Section 1
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Emerging Viruses
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Date created
Mar 14, 2020
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Section 1
(50 cards)
Emerging Viruses
Viruses that appear suddenly or that suddenly come to the attention of medical scientists. Class examples: HIV and AIDS
Adenoviruses
Type of virus that infects the respiratory tract in animals. Capsid is arranged in a polyhedral with a glycoprotein spike at each vertex.
Reproductive Cycle of an Animal Virus Equipped with a Viral Envelope with an RNA Genome
1. Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, promoting viral entry into the cell. 2. The capsid and viral genome enter the cell by fusion of the envelope with the cell's plasma membrane or by endocytosis and digestion of the capsid by cellular enzymes releases the viral genome. 3. The viral genome functions as a template for synthesis of complementary RNA strands by a viral enzyme and copies continue to be made from subsequent copies. 4. Complementary RNA strands also function as mRNA, which is translated into both capsid proteins in the cytosol and glycoproteins for the viral envelop in the ER. 5. Vesicles transport envelope glycoproteins to the plasma membrane. 6. A capsid assembles around each viral genome molecule. 7. Each new virus buds from the cell with an envelope studded with glycoproteins embedded in the membrane.
Retroviruses
The RNA animal viruses with the most complicated reproductive cycles (class VI). E.g. HIV. They have reverse transcriptases, which provide an RNA to DNA information flow, the opposite of the usual direction. Enveloped viruses that contain two identical molecules of single-stranded RNA and two molecules of reverse transcriptase.
Reverse Transcriptase
Enzymes in retroviruses that transcribe an RNA template into DNA, providing an RNA to DNA information flow, which is the opposite of the usual direction.
Prophage
The viral DNA that has become integrated into the host cell's (bacteria's) chromosome during the lysogenic cycle. One prophage gene codes for a protein that prevents transcription of most of the other prophage genes, leaving the phage genome mostly silent within the host (though in some cases, it can cause a change in the host cell's phenotype).
Virus' Effects on Cells and Symptoms
Some may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes; some cause host cells to produce toxins that lead to disease symptoms; some have molecular components that are toxic, such as their envelope proteins. How much damage a virus causes depends partly on the ability of the infected tissue to regenerate by cell division. Many of the temporary symptoms associated with viral infections, such as fever and aches, actually result from the body's own efforts at defending itself against infection.
Transformation in E. Coli
E. Coli and some other types of bacteria seem to lack the mechanism of transformation. However, placing E. Coli into a culture medium containing a relatively high concentration of Ca ions will artificially stimulate the cells to take up small pieces of DNA. This is used in biotechnology.
Plasmid
Small, circular, self-replicating DNA molecule separate from the bacterial chromosome. Smaller than the other bacterial chromosomes. Some can undergo reversible integration into the cell's chromosome. Have a small number of genes that under normal conditions are not required for the survival and reproduction of the bacterium but they can confer advantages for bacteria living in stressful environments.
Replication of Bacterial Chromosomes
From a single origin or replication, DNA synthesis progresses in both directions around the circular chromosome. Enzymes that cut, twirl, and reseal the double helix prevent the DNA from tangling.
Plant Viruses
Mostly have an RNA genome. Many have rod-shaped capsids while others have a polyhedral capsid. Once a virus enters a plant cell and begins reproducing, viral components can spread through the plant by passing through the plasmodesmata--proteins encoded by the viral genes are capable of altering the diameter of plasmodesmata to allow passage of viral proteins or genomes. Scientists have not yet devised cures for most viral plant diseases.
Antiviral Drugs
Mostly resemble nucleosides and as a result interfere with viral nucleic acid synthesis.
Vertical Transmission
One of the major routes by which plant viral disease spread. In this route, a plant inherits a viral infection from a parent. Can occur in asexual propagation (e.g. by taking cuttings) or in sexual reproduction via infected seeds.
Binary Fission
Mechanism by which bacterial cells divide. Preceded by replication of bacterial chromosomes. An asexual process--producing genetically identical offspring.
Three Processes That Bring Together Bacterial DNA from Different Individuals
Transformation, transduction, and conjugation.
Bacteriophages (or phages)
Viruses with some of the most complex capsids found. Their capsids have elongated icosahedral heads (having 20 triangular facets) enclosing their DNA. Attached to the head is a protein tail piece with fibers that the phages use to attach to a bacterium (seen in the "T-even" phages: T2, T4, and T6). The tail piece consists of a tail sheath and tail fibers. Infect bacteria.
