Chapter 19 (Viruses Replicate Only in Host Cells (Replicative Cycles of…
Viruses Replicate Only in Host Cells
viruses lack metabolic enzymes and equipment for making protein such as ribosomes thus only can replicate in a host cell.
each virus can infect cells of a limited number of host this is the host range.
Viruses identify host cells by a fit between viral surface proteins and specific receptor molecules on the outside of cells
Viruses either fuse through the host cells plasma membrane or taken up by endocytosis in which the virus commander the host reprogramming the cell to copy the viral genome and manufacture viral proteins. The host provides the nucleotides for making viral nucleic acids as well as enzymes ribosomes tRNAs amino acids ATP and other components needed for making the viral proteins.
ends with thousands of viruses exploding from the host cell after forcing the cell to making the shell of the virus and replicating it.
Phages have two mechanism to replicate.
The host dies in the replication
refers to the last stage of infection which the bacterium lyses breaks open and releases the phages that were produced within the cell
a phage that only replicates by lytic cycle s a virulent phage
allows replication of the phage genome without destroying the host
Phages that can do both are called temperate phages
a DNA molecule is incorporated into a specific sit on th chromosome by viral proteins that break both circular DNA molecules and join them to each other
prophage- viral DNA
prophages are capable of generating active phages that lyse their host cells. Caused by an environmental signal such as chemical and high energy radiation.
Lysogeny spares bacteria from extinction
natural selection favors bacterial mutants with surface proteins that are no longer recognized as receptors by a particular type of phage.
when phage DNA enter a bacterium the DNA is often identified as foreign and cut up by cellular enzymes called restriction enzymes named for the ability to restrict a phages replication in a bacteria
The last defense for bacteria is the Crispr-Cas system where the Cas cuts DNA that the phages placed inside the bacterium.
Replicative Cycles of Animal Viruses
can only replicate inside host cells by utilizing viral glycoproteins that bind to specific receptor molecules
key variables the genome weather it is double or single stranded, another is the presence or absence of a membranous envelope
typically with viruses with RNA genomes have an envelope
membranous outer layer used to enter the host cell.
the cells plasma membrane is what forms the envelope around the virus.
Animal virus with the most complicated replicative cycles are retroviruses. Have an enzyme called reverse transcriptase that transcribes an RNA template into DNA an RNA DNa formation flow that is the opposite of the usual direction.
When the RNA is transcribed and turned into DNa and integrates in the DNA of a chromosome (or provirus) never leaves the host's genome.
A vaccine is a harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen
Viruses and prions are formidable pathogens in animals and plants
Viruses that suddenly become are referred to as emerging viruses
pandemic-a disease spread over a whole country
Three process contribute to the emergence of viral disease
mutation of existing viruses
dissemination of a viral disease from a small isolated human population
the spread of existing virsus from other animals
Viruses mutate in their replication since there is no enzymes to check the RNA before being turned to DNA
Viruses in plants is commonly seen through discoloration
prions- are proteins that cause a number of degenerative brain diseases in various animal species
Prions act slow take about 10 years to incubate and prions are virtually indestructible
A Virus Consists Of a Nucleic Acid Surrounded By a Protein Coat.
Structure of Viruses
tiniest viruses are 20 nm
infectious particle consisting of nucleic acid enclosed in a protein coat some surrounded by a membranous envelope
Genomes consist of double stranded DNA single stranded DNA double stranded RNA or single stranded RNA
organized as a single linear or circular molecule of nucleic acid some viruses consist of multiple molecules of nucleic acid.
protein shell is enclosed in a capsid made from capsomers
capsid shapes-rod shaped, polyhedral, or complex in shape.
helical viruses shaped like a helix, icosahedral viruses arranged in a polyhedral with 20 triangular facet.
some viruses have envelopes that surround the capsule called the viral envelope that helps them infect their host since they are derived from the membranes of the host cell containing phospholipids, membrane proteins, proteins, and glycoproteins.
Bacteriophages- viruses that infect bacteria or phages