Ch. 19- VirusesCh. 19- Viruses- Viruseso A virus is an infectious particle consisting of genes packaged in a protein coat o Viruses are much simpler in structure than even prokaryotic cells o Viruses cannot reproduce or carry out metabolism outside of a host cell.- Structure of Viruses o Viruses are not cells o A virus is a very small infectious particle consisting of nucleic acid (either DNA or RNA) enclosed in a protein coat and, in some cases, a membranous envelope - Viral Genomeso Viral genomes may consist of either  Double- or single-stranded DNA, or  Double- or single-stranded RNA o Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA viruso The genome is either a single linear or circular molecule of the nucleic acid o Viruses have between three and several thousand genes in their genome- Capsids and Envelopes o A capsid is the protein shell that encloses the viral genome o Capsids are built from protein subunits called capsomeres  A capsid can have a variety of structureso Some viruses have accessory structures that help them infect hostso Viral envelopes (derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals o Viral envelopes contain a combination of viral and host cell moleculeso Bacteriophages, also called phages, are viruses that infect bacteria o They have the most complex capsids found among viruses o Phages have an elongated capsid head that encloses their DNA o A protein tail piece attaches the phage to the host and injects the phage DNA inside - The Lytic Cycleo The lytic cycle is a phage replicative cycle that culminates in the death of the hostcell o The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses o A phage that reproduces only by the lytic cycle is called a virulent phage  Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA  An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode  Phages that use both the lytic and lysogenic cycles are called temperate phages - RNA as Viral Genetic Materialo The broadest variety of RNA genomes is found in viruses that infect animalso Retroviruses use reverse transcriptase to copy their RNA genome into DNA  HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome) o The viral DNA that is integrated into the host genome is called a proviruso Unlike a prophage, a provirus remains a permanent resident of the host cell o RNA polymerase transcribes the proviral DNA into RNA molecules o 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 Viruseso Viruses do not fit our definition of living organisms o Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid o Candidates for the source of viral genomes include plasmids and transposons o Plasmids, transposons, and viruses are all mobile genetic elements - Viral Diseases in Animals o Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes o Some viruses cause infected cells to produce toxins that lead to disease symptoms o Others have molecular components such as envelope proteins that are toxic.o Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful 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 - Emerging Viruseso Emerging viruses are those that suddenly become apparent o In 2009, a general outbreak (epidemic) of a flu- like illness appeared in Mexico and the United States, caused by an influenza virus named H1N1o Flu epidemics are caused by new strains of influenza virus to which people have little immunity o Viral diseases in a small isolated population can emerge and become global o New viral diseases can emerge when viruses spread from animals to humans o Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity o Pandemics: global epidemics- Viral Diseases in Plantso More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots o Most plant viruses have an RNA genome o Many have a helical capsid, while others have an icosahedral capsid - Viriods and Prionso Viroids are small circular RNA molecules that infect plants and disrupt their growth.o Prions are slow-acting, virtually indestructible infectious proteins that cause braindiseases in