Pathogen
A disease causing agent.
Reproductive Cycle of an Animal Virus Equipped with a Viral Envelope with a Double-stranded DNA Genome
Similar to the of an animal virus equipped with a viral envelope with an RNA genome except that the envelope is derived from the nuclear membrane of the host. E.g. the herpesvirus. These viruses also reproduce within the host's nucleus, using a combination of viral and cellular enzymes to replicate and transcribe their DNA.
Two Major Routes that Plant Viral Disease Spread By
Horizontal transmission and vertical transmission
How Viruses Synthesize New Viral Genomes
Most DNA viruses use the DNA polymerase of the host cell to synthesize new genomes along the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses use special virus-encoded polymerases that can use RNA as a template. After all the components have been synthesized, the new viruses usually assemble spontaneously. The simplest type or viral reproduction ends with the exit of hundreds or thousands of viruses from the infected host cell, usually destroying the host--causing some of the symptoms of viral infection.
Sex Pilus
A bacterial appendage that is used during conjunction.
Horizontal Trasmission
One of the major routes by which plant viral disease spread. In this route, a plant is infected from an external source of the virus. Since the virus must get past the plant's outer protective layer of cells (epidermis), plants that are damaged by by wind, injury, or insects are more susceptible to viral infections. Insects can be carriers. Farmers can act in the transmission through tools that they use.
HIV
A retrovirus.
Structure of a Virus
Only 20nm long--smaller than a ribosome. Not cells but rather infectious particles consisting of nucleic acid enclosed in a protein coat, in and some cases a membranous envelope. Genomes can consist of double-stranded DNA, double-stranded RNA, or single-stranded RNA/DNA. Their genomes are organized into a single linear or circular molecule of nucleic acid. Enclosed in a capsid
Lytic Cycle
A phage reproductive cycle that culminates in the death of the host cell (bacteria). Steps: 1. The phage uses its tail fibers to bind to specific receptor sites on the outer surface of the host cell. 2. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell's DNA is hydrolyzed. One of the first phage genes translated after the viral DNA enters the host cell codes for an enzyme the degrade the host cell's DNA (phage DNA is protected from this because it contains a modified form of cytosine that is not recognized by the enzyme). 3. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes within the cell. 4. Three separate sets of proteins self-assemble to form phage heads, tails and tail fibers. The phage genome is packaged inside the capsid as the head forms. 5. The phage directs production of an enzyme that damages the host cell wall, allowing fluid to enter, the cell to swell and then ultimately burs (lyse)t, releasing the phages.
Lyses
Breaks open.
Viroids
A class of pathogens. They are circular RNA molecules that infect plants. Do not encode proteins but can replicate in host plant cells, using cellular enzymes. The small RNA molecules seem to cause errors in the regulatory systems that control plant growth. Symptoms for plants include abnormal development and stunted growth.
Transduction
A process that brings together bacterial DNA from different individuals. In this process, phages carry bacterial genes from one host cell to another as a result of atypical behavior in the phage reproductive cycle; a small piece of the host cell's degraded DNA is accidentally packaged within a phage capsid in place of the phage genome.
Tobacco Mosaic Virus (TMV)
A virus that stunts the growth of tobacco plants and gives their leaves a mosaic colorization. Have a rigid-rod shaped capsid made from over a thousand molecules of a single type of protein in a helix.
Influenza Virus
A type of virus found in animals. Have a membranous envelope surrounding the capsids that help in their infection of a host. Outer envelope is studded with glycoprotein spikes. Genome consists of eight different RNA molecules, each wrapped in a helical capsid.
Host Range
The limited range of host cells that each type of virus can infect. Host specificity results from the evolution of recognition systems by the virus--"lock and key" fit between proteins on the outside of the virus and specific receptor molecules on the surface of the host cells.
Two Mechanisms by Which Phages Reproduce
The lytic cycle and the lysogenic cycle.
Transformation
A process that brings together bacterial DNA from different individuals. The process is the alteration of bacterial cell's genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment. Many bacteria have cell surface proteins that recognize and transport DNA from closely related species into the cell, which can then incorporate the foreign DNA into the genome.
Restriction Enzymes
(Restriction Endonucleases) A cellular enzyme (found in bacteria) that cuts up phage DNA after it has been detected as foreign by the host bacteria. Just as natural selection favors bacteria with effective restriction enzymes, it also favors bacteria mutants that are resistant to these enzymes; thus, the parasite-host relationship is in constant evolutionary flux.
