwide Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes Some viruses cause infected cells to produce toxins that lead to disease symptoms Smaller, less complex entities called viroids and prions also cause disease in plants and animals

Vaccines vaccines- are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen –can prevent certain viral illnesses Hepatitis B, A Polio (IPV) Rotavirus MMRV- Measles, Mumps, Rubella, Varicella (chicken pox) HPV Other crucial childhood vaccines that protect against bacteria:  DTaP (Diptheria, Tetanus, Pertussis [whooping cough])  Hib (Haemophilus influenzae type b)  Pneumococcal

HIV can be treated with the drug AZT. AZT has a 100- to 300-fold greater affinity for the HIV reverse transcriptase, as compared to the human DNA polymerase antiviral drugs- can help to treat, though not cure, viral infections by interfering with viral replication Viral infections cannot be treated by antibiotics