Chapter 19 Flashcards
A virus is an infectious particle consisting of genes packaged in a protein coat
Viruses are much simpler in structure than even prokaryotic cells
Viruses cannot reproduce or carry out metabolism outside of a host cell
Viruses were detected indirectly long before they were actually seen
Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration
In the late 1800s, some researchers hypothesized that a particle smaller than bacteria caused the disease
In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)
Viruses are not cells
A virus is a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
Viral genomes may consist of either
Double- or single-stranded DNA, or
Double- or single-stranded RNA
Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
The genome is either a single linear or circular molecule of the nucleic acid
Viruses have between three and several thousand genes in their genome
Some viruses have accessory structures that help them infect hosts
Viral envelopes (derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals Viral envelopes contain a combination of viral and host cell molecules
Bacteriophages, also called phages, are viruses that infect bacteria
They have the most complex capsids found among viruses
Phages have an elongated capsid head that encloses their DNA
A protein tail piece attaches the phage to the host and injects the phage DNA inside
Viruses are obligate intracellular parasites, which means they can replicate only within a host cell
Each virus has a host range, a limited number of host cells that it can infect
Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Phages are the best understood of all viruses
Phages have two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle
The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses
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
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated into the host cell’s chromosome
This integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
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
There are two key variables used to classify viruses that infect animals
An RNA or DNA genome
A single-stranded or double-stranded genome
Whereas few bacteriophages have an envelope or an RNA genome, many animal viruses have both
Many viruses that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are derived from the host cell’s plasma membrane as the viral capsids exit
Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane
The broadest variety of RNA genomes is found in viruses that infect animals
Retroviruses use reverse transcriptase to copy their RNA genome into DNA
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)
The viral DNA that is integrated into the host genome is called a provirus
Unlike a prophage, a provirus remains a permanent resident of the host cell
RNA polymerase transcribes the proviral DNA into RNA molecules
The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell
Viruses do not fit our definition of living organisms
Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid
Candidates for the source of viral genomes include plasmids and transposons
Plasmids, transposons, and viruses are all mobile genetic elements
The largest virus yet discovered is the size of a small bacterium, and its genome encodes proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis
There is controversy about whether this virus evolved before or after cells
Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide
Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
Some viruses cause infected cells to produce toxins that lead to disease symptoms
Others have molecular components such as envelope proteins that are toxic
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 viruses are those that suddenly become apparent
In 2009, a general outbreak (epidemic) of a flu-like illness appeared in Mexico and the
United States, caused by an influenza virus named H1N1
Flu epidemics are caused by new strains of influenza virus to which people have little immunity
Viral diseases in a small isolated population can emerge and become global
New viral diseases can emerge when viruses spread from animals to humans
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity
These strains can cause pandemics, global epidemics
The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu”
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
Most plant viruses have an RNA genome
Many have a helical capsid, while others have an icosahedral capsid
Plant viruses spread disease in two major modes
Horizontal transmission, entering through damaged cell walls
Vertical transmission, inheriting the virus from a parent
Viroids are small circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version
Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions
