2Discovery of VirusesA virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids.Viruses can reproduce only by infecting living cells.
3Structure and Composition Viruses differ widely in terms of size and structure.Most viruses are so small they can be seen only with the aid of a powerful electron microscope.
4Structure and Composition Most viruses have proteins on their surface membrane or capsid that bind to receptor proteins on the host cell.The proteins “trick” the cell to take the virus, or in some cases just its genetic material, into the cell.Once inside, the viral genes are eventually expressed and may destroy the cell.
5Structure and Composition Most viruses infect only a very specific kind of cell.Plant viruses infect plant cells.Most animal viruses infect only certain related species of animals.viruses that infect bacteria are called bacteriophages.
6Viral Infections What happens after a virus infects a cell? Inside living cells, viruses use their genetic information to makemultiple copies of themselves.Some viruses replicate immediately, while others initiallypersist in an inactive state within the host.
7Types of Viral Infections Lytic infectionLysogenic Infection
8Lytic InfectionsIn a lytic infection, a virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse.Bacteriophage T4 is an example of a bacteriophage that causes such an infection.
9Lytic InfectionsBacteriophage T4 has a DNA core inside a protein capsid that binds to the surface of a host cell.
10Lytic InfectionsThe virus injects its DNA into the cell.
11Lytic InfectionsControlled by viral genes, the host cell’s metabolic system makes copies of viral nucleic acid.
12Lytic InfectionsThe host cell lyses, releasing hundreds of virus particles that go on to infect other cells.
13Lytic InfectionsA lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host.First, the outlaw eliminates the town’s existing authority.In a lytic infection, the host cell’s DNA is chopped up.
14Lytic InfectionsNext, the outlaw demands to be outfitted with new equipment from the local townspeople.In a lytic infection, the viruses use the host cell to make viral DNA and viral proteins.
15Lytic InfectionsFinally, the outlaw forms a gang that leaves the town to attack new communities.In a lytic infection, the host cell bursts, releasing hundreds of virus particles.
16Lysogenic InfectionSome bacterial viruses cause a lysogenic infection.
17Lysogenic InfectionIn a lysogenic infection a host cell is not immediately taken over.
18Lysogenic InfectionThe viral nucleic acid is inserted into the host cell’s DNA.
19Lysogenic InfectionThe viral DNA is then copied along with the host DNA without damaging the host.
20Lysogenic InfectionViral DNA multiplies as the host cells multiply.
21Lysogenic InfectionIn this way, each generation of daughter cells derived from the original host cell is infected.
22Lysogenic InfectionBacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage.
23Lysogenic InfectionThe prophage may remain part of the DNA of the host cell for many generations.
24Lysogenic InfectionInfluences from the environment—radiation, heat, etc—trigger the prophage to become active.
25Lysogenic InfectionIt then removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.
26A Closer Look at Two RNA Viruses About 70 percent of viruses contain RNA rather than DNA.In humans, RNA viruses cause a wide range of infections, from relatively mild colds to severe cases of HIV.Certain kinds of cancer also begin with an infection by viral RNA.
27The Common ColdCold viruses attack with a very simple, fast-acting infection.A capsid settles on a cell, typically in the host’s nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.
28The Common ColdThe host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it into capsids and other viral proteins.The new capsids assemble around the viral RNA copies, and within 8 hours, the host cell releases hundreds of new virus particles to infect other cells.
29Common Cold Video- How the Flu Virus infects A virus infects a host cell and then uses it to make more virusesThe viruses then infect more cells, until the host is weakened or killedHere is a video that explains how a common cold virus infects cells
30HIVThe deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV).HIV belongs to a group of RNA viruses that are called retroviruses.The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.
31HIVWhen a retrovirus infects a cell, it makes a DNA copy of its RNA.
32HIVThe copy inserts itself into the DNA of the host cell.
33How HIV Infects the Cell Video HIV virus attacks a cell and then uses it to make more viruses
34HIVRetroviral infections are similar to lysogenic infections of bacteria. Much like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.
35Viruses and CellsAll viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process.Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts.
36Viruses and CellsViruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.
37Viruses and CellsSome of the main differences between cells and viruses are summarized in this chart.
38Viruses and CellsAlthough viruses are smaller and simpler than the smallest cells, it is unlikely that they were the first living organisms.Because viruses are dependent upon living organisms, it seems more likely that viruses developed after living cells.The first viruses may have evolved from the genetic material of living cells. Viruses have continued to evolve, along with the cells they infect, for billions of years.