Chapter 19 - Viruses Flashcards
Give a brief description of viruses
- They can’t independently reproduced (this is why they aren’t alive)
- Viruses lead “a kind of borrowed life” between life-forms and chemicals
- The origins of molecular biology lie in early studies of viruses that infect bacteria
What is the structure of a virus?
- A virus consists of a nucleic acid surrounded by a protein coat
- Viruses are not cells
- Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope.
What are viral genomes ?
Viral Genomes may consist of either:
- Double stranded or single stranded DNA
- Double stranded or single stranded RNA
Depending on its type of nucleic acid, a virus is called a DNA virus or RNA virus
Define:
- Capsid
- Capsomere
a Capsid…
..is the protein shell that enclosed the viral genome.
Capsomeres….
..are Protein subunits that build the capsids.
A capsid can have various structures (see picture)
Some viruses have _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ that help them infect hosts
These _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ surround the _ _ _ _ _ _ of influenza viruses and many other viruses found in animals
Some viruses have membranous envelopes that help them infect hosts
These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals
Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules
Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules
What is a bacteriophage?
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 enclosed their DNA
- A protein tail piece attaches the phage to the host and injects the phage DNA inside
Define Host Range (in terms of viruses)
Each virus has a host range,
a limited number of host cells that it can infect
Some viral host ranges are broad
- Example, West Nile virus can infect mosquitoes, birds, horses and humans
Other viral host ranges are more narrow
- Example: measles virus can only infect humans
Furthermore, infections of multicellular eukaryotes is usually limited to a specific type of tissue
- Example: cold viruses infect respiratory epithelium
- HIV infects certain types of white blood cells
Viruses are ………….., which means they can reproduce only within a host cell
Viruses are obligate intracellular parasites which means they can reproduce only within a host cell
Once a viral genome has entered a cell…
Once a viral genome has entered a cell… the cells begins to manufacture viral proteins.
The virus makes use of the host enzymes, ribosomes, tRNA’s, amino acids, ATP and other molecules.
Viral nucleic acid molecules capsomeres spontaneously self-assemble into new viruses.
Phages best understood out of all viruses. They have two reproductive mechanisms, what are they ?
1. The lytic Cycle
The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell
- The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses
- A phage that reproduces only by the lytic cycle is called a virulent phage
- Natural selection favours bacterial mutants with receptors that cannot be recognized by phage
- Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
2. The lysogenic Cycle
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
- Prophage: genetic material of the virus
What is the first mechanic mechanism of phages?
The lytic Cycle
The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell
- The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses
- A phage that reproduces only by the lytic cycle is called a virulent phage
- Natural selection favours bacterial mutants with receptors that cannot be recognized by phage
- Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
What is the second mechanical mechanism for phages?
2. The lysogenic cycle
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
- Prophage: genetic material of the virus
An _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 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 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
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
Examples of diseases caused by bacteria that are infected by temperate phages
- Diptheria, Botulism, Scarlett Fever
- 0157:H7 strain of E. coli
What are the two key variables used to classify viruses that infect animals?
1. DNA or RNA?
2. Single stranded or Double stranded?
- Many viruses that infect animals have _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
- Viral glycoproteins on the envelope bind to specific receptor molecules ……….
- Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit.
- Other viral membranes form the host’s _ _ _ _ _ _ envelope and are then replaced by an envelope made from Golgi apparatus membrane.
Many viruses that infect animals have membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit.
Other viral membranes form the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane.
REVIEW 1 of each, PT1 is in question image. (continunation) PT2 is in answer image
PT2
How do retroviruses copy their RNA genome into DNA?
Retroviruses use reverse transcription to copy their RNA genome into DNA
RNA –> DNA is reverse of normal path of information: hence, “retrovirus”
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 _ _ _ _ _ _ _ _.
Unlike a prophage, a _ _ _ _ _ _ _ remains a permanent resident of the host cell.
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.
- The host’s RNA polymerase transcribes the proviral DNA into RNA molecules.
- The RNA molecules function as mRNA for synthesis of viral proteins as genomes for new virus particles released from the cell.
Why don’t viruses fit our definition of a living organism?
Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid.
- Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments.
- Plasmids, transposons, and viruses are all mobile genetic elements
Define
Rhinovirus
Poliovirus
why do people typically recover from a rhinovirus oppopsed to a poliovirus?
people typically recover from Rhinovirus (common cold) because epithelium of the respiratory tract can efficiently repair iteself, replacing damaged cells via mitosis.
Poliovirus however, infects the mature nerve cells, and therefore making damage permanent because these cells do not divide and cannot be replaced.
What is a vaccine?
Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
- Vaccines stimulate antibody production to prepare the immune system for a potential infection
- Viral infections cannot be treated by antibiotics
- Antiviral drugs can help to treat, though not cure, viral infections
How are flu epidemics caused?
Flu epidemics
are caused by new strains of influenza virus to which people have little immunity.
- Viral disease in a small isolated population can emerge and become global.
- This has been enhanced by globalization.
- New viral disease can emerge when viruses spread from animals to humans.
- Viral strains that jump species can exchange genetic information with other virsuses to which humans have no immunity.
What is an emerging virus?
Emerging viruses
are those that appear suddenly or suddenly come to the attention of scientists
- Severe acute respiratory syndrome (SARS) suddenly appeared in China (2003)
- It then spread worldwide (including a major outbreak in Toronto) killing nearly one thousand people
- Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory
How does a virus jump from one species to another?
- Consider and influenza virus…
- Certain flu viruses can infect a wide range of animals
- Two related viruses infect a host
- For example, a pig’s cell is infected with a “human virus” and a “pig virus”
- During reassembly and emergence
- Recombination and mutation of genetic material
- Packing into a novel capsid (envelope)
- Humans would have never been exposed to this new virus and will have no immunity.
