Viruses: a kind of “borrowed life” HIV infected T-cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Structure not cells small infectious particles w/ – DNA or RNA enclosed in a protein coat (capsid) – in some cases, a membranous envelope

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Genomes genomes may be either – Double or single stranded DNA, or – Double or single stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings help viruses infect hosts surround the capsids of influenza viruses and many other viruses found in animals derived from the host cell’s membrane – contain a combination of viral and host cell molecules Membranous Envelopes

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings AKA phages are viruses that infect bacteria most complex capsids found among viruses – w/ elongated capsid head that encloses their DNA – A protein tail piece attaches to host and injects the phage DNA inside Bacteriophages

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Reproduction obligate intracellular parasites – can reproduce only within a host cell Each virus has a host range – a limited number of host species that it can infect

Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating VIRUS DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA Viruses use the host’s cellular machinery to reproduce itself enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Two Reproductive Mechanisms 1.the lytic cycle 2.lysogenic cycle

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lytic Cycle A reproductive cycle that kills the host cell by lysing it or bursting it open with new viral progeny Bacteriophages and other viruses can reproduce in this way

Fig Attachment 1

Fig Entry of phage DNA and degradation of host DNA Attachment 1 2

Fig Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 3

Fig Phage assembly Assembly Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment HeadTailTail fibers 3

Fig Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lysogenic Cycle At the start, this cycle replicates viral genome without destroying the host The viral DNA molecule is inserted into the host cell’s chromosome – This integrated viral and host DNA is known as a prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings An environmental signal can trigger the virus genome to exit prophage and switch to the lytic mode The Lysogenic Cycle

Fig Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Classification two key variables used to classify viruses – DNA or RNA? – Single-stranded or double-stranded?

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings RNA Viruses (Retroviruses) Retroviruses use reverse transcriptase to copy their RNA genome into DNA – Ex. HIV

Glycoprotein Reverse transcriptase HIV RNA (two identical strands) Capsid Viral envelope HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus The viral DNA that is permanently integrated into the host genome is called a provirus (unlike prophage)

HIV Membrane of white blood cell HIV entering a cell 0.25 µm New HIV leaving a cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Diseases Viruses damage or kill cells and tissues: – reproduction – release of hydrolytic enzymes from lysosomes Some viruses cause infected cells to produce toxins that lead to disease symptoms Others have envelope proteins that are toxic

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings derivatives of pathogens 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 Vaccines

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Emerging Viruses appear suddenly and come to the attention of scientists Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Flu epidemics are caused by new strains of influenza virus to which people have little immunity New viral diseases can emerge when viruses spread from animals to humans Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings These strains can cause pandemics, global epidemics The “avian flu” is a virus that recently appeared in humans and originated in wild birds

(a) The 1918 flu pandemic (b) Influenza A H5N1 virus (c) Vaccinating ducks 0.5 µm