Lysogenic Cycle
A phage reproductive cycle in which the genome is replicated without destroying the host (bacteria). In this cycle, the phage attaches to a host cell and injects its DNA, which then circularizes. The DNA molecules is then incorporated by genetic recombination (crossing over) into a specific site on the host cell's chromosome, becoming a prophage. One prophage gene codes for a protein that prevents transcription of most of the other prophage genes, leaving the phage genome mostly silent within the host (though in some cases, it can cause a change in the host cell's phenotype). Every time the host cell prepares to divide, the phage DNA is replicated along with the host DNA and is passed on to daughter cells. At some point, the phage genome can exit the host;s chromosome and initiate the lytic cycle; usually triggered by some environmental signal like radiation or the presence of a certain chemical.
Nucleoid
The dense region of DNA in bacteria that is not bounded by membrane like the nucleus of a eukaryotic cell.
Capsomeres
Large protein subunits that are the building blocks of capsids.
Prions
Proteins that appear to cause a number of degenerative brain disease in various animal species. Causes mad cow disease, Creutzfeldt-Jakob Disease in humans, and scrapie in sheep. Most likely transmitted through food. Very slow acting. Incubation period until symptoms appear is about 10 years. Virtually indestructible; not destroyed or deactivated by heating to normal cooking temperatures. No known cure for prion disease. Most likely a misfolded form of a protein normally present in the brain which converts the normal protein like itself to the prion version of itself.
Reproductive Cycle of Retroviruses
After the virus enters the cell, its reverse transcriptase molecules are release into the cytoplasm and catalyze the synthesis of viral DNA, which then enters the cell's nucleus and integrates into the DNA of a chromosome, becoming a provirus. The host's RNA polymerase transcribes the proviral DNA into RNA, which can function both as mRNA for the synthesis of viral proteins and as genomes for new virus particles released from the cell. Envelope glycoproteins are made in the ER.
Viral Envelopes
Membranous envelopes that surround the capsids of some viruses. Derived from the membrane of the host cell and contain host cell phospholipids and membrane proteins but also contain proteins and glycoproteins of viral origin.
Phage Lambda
A temperate phage widely used in biological research. Resembles a T4 phage but its tail has only one short tail fiber.
Vaccines
Harmless variants or derivative of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen. The immune system is the basis for this major medical tool that prevents viral infections.
Virus (general concept)
Obligate intracellular parasites: they can only reproduce within a host cell. Lack metabolic enzymes, ribosomes, and other equipment for making proteins; are merely packaged sets of genes in transit from one host to another. All have a host range. After a virus has its host cell assemble the viral nucleic acids and capsomeres, their assembly into new viruses is usually spontaneous.
Virulent Phage
A phage that reproduces only by a lytic cycle.
Capsid
The protein shell that encloses a virus. Depending on the virus, it may be rod-shaped, polyhedral, or more complex such as a polyhedral head and a tail apparatus. Built from large numbers of protein subunits called capsomeres.
Temperate Phages
Phages capable of using both the lysogenic cycle and lytic cycle within a host.
How T-even Phages Infect a Cell
Use their elaborate tail apparatus to inject DNA into a bacterium. Once inside, the viral genome can commandeer its host, reprogramming the cell to copy the viral nucleic acid and manufacture viral proteins. The host provides the nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins provided by the viral DNA.
Conjunction
A process that brings together bacterial DNA from different individuals. The direct transfer of genetic material between two bacterial cells that are temporarily joined. The transfer is one way: one cell donates DNA and its mate receives the DNA. The donor uses appendages called sex pili to attach to the recipient, which retract once they've made contact, bringing the two bacteria together. A temporary cytoplasmic mating bridge then forms between the two cells, providing an avenue for DNA transfer. Process involves a special type of DNA replication so that each bacteria has a copy of the shared genome.
3 Processes Contributing to the Emergence of Viral Diseases
1. The mutation of existing viruses; RNA viruses ten to have an unusually high rate of mutation because errors in replicating their RNA genomes are not corrected by proofreading. 2. The spread of existing viruses from one host species to another. 3. The spread of a viral disease from a small, isolated population can leave to widespread epidemics.
Provirus
The integrated retrovirus DNA into an animal host cell's DNA in the nucleus. Never leaves the host's genome, remaining a permanent resident.
Section 2
(20 cards)