Chapter 13 Flashcards
Viruses
Obligate intracellular parasitic molecules; replication can only occur within another living cell, considered non-living; 5,000 known to man
Central Core of a Virus
- Genome: The nucleic acids found inside a virus will store information that will direct the host cell to replicate the viral parts (numerous times)
- Enzymes: Some are carried along with the nucleic acid; used to allow entry to a cell or to replicate its own nucleic acids (e.g., DNA and RNA polymerase)
Capsid
Outer protein coat found on all viruses; constructed from identical building blocks (capsomeres) which are composed of smaller protein molecules
Capsomeres
Form viral capsids spontaneously, forming viruses into nearly perfect geometric shapes; rod-shaped form a helical capsid
Icosahedron
Capsid shape; 3-dimensional, 20-sided polygon with 12 evenly spaced corners
Spikes
Proteins that project outward into the environment and are essential for viruses to attach to a host cell
Enveloped Viruses
Viruses that have a bilayer surrounding their capsid
Naked Virus
Type of virus that does not have an envelope
Functions of Capsids and Envelopes
- Both protect the nucleic acid from various enzymes and chemicals outside the host cell
- Both allow the virus to attach to host cells and help the virus penetrate the host cell
Host Range
The organisms or cell types a virus can infect; usually limited to only a few cell types or species or even strains
Productive State
New viruses are being produced by the host cell:
- Host cell destroyed by lysis when new viruses are released (lytic cycle)
- Host cell continues to divide while releasing viruses
Latent State
No new viruses are produced by host cell:
- Viral nucleic acid is incorporated into host chromosome
- Host cell continues to divide, replicating viral DNA with each new cell (lysogenic cycle)
Superfamilies
Broadest category into which viruses are divided into:
- Those that contain DNA (7 families)
- Those that contain RNA (13 families)
- Family names end with Latin root: viridae (capitalized and underlined)
Characteristics Used to Classify Viruses into Families
- Shape of capsid
- Overall viral size
- DNA/RNA viruses
- Presence of cell envelope
- Type of host cell it will infect
- How the virus effects the host cell (e.g., productive, latent)
Bacteriophages
Viruses that infect bacteria
Adsorption (Bacteria)
The bacteriophage’s spikes on the capsid (all are naked) attach to bacterial receptor proteins; viruses are not capable of locomotion, they must collide with a host cell by chance; this must occur before any other productive phase of infection can occur
Penetration (Bacteria)
Entrance of viral nucleic acids into cytoplasm of bacterial host cell:
- The enzyme lysozyme digests its way through a small portion of bacterial cell wall
- Viral nucleic acids are injected through the cell membrane of the host
- Viral nucleic acid enters cytoplasm but capsid remains on the outside of the cell
Transcription of Viral DNA and Production of New Viral Parts (Bacteria)
- DNA of virus is transcribed and translated using the machinery of the host cell
- Phage-induced proteins are produced
- The phage DNA is replicated and phage capsid is produced
- All virus particles are produced independently of one another, including all DNA, capsomeres, binding proteins, etc.
Phage-Induced Proteins
Contain new genes that are unique to the virus and not the host cell; code for the production of proteins that perform specific viral functions
Nuclease
An enzyme that breaks apart and destroys host cell DNA; essential for the replication of viral DNA and capsid
Maturation (Bacteria)
Assembling virus particles into mature viruses:
- Some particles spontaneously assemble themselves
- Others use enzymes to join the particles together
Mature Viruses are Released (Bacteria)
- Lysozyme is produced and begins to digest cell wall from inside out
- Osmotic pressure causes the cell to rupture expelling the mature viruses
Burst Size
The number of phages released per cell when it lysis due to viral infection (approximately 200)
Adsorption (Animal)
A recognition process between a virus and a host cell that results when the virus attaches to the external surface of the host cell
Receptor Sites
Specific proteins found on the cell membrane of a host that a virus attaches to; may have to attach to two (virus may have more than one type of spike); normally used by the host cell for other functions
Penetration/Uncoating (Animal)
Entrance of the virus into the host cell
Endocytosis
Process by which cells take up particles by enclosing them in a vesicle pinched off from the cell membrane; virus is engulfed in a vesicle of the host cell’s making
Uncoating
A process in which enzymes found within the host cell will dissolve the capsid and envelope (if present) and release the nucleic acid strands into the cytoplasm
Fusion
The viral envelope fuses with the cell membrane of the host; only occurs with enveloped viruses; only capsid and genome enters host’s cytoplasm; capsid is uncoated releasing the nucleic acid
Transcription of Viral DNA and Production of New Viral Parts (Animal)
The copying and expression of the viral genome using the hosts equipment that results in the production of various virus components:
- The viral genome is expressed and copied
- Viral enzymes, necessary for the production of viral parts, are produced
- Viral parts are constructed, including capsomeres and spikes
Maturation (Animal)
Assembly of individual viral parts into a whole virus:
- Viral enzymes and nucleic aced strands are surrounded by capsomere components
- If the virus is enveloped, then spikes are placed within the cell membrane of the cell
Release (Animal)
Escape from the host cell to the environment; burst size - 3,000 to 50 million from one cell
Budding
Process enveloped viruses use to escape a cell:
- A vesicle (cell membrane) is placed around the virus by the cell (membrane contains spikes)
