Chapter 19 AP Bio: Viruses

Viroids

small circular single RNA molecules that infect plants they do not encode proteins but can replicate in host plant cells apparently using host cell enzymes. They seem to cause errors in the regulatory systems that control plant growth the typical signs are abnormal development and stunted growth. One viroid disease called cadang-cadang has killed more than 10 million coconut palms in the Philippines.

Tobacco mosaic disease

stunts growth of tobacco plants and gives their leaves a mosaic coloration. Adolf Mayer a German 👨‍🔬 discovered it would be transmitted by rubbing sap onto healthy plants. Mary suggested that the disease was caused by an unusually small bacteria that were in visible under a microscope this was tested later by Dimitri Ivanowski a Russian biologist who passed the leaves through a filter designed to remove bacteria but it's still produced the disease. Beijerinck tried to replicate it in test tubes but it could not so he imagined a particle much smaller and simpler than a bacterium-a virus. Wendell Stanley crystallized the particle, known as TMV.

Reverse transcriptase

transcribes an RNA template into DNA, providing an RNA to DNA information flow, the opposite of the usual direction.

Emerging viruses

viruses that appear suddenly or are new to medical scientists. HIV the aids virus is a classic example which appeared in San Francisco in the early 1980s seemingly out of nowhere although later studies uncovered a case in the Belgian Congo in 1959. The Ebola virus recognize initially in 1976 in central Africa is one of several emerging viruses that cause hemorrhagic fever and often fatal syndrome (set of symptoms) characterized by fever, vomiting, massive bleeding, and circulatory system collapse. A number of others cause encephalitis inflammation of the brain. One is west Nile virus which appeared in North America for the first time in 1999 and has spread to all 48 contiguous states.

How does a viral infection begin?

when a virus binds to a host cell and the viral genome makes its way inside. The mechanism of entry depends on the type of virus and sell. T even phase is use their elaborate tail apparatus to inject DNA into a bacterium other viruses are taken up by endocytosis or in the case of enveloped viruses by fusion of viral envelope with the plasma membrane.

Mimivirus

A double-stranded DNA virus with and I casa he drop caps that that is 400 nm in diameter. It is the size of a small bacterium so is named for mimicking microbe. It's genome contains 1.2 million bases and an estimated 1000 genes. Some of the genes appear to code for products previously thought to be hallmarks of cellular genomes including proteins involved in translation, DNA repair, protein folding, and polysaccharides synthesis. The researchers who described it propose that it most likely evolved before the first cells and developed an exploitative relationship with them. Others disagree maintaining that the virus involved more recently than cells and has simply been efficient at scavenging jeans from its hosts. The question of whether some viruses deserve their own early branch on the tree of life may not be answered for some time.

Vaccine

A harmless variant or derivative of a pathogen that stimulates a host's immune system to mount defenses against the pathogen. Smallpox was eradicated by a vaccination program carried out by the world health organization. The very narrow host range of the smallpox virus was a critical factor in the success of this program because it only infects humans. Effective vaccines are also available to protect against rubella, mumps, hepatitis B and others.

virulent phage

A phage that reproduces only by a lytic cycle.

Vertical transmission

A plant in here it's a viral infection from a parent. It can occur in a sexual propagation through cuttings or in sexual propagation via infected seeds.

What else supports this theory

A viral genome can have more in common with the genome of its host then with the genomes of viruses that infect other hosts. Some viral genes are essentially identical to the genes of the host. However recent sequencing has shown that the genetic sequences of some viruses are quite similar to those of seemingly distantly related viruses such as animal and plant viruses. This may reflect the persistence of groups of viral jeans that were favored by natural selection during early evolution of viruses and the eukaryotic cells that served as their hosts.

How can a protein which cannot replicate it self be a transmissible pathogen?

