17 1 The Viruses 17 1 The Viruses cont Our understanding of disease genetics and some of the characteristics of life has come from the study of viruses The outer membrane of some animal viruses may include an outer membrane envelope with spike shaped glycoproteins Viruses are extremely small and noncellular and are not included in the classification of living organisms This outer membrane is a piece of the host s plasma membrane which also contains viral proteins Viruses consist of two primary structures An outer capsid An inner core of either DNA or RNA 17 1 The Viruses cont The interior of the virus also contains a variety of proteins 17 1 The Viruses cont The debate over whether viruses are living organisms has focused on several facts Viruses are obligate intracellular parasites meaning that they can only reproduce inside a living cell Viruses can be synthesized chemically in the laboratory Viruses have a genome that is subject to mutation and controls viral reproduction Viral Reproduction Viruses are specific to a particular host cell Once inside the host the viral genome takes over the cell and uses the host s enzymes ribosomes tRNA and ATP to synthesize new viral particles Reproduction of Bacteriophages A bacteriophage or phage is a virus that reproduces in a bacterium There are two possible cycles of phage reproduction A lytic cycle A lysogenic cycle 1 Reproduction of Bacteriophages cont The lytic cycle has five stages Attachment Penetration Biosynthesis Maturation Release Reproduction of Bacteriophages cont Penetration is the stage during which a viral enzyme breaches the bacterial cell wall so that the viral DNA can be injected During the biosynthesis stage the virus deactivates all host genes not needed for viral reproduction and initiates the synthesis of viral components The attachment stage involves the attachment of the capsid molecule to the receptor on the surface of the host cell Reproduction of Bacteriophages cont The assembly of new viral particles occurs during maturation During the release stage viral lysozymes rupture the bacterium to release the new virus particles The lytic cycle causes the death of the host bacteria Reproduction of Bacteriophages cont In the lysogenic cycle the host cell may not immediately lyse because the phage is latent After attachment and penetration integration occurs as the viral DNA becomes incorporated into the host DNA This latent viral DNA is called a prophage Reproduction of Bacteriophages cont Reproduction of Bacteriophages cont The prophage is replicated along with the host DNA and is passed along to all daughter cells Daughter cells with a prophage are lysogenic cells These lysogenic cells can be triggered to enter and complete the lytic cycle 2 Plant Viruses Plant viruses infect plants through damaged tissues Plant viruses spread through the plant by migrating through the plasmodesmata which interconnect plant cells Plant viruses are transmitted by several mechanisms Insects Pruning and propagating tools Seeds and pollen Viral diseases cannot be controlled with chemicals but may be with biotechnology Viral diseases cannot be controlled chemicals biotechnology Animal Viruses The reproduction of animal viruses is similar to that of bacteriophages but with some differences When animal viruses undergo attachment the envelope fuses with the plasma membrane so the virus can enter by endocytosis A virus that enters by endocytosis is uncoated as the capsid is removed The uncoated viral genome then begins the biosynthesis stage The new viral particles are released from the host cell as they bud picking up a viral capsid Animal Viruses cont Retroviruses RNA animal viruses that have a DNA stage like HIV are called retroviruses Retroviruses have an enzyme called reverse transcriptase which carries out transcription to form a cDNA from RNA The term cDNA indicates that the DNA is a copy of the viral genome The single stranded cDNA replicates to become double stranded DNA that integrates into the host genome New retroviruses are produced as the viral DNA is transcribed Emerging Viruses Emerging Viruses cont Besides HIV there are other viruses that are becoming problematic worldwide West Nile virus The SARS virus Hantavirus Ebola virus One reason that these viruses are becoming more important is that their range has changed recently Several other factors increase the incidence of viral disease Viruses have high rates of mutation Some viruses can jump from one host species to another The mode by which the virus is transmitted can also change 3 Drug Control of Human Viral Diseases The available antiviral drugs control viral infections in several ways Some antiviral compounds are structurally similar to nucleotides and interfere with viral reproduction Some drugs block reverse transcriptase Protease inhibitors are used to block maturation of viral proteins General Biology of Bacteria 17 3 Prokaryotes The first cellular organisms on the planet were single cell prokaryotes There are two types of prokaryotes Bacteria Archaea General Biology of Bacteria cont Bacteria can have several shapes Rod shaped called bacilli Spherical called cocci A curved rod called vibrio Spiral shaped called spirillium or a spirochete Some bacteria can form doublets and are therefore called diplococci or diplobacilli General Biology of Bacteria cont General Biology of Bacteria cont Bacteria have a simple structure A single closed circle chromosome contained within the nucleoid Additional circular DNA molecules plasmids Ribosomes for protein synthesis An outer cell wall reinforced with peptidoglycan For some bacteria a flagella for locomotion 4 General Biology of Bacteria cont General Biology of Bacteria cont Bacteria and archaea reproduce by binary fission The circular chromosome replicates and separates The enlarged cell is partitioned by the plasma membrane and cell wall forming two identical cells Binary fission is not a mitotic process General Biology of Bacteria cont Some bacteria can undergo conjugation using sex pili Conjugation allows for a form of sexual recombination as genetic information is exchanged between bacteria Bacteria can undergo transformation when they incorporate DNA from the environment into their own genome General Biology of Bacteria cont General Biology of Bacteria cont When bacteriophages transmit viral DNA from one bacterial cell to another the process is called transduction When bacteria experience unfavorable environmental conditions