- Viruses are released slowly without the sudden destruction of the cell
- Can be destructive (not always) to the cell because of the accumulation of damage to cell membrane or the membrane of organelles; can shut down metabolism and genetic expression; toxicity of viral components
Virulent Viruses
Viruses that will always be productive and cause cell lysis; a rapid, severe, and destructive virus
Latent Virus
A nonactive virus which is in a dormant state within a cell; do not lyse host cells; incorporate their DNA directly into the chromosomes of the host cell (lysogenic cycle); e.g., herpes virus
Prophage
A bacteriophage that is latent (lamda virus infects E. coli)
Provirus
An animal virus that is latent (HIV, herpes)
Temperate Viruses
Viruses that are capable of causing lysis or becoming latent; whether the virus will lysis the cell or become latent is usually due to chance but modifying environmental conditions may play a role in which type is more often expressed
Lysogenic Conversion
With a latent virus, some genes are expressed and the living host cell acquires new proteins, and hence, new characteristics that it does not normally have
Repressor Gene
A viral gene that is expressed by a latent virus; codes for a repressor protein that prevents transcription of a gene that produces an excise enzyme
Excise Enzyme
An enzyme that has the ability to remove viral DNA from a host chromosome; once viral DNA is removed from the host DNA, the virus becomes virulent and new viruses are produced and assembled
Induction
The process of converting a latent virus into a virulent one; Lysogenic cycle —-> Lytic cycle
Acute Infections
Viral infections that cause disease symptoms for short periods of time:
- Are productive
- Can cause extensive tissue damage and cell death, but usually localized
- Host organism can recover and does not usually die from exposure
- Much of the damage can be repaired after recovery
- The body can develop life-long immunity
- Ex. mumps, measles, influenza (flu), cold viruses
Persistent Infections
A specific virus is present within the host for long periods of time (life-long in some cases):
- Can be productive or latent depending on the state of the disease
- Disease symptoms may not be apparent
- The host is a persistent source of infective viruses
- Ex. hepatitis C, HIV
Late Complications Following Acute Infection
A chronic infection where a person develops an acute viral disease, recovers, and years later will relapse (possibly with other disease symptoms); e.g, acute measles, 1-10 years later some develop subacute sclerosing panencephalitis (SSPE), which is a fatal brain disorder
Latent Infections
A chronic infection where a person develops an acute viral disease, recovers, and days, months, or years later will develop another, similar infection:
- Cannot be detected until disease onset
- e.g., herpes simplex virus 1 causes cold sores, fever blisters; as the virus heals, some viruses leave the mucous membrane of the mouth and enter neurons (will not harm); viruses are prevented from replicating
- Stress, menstruation, fever, sunburn, can all trigger the movement of viruses from the neuron, reentering the mucosa and cause more blisters``
Chronic Infections
The virus is present at all times, where the person may or may not show symptoms of the disease, but the host is continually affected; e.g., hepatitis B & C, after an acute phase (nausea, fever) the virus persists and can slowly cause cirrhosis of the liver
Slow Infections
The virus number slowly increases over a long period of time, finally accumulating to a lethal point where the fatal disease is caused; e.g., HIV… no symptoms… AIDS
Cancer
Abnormal condition of unregulated cell division
Tumor
A mass of newly dividing cells:
- Benign: cells remain within a localized region
- Malignant: the cells migrate to other locations (metastasize)
Proto-Oncogenes (accelerator)
Type of regulatory gene that activates gene transcription and increases the rate of cell division; if this gene is always turned on then the cell can become a cancer cell
Tumor Suppressor Genes (brake)
Type of regulatory gene that suppresses cell division; if this gene is turned off, then a cell can become a cancer cell
How Regulatory Genes are Altered
- Mutation: cause 80% of all human cancer (50% on tumor suppresor, 30% on proto-oncogenes)
- Viruses: cause 15% of all human cancers (probably more)
- The rest may result from genetic predisposition
Oncoviruses
Viruses that invade a host and transform it into a cancer cell; latent and will incorporate its genetic material into the DNA of the host cell
Oncogenes
Genes of viral DNA that disrupt the normal functioning of proto-oncogenes in host cells and produce cancer cells (proto-oncogenes permanently turned on)
Retroviruses
Cancer causing virus; single strand of RNA; uses enzyme reverse transcriptase to form a double stranded DNA molecule and then it is incorporated into host chromosome; these viral proteins can modify host cell function
Viroid
A circular molecule of RNA without a capsid:
- Don’t produce proteins
- Cause several economically important plant diseases
- None known to infect animals
- 1/10 the size of the smallest plant virus
- How they cause disease is a mystery but one theory is that it interacts in some way with the host genome, changing the expression of the host genes to cause disease
Prions
Infectious agent composed only of protein:
- Affect the central nervous system
- Normal cellular protein are destroyed by the host cell proteases
- Infectious agent’s proteins are not easily broken down and accumulate in the cell
- Not known exactly how it causes disease but one theory is that the infectious agent’s proteins attach to normal proteins and convert their shape into the agent’s shape
Protease
An enzyme that breaks down proteins
Induction Triggers
Often unknown, but usually related to:
- decreased immunity
- stress
- fever
- UV exposure