According to the leading model a prion is a miss folded form of a protein normally present in brain cells. When it gets into a cell containing the normal form of the protein it somehow converts normal protein molecules to the miss folded prion versions several prions then aggregate into a complex that can convert other normal proteins into prions which join the chain. This aggregation interferes with normal cellular functions and causes disease symptoms. This model was greeted with much skepticism when it was first proposed by Stanley Prusiner in the early 80s but is now widely accepted.

Icosahedral viruses

Adenoviruses which in fact the respiratory tract of animals have 252 identical protein molecules arranged in a polyhedral Said what 20 triangular facets these and other similarly shaped viruses are referred to as this.

Victims of H1N1

Although it was declared a pandemic it's tall was significantly lower than that of the 1918 flu. However 79% of the confirmed swine flu cases in 2009 occurred in people under 30 years of age and the highest mortality rates occurred in people under 64 opposite to the seasonal flu. Some scientist hypothesize that the 1918 flu virus was the ancestor of most subsequent H1N1 epidemic causing viruses including that responsible for the 2009 pandemic. Older people are more likely to have been exposed to earlier H one N one viruses and could have probably build up immunity to them this would explain why contracting the 2009 virus was more deadly for young people.

Virus

An infectious particle incapable of replicating outside of a cell, consisting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope. Little more than genes in a protein coat.

Animal viruses with envelopes

And animal virus with an envelope or outer membrane uses it to enter the host cell. Protruding from the outer surface are viral glycoproteins that bind to the specific receptor molecules on the surface of the cell. Ribosomes bound to the ER of the host cell make the protein parts of the envelope glycoproteins; cellular enzymes in the ER and gold J apparatus then add the sugars. The resulting viral glycoproteins embedded in Hosell derive membrane are transported to the cell surface in a process like XO cytosis new Seeds are wrapped in membrane as a bed from the south. The viral envelope is derived from the host cells plasma membrane although some of the molecules of this membrane are specified by viral jeans. The envelope viruses are now free to infect others sells this does not necessarily kill the host cell.

lytic cycle

Culminates in the death of the host cycle. The term refers to the last stage of infection in which the bacterium laces and releases the phages that were produced. Each can then in fact a healthy cell and a few successive cycles can destroy an entire bacterial population in a few hours.

Transposons

DNA segments that can move from one location to another within a cells genome.

Characteristics of prions

Do you are especially alarming. First they act very slowly with an incubation period of at least 10 years before symptoms develop. This prevents sources of infection from being identified until long after the first cases appear allowing many more infections to occur. Second they are virtually indestructible they are not destroyed or deactivated by heating to normal cooking temperatures. There is no known cure for these diseases in the only hope for effective treatments lives in understanding the process of infection.

Are viruses alive?

Early on people thought they were chemicals, the root of virus means poison in line. In the 1800s researchers propose that they were the simplest of living forms however they cannot reproduce or carry out metabolic activities outside of a host cell so most would probably agree that they are not alive but exist in a shady area between life forms and chemicals. Viruses lead a kind of borrowed life.

Prions

Even more surprising is the evidence for infectious proteins which appear to cause a number of degenerative brain disease is in various animals. These include scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans which has caused the death of some 150 people in great Britain. Prions are most likely transmitted in food such as when people eat prion laden beef from cattle with mad cow disease. Kuru, another human disease caused by prions was identified in the early 1900s among the South Fore natives of new guinea. This epidemic peaked in the 1960s puzzling scientists who at first thought the disease had a genetic basis eventually anthropological investigations ferreted out how the disease was spread: ritual cannibalism.

Why have phages not exterminated all bacteria?

Face treatments have been used medically in some countries to help control bacterial infections in humans. However natural selection favors bacterial mutants with receptors that are no longer recognized by a particular type of fish second one phase DNA successfully enters a bacterium the DNA is often identified as foreign and cut up bike cellular enzymes called restriction enzymes because they restrict the ability of the phage to infect the bacterium. The bacterial cells own DNA is metal he didn't anyway that protects attack by its own restriction enzymes but natural selection favors favors mutants that can bind the altered receptors or are resistant to particular restriction enzymes that's the parasite host relationship is in constant evolutionary flux. In addition instead of leasing their house sells many phases coexist with them in lysogeny.

Lytic cycle for phage lambda

Infection of an E. coli cell by phage lambdaBegins when the facia binds to the surface of the cell and in Jack's it's linear DNA gene them. Within the host the DNA molecule formed a circle what happens next depends on the replicative mode. During a lactic cycle the viral jeans immediately turned the host cell into a lambda producing factory in the cell soon lyses and releases the products. During a lysogenic cycle the DNA is incorporated into a specific site on the E. coli chromosome by viral proteins that break both circular DNA molecules and join them together.

Bacteriophages

Many of the most complex capsids are found among the viruses that infect bacteria called bacterial phages or simply phases. The first studied included seven that in fact E. coli these were named T1, T2 etc. in order of discovery. The three t-even phages turned out to be very similar in structure their capitals have elongated icosahedral heads closing the DNA attached to the head is a protein tail with fibers by which they attach to a bacterium.

What can spread viruses?

New roads and destruction of forests-brings humans into contact with animals and humans they might not have been in contact with before.

What happens once the virus enters

Once it begins replicating viral genomes and associated proteins can spread throughout the plant through plasmodesmata the cytoplasmic connection between plant walls. The passage of viral macromolecules from cell to cell is facilitated by Bireli encoded proteins that cause enlargement of plasmodesmata Scientists have not yet devised cures for most plant viral diseases.

Avian flu

Perhaps a greater long-term threat is the avian flu caused H5N1 virus carried by wild and domestic birds. The first transmission to humans was in 1997 118 people in Hong Kong were infected and six subsequently died. While the 2009 flu virus spread easily from human to human reports of human to human transmission of the avian flu are quite rare. More alarming is the overall mortality rate which is greater than 50%. Furthermore the host range of avian flu is expanding which provides increasing opportunities for different strains of the virus to reassort their genetic material and for new strains to emerge. If it involves so that it can spread easily from person to person it could be a threat like a the 1918 virus.

Temperate phages

Phages that are capable of using either the lytic or lysogenic cycle. Such as lambs which is used in research it looks like t4 but only one short tail fiber.

Plasmids

Small circular DNA molecules found in bacteria and in the unicellular eukaryotes yeast. Plasmids exist apart from the cells genome and can replicate independently of the genome and are occasionally transferred between cells.

Other types of envelope

Some are not derived from the plasma membrane. Herpesviruses are temporarily closed in membrane derived from the nuclear envelope of the host they then shut this membrane in the cytoplasm and acquire a new envelope from the membrane of the Golgi apparatus. These viruses have a double-stranded DNA genome and replicate with in the house sell nucleus using a combination of viral and cellular and designs to replicate and transcribed. With herpes copies of the viral DNA can remain behind as many chromosomes in the nuclei of certain nerve cells and remain latent until some sort of physical or emotional stress triggers a new round of active virus production. The infection of other cells by these new viruses causes the blisters characteristic of herpes such as cold sores.

Viral envelopes

Some viruses have accessory structures to help them in fact hosts. A membrane us envelope that surrounds the cop says of influenza viruses and many others found in animals. These are derived from the membranes of the host cell and contain host cell phospholipids and membrane proteins. They also contain proteins and glycoproteins of viral origin. Some viruses carry a few viral enzyme molecules within their capsid.

Host range

The limited range of host cells that each type of virus can infect and parasitize. This results from the evolution of recognition systems by the virus based on a lock and key fit between viral surface proteins and specific receptor molecules on the outside of cells.. Some viruses have broad ranges such as west Nile virus and equine encephalitis which can each in fact mosquitoes, birds, horses and humans. Other have narrow ones such as measles which can only in fact humans and viral infection of multicellular eukaryotes is usually limited to particular tissues the cold virus only infects the cells in the upper respiratory tract and the aids virus binds to receptors in only certain types of red white blood cells.

Phages

The most understood of all viruses but some are also the most complex. Research on them lead to the discovery that some double-stranded DNA viruses can replicate buy to alternative mechanisms the lyctic and the lysogenic cycle.

Capsid

The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complex in shape. They are built from a large number of protein subunits called capsomeres but the number of different kinds of proteins is usually small. Tobacco mosaic virus has a rigid rod shape capsid made of over 1000 molecules of a single type of protein are arranged in a helix. Rod shaped viruses are usually called helical viruses

What happens once the viral genome is inside?

The proteins in encodes can commandeer the host reprogramming the cell to copy the viral nucleic acid and manufacture of viral proteins the host provides the nucleotides for making barrel nucleic acid's and enzymes, ribozymes, tRNA, amino acids, ATP and others. Many DNA viruses use the DNA polymerize is of the Hosell to synthesize new genomes along the templates produced by the viral DNA. In contrast to replicate their genomes RNA viruses use virally and coded are in a polymerases that can use RNA as a template.

What is the third source of new viral diseases in humans?

The spread of existing viruses from other animals. Scientists estimate that about 3/4 of New human diseases originate like this. Animals that harbor and can transmit a particular virus but are generally unaffected by it or said to act as a natural reservoir for that virus. The 2009 flu pandemic was likely passed to humans from pigs and was originally called swine flu.

Why would they switch over

The term lysogenic implies that the pro phages are capable of generating active they just that lives there who sells this occurs when the lamb did genome is induce to exit the bacterial chromosome and initiate a lytic cycle. In environmental signal such as a certain chemical or high-energy radiation usually triggers the switchover.

How small are viruses?

The tiniest viruses are only 20 nm in diameter smaller than a ribosome millions could fit on a pin head even the largest one is barely visible under the light microscope. Stanley's discovery that some viruses could be crystallize was exciting and puzzling not even the simplest of cells can either get into regular crystals but what are they? Examining the structure of a virus more closely reveals that it is an infectious particle consisting of nucleic acid enclosed in a protein coat and for some viruses surrounded by a membranous envelope.

Types of flu

There are three types, types B and C which in fact only humans and have never caused an epidemic and type a which infects wide range of animals including birds pigs horses and humans. Influenza A strains have caused four major flu epidemic's in the last 100 years the worst was the first Spanish flu pandemic of 1918 to 1919 which killed about 40 million people.

How are strains of influenza A named?

They are given standardize names for example both the strain that cause the 1918 flu and the one in that cause the 2009 pandemic flu are called H one N one the name identifies which forms of 2 viral surface proteins are present hemagglutinin H and neuraminidase N there are 16 different types of hemagglutinin a protein that helps the flu virus attached to host cells and nine types of neuraminidase an enzyme that helps release new viral particles from infected cells. Water birds have been found that carry viruses with all possible combinations of H and N.

What do plasmids transposons and viruses all share

They are mobile genetic elements

Plant virus structure

They have the same basic structure and mode as animal viruses. Most discovered thus far including tobacco mosaic virus have an RNA genome. When you have a helical capsid while others have an icosahedral capsid.

What happens after the viral nucleic acid molecules and capsomeres are produced?

They spontaneously self a sample into new viruses. Researchers can separate the RNA in caps and mirrors of TMV and then reassemble complete viruses by mixing the components together under the right conditions. The simplest type of viral replicative cycle ends with the exit of hundreds or thousands of viruses from the infected host cell which often damages or destroys the cell. This, as well as the body's responses to the destruction caused many of the symptoms associated with viral infections. The viral progeny that exit the cell have the potential to in fact additional sells spreading it. There are many variations on this cycle.

How do such viruses burst out on the human scene giving rise to harmful diseases that were previously rare or even unknown?

Three processes contribute to the emergence of viral diseases. The first and perhaps the most important is the mutation of existing viruses. RNA viruses tent do you have an usually high rate of mutation because errors in replicating the RNA genomes are not corrected by proofreading. Some mutations change existing viruses into new genetic or ideas or strains that can cause disease even in individuals who are immune to the ancestral virus.

Other cool things about viruses

Unique genetic mechanisms that are interesting and help us understand how viruses cause disease. In addition the study of viruses had led to the development of techniques that enable scientists to manipulate genes and transfer them from organisms to another. These play in important role in basic research, biotechnology and medical applications. They are used as agents of gene transfer in gene therapy.

How can people be helped after they have been infected?

Vaccines can prevent certain viral illnesses but medical technology can do Little at present you cure most viral infections once they occurred. Antibiotics are powerless against viruses. They kill bacteria by inhibiting enzymes specific to bacteria but have no effect on eukaryotic or Virally encoded enzymes. However the few enzymes that are encoded by viruses have provided targets for other drugs. Most antiviral drugs resemble nucleosides and as a result interfere with viral nucleic acid synthesis. One is acyclovir which impedes herpes virus replication by inhibiting the viral polymerize that synthesizes viral DNA. Similarly azidothymidine (AZT) curbs HIV replication by interfering with the synthesis of DNA by reverse transcriptase. Much effort has gone into developing drugs against HIV in currently multi drug treatments sometimes called cocktails have found to be most effective. These treatments commonly include a combination of two nucleoside mimics and a protease inhibitor which interferes with an enzyme required for assembly of the viruses.

Horizontal transmission

Viral diseases of plants spread by two major routes in horizontal transmission a planet is infected from an external source of the virus. Because the invading virus must get past the plants outer protective layer of cells or the epidermis a plant becomes more susceptible to viral infection if it has been damaged by wind, injury, or herbivores. Herbivores especially in sex pose a double threat because they can also act as carriers of viruses. Moreover farmers in gardeners may transmit plant viruses inadvertently on pruning shears and other tools.

How did viruses originate?

Viruses have been found that in fact every form of life. Because they depend on cells for their own propagation it is likely that they are not the descendants of pre-cellular forms of life but evolved after the first cells appeared. Most molecular biologist think that viruses originated from naked bits of cellular nucleic acid's that moved from one cell to another perhaps via injured sell services. The evolution of genes coding for capsid proteins may have facilitated the infection of an injured cells. Candidates for the original sources of viral genomes include plasmids and transposons.

Viral symptoms

Viruses may damage or kill cells by causing the release of hydrolytic enzymes from life zones. Some viruses cause infected cells to produce toxins that lead to disease symptoms and some have a molecular components that are toxic such as envelope proteins. How much damage they caused depends partly on the ability of the infected tissue to regenerate by cell division. People recover completely from Kohl's usually because the epithelium of the respiratory tract which the viruses in fact can efficiently repair it self. Damage inflicted by poliovirus to mature nerve cells is permanent because the cells do not divide and usually cannot be replaced. Many of the temporary symptoms associated with viral infections such as fevers and aches actually result from the bodies own efforts are defending itself against infection not cell death caused by the virus.

Prophage

When integrated into the bacterial chromosome the viral DNA is known as a prophage. One prophage gene codes for a protein that prevents transcription of most other prophage genes does the phage genome is mostly silent with in the bacterium. Every time the cell prepares to divide it replicates the phage DNA along with its own impasses them on. One so can give rise to a large population this enables viruses to propagate without killing the host.

lysogenic cycle

allows replication of the phage genome without destroying the host.

RNA as viral genetic material

broadest variety of RNA genomes are found in animal viruses. Among the three types of single-stranded RNA genome Spohn in animal viruses the genome of class IV viruses can directly serve as mRNA and thus can be translated into viral proteins immediately after infection.

What is an important virus that is like this?

