chapter 19, viruses
structure of viruses
they consist of nucleic acid that enclosed by protein coats, for some have a membranous envelope
viruses can have double-stranded DNA, single-stranded DNA, double and single stranded RNA depending on the type of virus.
a DNA virus or an RNA virus based on the nucleic acid that makes up the genome
capsids and envelopes
capsids are the protein shell that enclose the viral genome
capsids are built from a large number of protein subunits called capsomeres
tobacco mosaic virus, rod-shaped capsid made from over thousands of molecules a single protein in a helix shape
adeno viruses, infect animals respiratory tract, proteins arranged polyhedral capsid that has 20 triangular facets called an icosahedron
there are some viruses that have structures that allow them to infect their hosts
influenza virus has a membranous envelope that surrounds the capsid
bacteriophage (phages), complex capsids found in viruses that infect bacteria
the capsids have icosahedral heads that enclose the DNA, attached to the head is a protein tail piece that have fibers in which the phages attach to a bacterial cell
viruses replicate only in host cells
viruses don't have metabolic enzymes and equipment in order to make proteins such as ribosomes
but viruses can only replicate within a host cell, b/c they are intracellular parasites
their is a limitation to what a virus can infect when it is in a host called the host range
viruses recognize the host cell by a "lock-and-key", allowing it to fit between viral surface proteins and specific receptors molecules on the outside of the cell
cycle of phages
the lytic cycle, lead up to the death of the host cell
it refers to the last stage of infection, the bacterium lyses and the phages are released that were produced
the phages infect the healthy cells, and the lytic cycle can destroy bacterial populations in just a few hours
a virulent phage is a page that replicates only by the lytic cycle
the lysogenic cycle, allows the replication of the phage genome w/ destroying the host
there are phages that can use both modes of replication with a bacterium are called temperate phages
replicative cycles of animal viruses
animal viruses have an envelope called the outer membrane allowing it to enter the host cell
on the outside of the envelope are viral glycoproteins that bind to a certain receptor on the host cell
cycle with the enveloped virus with a RNA genome
(1) ribosomes bound to (ER) of host cells make are able to make the protein parts of the envelope glycoprotein.
(2) the (ER) and the Golgi apparatus have cellular enzymes which add sugars
(3) resulting viral glycoproteins. embedded in the membrane derived from the host cell are transported tot eh cells surface
(4) new viral capsids are wrapped in membrane as they bud from the cell
(5) the enveloped viruses are free to infect other cells
the replicative cycle don't kill the host cell
RNA as viral genetic material
three types of single stranded RNA genomes in animal viruses
genome class of IV viruses serve as mRNA and are translated into viral protein after infection
evolution of viruses
an isolated virus in biologically inert, so it cant replicate its gene or make its own ATP
even though viruses cant replicate or carry out metabolic activities independently
viruses have been found that infect every form of life (bacteria, plants ,fungi, animals, archaea etc.) b/c they depend on cell for reproduction
evolution of genes coding for capsid proteins have allowed viruses to bind cell membranes, facilitating infection to uninjured sites
viral diseases in animals
viruses can damage or kill cells from the release of hydrolytic enzymes
viruses cause the infected cells to produce toxins, producing symptoms.
the damage that viruses cause depends on the ability of the infected tissue to regenerate
temporary symptoms of viral infections is a way the body defends itself against infection
vaccine, harmless variant of the pathogen that stimulate the immune system to mount defenses against the harmful pathogen
b/c antibiotics kill bacteria by inhibiting enzymes specific to bacteria, and have no effect on viral encoded enzymes
ex: smallpox a viral infection
emerging viruses
suddenly become apparent
ex: HIV and AIDS, Ebola, encephalitis
epidemic, wide outbreak in Mexico and United States known as influenza virus (H1N1)
pandemic, global epidemic
viruses burst on to the human scene in three ways
first is the mutation of existing viruses b/c RNA have a high mutation rate, but do not proofread and correct errors in replicating
second, mergence of viral is the spreading of viral disease in small isolated populations
some mutations change existing viruses into new genetic varieties
their use of genetic codes makes it hard to deny evolutionary connection
ex: AIDS went unnoticed until it began to spread
third, viral diseases in humans spread of existing viruses from other animals
animals that carry a virus are unaffected by it, and are considered natural reservoir for the virus
ex: swine flu, from pig to human
three types of influenza virus
type B, and C infect only humans never caused an epidemic
type A, infect a wide range of animals, birds, horses, pigs and humans
different strains of A are given names
name identifies tell us which form of two viral proteins are present, hemagglutinin (H), and neuraminidase (N)
(H) there are 16 different types, a protein that helps the flu virus attach to the host
(N) there are 9 types, enzyme that helps release new virus particles from infected cells
viral diseases in plants
common sights of vial infection in plants
bleached or brown spots on leaves or fruits, stunted growth, damaged flower or roots
viral diseases spread either horizontal or vertical tranmisson
horizontal a plant is infected by from external source of the virus, it has to get past the protective layer
vertical, inheriting viruses from parent
a plant becomes more vulnerable to viral infection if it was damaged by wind, injury or herbivores
herbivores are a threat b/c the act as carriers of viruses
occur in asexual propagation or in sexual reproduction
virus enter plant cell and replicates genomes and proteins it can spread throughout the plant by means of plasmodesmata
plasmodesmata, the cytoplasmic connections that penetrate the walls between adjacent pant cells
passage of viral macromolecules cell to cell, is facilitated by virally encode proteins that enlarge the plasmodesmata
viroids and prions
viriods circular RNA molecules that infect plants
they don't encode proteins but use host cell enzymes
viriods cause errors in the regularity hat control plant growth
typical signs are abnormal development and stunted growth
prions an infectious protein
causing degenerative brain diseases in animal species
ex: mad cow disease, scrapie in sheep, Creutzfeldt-Jakob disease in humans
prions act slowly with an incubation period of about ten years before symptoms develop
secondly prions are indestructible
prions are misfolded versions of normal brain proteins
the prions convert the normal proteins to the misfolded prion
prion aggregation interferes with normal cellular functions and cuases disease symptoms
lysogenic signifies prophages capable of generating active phages that lyse their own host cells
a switch over from lysogenic to lytic cycle is triggered by chemical or high energy radiation
class V, the RNA genome serves instead as a template for mRNA synthesis
the RNA genome is transcribed into RNA strands, functioning as mRNA and as a template for synthesis of additional copies of genomic RNA
viruses that use RNA genome as template for mRNA transcription require RNA to RNA synthesis
most complicated RNA animals viruses is the retrovirus
virus equipped with an enzyme called reverse transcription
transcribes the RNA template into DNA, RNA to DNA information flow the opposite of the usual direction
ex: HIV that causes AIDS