BIOL. 1406 Chapter 19 Viruses Flashcards
How does a virus make more viruses?
A generic virus containing a viral genome enters the host cell and releases the genome;
The viral genome is replicated by enzymes;
Copies of genome are transcribed by host enzymes;
Viral mRNA is translated into viral proteins;
New viruses are released form the cell.
With some viruses, the host cell bursts, releasing new viruses.
With other viruses, the host cell remains alive, releasing new viruses.
Viruses
Cellular parasites that exist by manipulating cellular processes.
Scientists use molecular tools to manipulate cells.
Virus
An infectious particle, consisting of viral genome that is packaged into a protein coat and, in some cases, a membranous envelope
Cannot reproduce or carry out processes outside of host cell
Viral Genomens
Single/double-stranded DNA or RNA;
Either a single linear or circular molecule of the nucleic acid
Capsid
The protein shell that encloses the viral genome;
Made up of small subunits - capsomeres:
Can have a variety of structures; may be helical or icosahedral
Viral envelopes
Derived from membranes of host cells; surround the capsid of influenza and many other viruses found in animals
Bacteriophages
Phages;
Viruses that infect bacteria;
They 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
Host Range
A number of species it can infect;
some viruses have a broad one while others can only infect one species
Two alternative reproductive mechanisms of phages
Lytic and lysogenic cycles
Lytic Cycle
A phage replica give cycle that culminated during host cell’s death;
Produces new phages and breaks apart the host cell wall, releasing the progeny viruses
Virulent phage
A phage that reproduces by the lytic cycle
Lysogenic cycle
phage reproductive cycle that replicates viral DNA without destroying the host cell;
viral genome is incorporated in host cell’s DNA;
every time a cell divides, cell’s DNA replicates with the phage’s DNA shared by the two daughter cells;
an environmental signal can trigger bacteria to enter the lytic cycle;
some prophages are expressed during lysogeny, while others secrete bacterial toxins harmful to humans
Temperate Phages
Bacteriophages that use both the lytic and lysogenic cycles
Prophage
The integrate viral DNA inside the host’s genome
Bacterial Defenses Against Phages
natural selection favors mutants with surface proteins that are not recognized as receptors by a particular type of phage;
foreign DNA can be identified as such and cut up by restriction enzymes (inside cell);
bacteria and archaea can protect themselves with the use CRISPR-Cas9
CRISPR-Cas9
a DNA sequence inside bacteria that contains clustered, regularly, interspaced, short, palindromic repeats;
each repeat codes for a viral genome that has entered host’s cell;
once transcribed, it attaches to the Cas9 protein and finds the complementary viral genome that entered the cell;
the protein cuts up the invaded agent, disabling it;
the technique may be used to alter, delete, or insert nucleotides
Two key variables that are used to identify animal viruses
an RNA or DNA genome that can be single- or double-stranded;
presence or absence of a membranous envelope;
Whereas few bacteriophages have an envelope, many animal viruses do.
Whereas few bacteriophages have an RNA genome, many animal viruses do.
Viral Envelopes
many viruses that infect animals have a membranous envelope;
after synthesis, capsids acquire an envelope from host’s cell membrane;
others exit out of the nuclear envelope with an envelope that is replaced by the Golgi Apparatus
Viral RNA Genome
broadest variety found in viruses that infect animals;
Genome is retroviruses that use reverse transcriptase to synthesize DNA from RNA
Retrivirus
contains a viral RNA;
uses reverse transcriptase to copy DNA from an RNA ;
HIV (Human Immunodeficiency Virus) that causes AIDS (Acquired Immunodeficiency Syndrome)
Provirus
the viral DNA that is integrated into the animal host’s cell;
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 genomes for viral particles released from cell
Evolution of viruses
Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid;
candidates for the source of viral genome include plasmids and transposons;
(plasmids, transposons, and viruses are all mobile genetic elements)
Formidable pathogens in animals and plants
viruses and prions
Prion
a misfolded protein that can transmit its misfoldedness to normal variants of the same protein and trigger cellular death
Vaccine
A harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen;
can prevent certain viral illnesses;
viral infections cannot be treated with antibiotics;
antiviral drugs can help to treat, not cure, viral infections by inhibiting synthesis of viral DNA and by interfering with viral assembly
Causes of rapidly emerging viral disease in humans
1) mutations of existing viruses into new ones that can spread more easily;
2) spread of viral disease from a small, isolated human population
3) spread of existing viruses in animals (3/4 of human disease originate this way)
Flu Epidemics
cause by type A influenza viruses;
infect a wide variety of animals: birds, pigs, horses, and humans;
given standardized names based on the viral surface proteins hemagglutin (HA) and neuraminidase (NA);
18 types of HA and 11 types of NA have been identified
H5N1 strain
an influenza strain, circulating among people that is very difficult;
very difficult to mount an effective immune response to it;
not transmitted from person to person
H1N1
a deadly strain;
swine flu;
combination of swine, avian, and human influenza genes
Pandemic
A global epidemic
Influenza viruses
flu viruses;
have nine RNA segments in their genome, leading to many new genetic combinations;
have a high rate of mutation;
normal seasonal flu viruses are not considered emerging viruses because variants of these viruses have been circulating among humans for a long time;
these viruses undergo mutation and reassortment;
variations that are most likely to occur each year are selected to generate vaccines
Plant Viruses
most have an RNA genome;
many have a helical capsid, while other have an icosahedral capsid;
spread by two major routes:
horizontal transmission and vertical transmission
Horizontal Transmission
virus, entering through damaged cell’s walls
Vertical Transmission
virus, inherited from a parent
Prions
infectious, incorrectly-folded proteins that appear to cause generative brain diseases in animals;
can be transmitted in food, act slowly, and are virtually indestructible;
able to convert a normal form of protein into misfolded version;
when several prions aggregate into a complex that can covert proteins into prions, which join the chain;
involved in diseases such as Alzeihmer’s and Parkinson’s