HIV or human immunodeficiency virus the retro virus that causes AIDS or acquired immunity deficiency syndrome. HIV and other retroviruses are enveloped viruses that contain two identical molecules of single-stranded RNA into molecules of reverse transcriptase. After HIV enters a Hosell it's reverse transcriptase molecules are released into the cytoplasm where they catalyze synthesis of viral DNA the newly made viral DNA that enters the cells nucleus and integrates into the DNA of a chromosome. This integrated DNA called a provirus never leaves the hosts genome remaining a permanent resident of the cell. The hosts are in a polymerize transcribes the proviral DNA into RNA molecules which can function as mRNA for the synthesis of viral proteins and is genomes for the new viruses that will be assembled and released from the cell.

H1N1 Epidemic

In April 2009 a general outbreak or epidemic of a flu like illness appeared in Mexico and the US. It was quickly identified as an influenza virus related to viruses that cause the seasonal flu. This was named H1N1. The viral disease spread rapidly prompting the world health organization to declare a global epidemic or pandemic in June. By November the disease had reached 207 countries infecting over 600,000 people and killing almost 8000.

Can they alter the host?

In addition to the gene for transcription preventing protein if you other prophage jeans may be expressed during lysogeny. Expression may alter the hosts phenotype which can have important medical significance. The three species of bacteria that causes diphtheria, botulism, and scarlet fever would not be very harmful without certain prophase genes that cause the host bacteria to make toxins. The difference between E. coli strain that resides in our intestines and the one that causes food poisoning is the presence of pro phages in the strain.

Animal viruses

Many variations on the basic scheme of viral infections and replication are present among animal viruses. One key variable is the nature of the viral genome which is the basis for coming classification of viruses. Single-stranded RNA viruses are further classified into three classes according to how the RNA genome functions. Few bacterial phages have an envelope or are in a genome where as many in all viruses have both nearly all with RNa genomes have an envelope and some with DNA do too.

Viral genomes

Many viruses genomes consist of a double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA depending on the virus. A virus is called a DNA or RNA viruses based on the kind of nucleic acid in its genome. In either case the genome is usually organized as a single linear or circular molecule of nucleic acid although some viruses consist of multiple molecules of nucleic acid this most Paris is noon have only four jeans in there genome or the largest have 700 to 1000.

Value of viruses

Molecular biology was born in the laboratories of biologist studying viruses that infect bacteria. Experiments with viruses provided important evidence that genes are made of nucleic acid's in they were critical in working out the molecular mechanisms of the fundamental processes of DNA replication and transcription and translation.

Viral diseases in plants

More than 2000 types are known and together they account for an estimated annual loss of $15 billion worldwide do you to their distraction of agricultural and horticultural crops. Common signs include bleached or brown spots on leaves and fruits stunted growth and damage flowers or routes all tending to diminish the yield and quality of crops.

What was the likely cause of both of these pandemics?

That the virus mutated as it passes from one host species to another. When an animal like a pig or bird is infected with more than one strain of flu virus the different strains can undergo genetic recombination if the RNA molecules making up their genomes mix and match during viral assembly. Pigs are thought to have been the breeding ground for the 2009 flu which contains sequences from bird pig and human flu viruses. Coupled with mutation these reassortment can lead to the emergence of a virus strain that is capable of infecting human cells. Humans who have never been exposed to that particular strain will lack immunity in the recombinant virus has the potential to be highly pathogenic if it recombines with viruses that circulate widely among humans it may acquire the ability to spread easily from to another.

Retroviruses

The RNA animal viruses with the most complicated replicative cycles are retroviruses class VI these viruses are equipped with an enzyme called reverse transcriptase.

Class V

The RNA genome to serves as a template for mRNA synthesis and is then transcribed into complementary RNA strands which function both as mRNA and as templates for the synthesis of additional copies of genomic RNA. All viruses that require RNA to RNA synthesis to make mRNA is a viral enzymes capable of carrying out this process there are no such enzymes in most cells. This is package with the genome inside the viral capsid.

What is a second process

The dissemination of a viral disease from a small isolated human population. AIDS went unnamed and virtually unnotice for decades before it began to spread around the world. In this case technological and social factors including affordable international travel, blood transfusions, sexual promiscuity in the abuse of intravenous drugs allowed a previously rare human disease to become a global scourge.