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)
