Polio Virus Life Cycle

Treatment of HIV

The aim of antiretroviral treatment is to keep the amount of HIV in the body at a low level. This stops any weakening of the immune system and allows it to recover from any damage that HIV might have caused already. The drugs are often referred to as: antiretrovirals, ARVs, anti-HIV or anti-AIDS drugs.

HIV Replication

Lamda Phage Replication Cycle

Enterobacteria phage λ (lambda phage, coliphage λ) is a bacterial virus, or bacteriophage, that infects the bacterial species Escherichia coli (E. coli). It was discovered by Esther Lederberg in 1950 when she noticed that streaks of mixtures of two E. coli strains, one of which treated with ultraviolet light, was "nibbled and plaqued". This virus has a temperate lifecycle that allows it to either reside within the genome of its host through lysogeny or enter into a lytic phase (during which it kills and lyses the cell to produce offspring).

The phage particle consists of a head (also known as a capsid), a tail, and tail fibers (see image of virus below). The head contains the phage's double-strand linear DNA genome. During infection, the phage particle recognizes and binds to its host, E. coli, causing DNA in the head of the phage to be ejected through the tail into the cytoplasm of the bacterial cell. Usually, a "lytic cycle" ensues, where the lambda DNA is replicated and new phage particles produced within the cell. This is followed by cell lysis, releasing the cell contents, including virions that have been assembled, into the environment. However, under certain conditions, the phage DNA may integrate itself into the host cell chromosome in the lysogenic pathway. In this state, the λ DNA is called a prophage and stays resident within the host's genome without apparent harm to the host. The host is termed a lysogen when a prophage is present. This prophage may enter the lytic cycle when the lysogen enters a stressed condition.

Entry of Virus into Host Cell

Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below.^[1] Despite the variation among viruses, the generalities are quite similar. However, the specifics are varied.

Mechanism for Releasing Enveloped Viruses

Abstract

Many enveloped viruses complete their replication cycle by forming vesicles that bud from the plasma membrane. Some viruses encode “late” (L) domain motifs that are able to hijack host proteins involved in the vacuolar protein sorting (VPS) pathway, a cellular budding process that gives rise to multivesicular bodies and that is topologically equivalent to virus budding. Although many enveloped viruses share this mechanism, examples of viruses that require additional viral factors and viruses that appear to be independent of the VPS pathway have been identified. Alternative mechanisms for virus budding could involve other topologically similar process such as cell abscission, which occurs following cytokinesis, or virus budding could proceed spontaneously as a result of lipid microdomain accumulation of viral proteins. Further examination of novel virus–host protein interactions and characterization of other enveloped viruses for which budding requirements are currently unknown will lead to a better understanding of the cellular processes involved in virus assembly and budding.

Replication Cycle of a Retrovirus

Replication of a Positive + Sense Strand of Lytic RNA PhagE

Viruses are differentiated based on how they reproduce within a cell. There are several types including singled-stranded positive-sense RNA viruses (ss+RNA), single-stranded negative-sense RNA viruses, double-stranded RNA viruses, retroviruses, and double-stranded DNA viruses. The animation shows reproduction of ss+RNA viruses within a cell.

In the first step of viral replication, early proteins are made. An example of an early protein is viral replicase. This is an RNA-dependent RNA polymerase (RdRp), meaning that it uses RNA as a template to make more RNA. In step 2, the RdRp copies the original positive-sense RNA strand to make a double-stranded RNA replicative complex. The newly made negative-sense RNA strand is used as a guide to make more single strands of positive-sense RNA (step 3). These single strands of positive-sense RNA can be used as mRNA to make the structural proteins (step 4) or will be packaged into the final virions. Step 5 shows that the structural proteins and the positive-sense single strands of RNA are packaged together (as indicated by the dotted green arrow) to bud out or exit the cell when it lyses (explodes).

Antiviral drug

Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses. Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit their development.

Antiviral drugs are one class of antimicrobials, a larger group which also includes antibiotic (also termed antibacterial), antifungal and antiparasitic drugs, or antiviral drugs based on monoclonal antibodies. Most antivirals are considered relatively harmless to the host, and therefore can be used to treat infections. They should be distinguished from viricides, which are not medication but deactivate or destroy virus particles, either inside or outside the body. Antivirals also can be found in essential oils of some herbs, such as eucalyptus oil and its constituents.

Interesting facts about Ebola Virus

Interesting facts about Ebola Virus

1. EBOV carries a negative-sense RNA genome in virions that are cylindrical/tubular, and contain viral envelope, matrix, and nucleocapsid components.
2. Ebola first appeared in 1976 in 2 simultaneous outbreaks, in Sudan, and in Democratic Republic of Congo with 151 and 280 deaths respectively.
3. Ebola Virus Disease (EVD) outbreaks have a case fatality rate of up to 90%.
4. Ebola Virus is not transmitted through air. The virus is transmitted to people from wild animals and spreads in the human population through human-to-human transmission.
5. First picture of Ebola Virus was made in 1976 with the magnification of 160,000X
6. There are 5 strains of Ebola virus
   a. Ebola Gabon
   b. Ebola Reston
   c. Ebola Zaire
   d. Ebola Sudan
   e. Ebola Ivory Coast
   Four of the strains can cause severe illness in humans and animals. Reston virus, has caused illness in some animals, but not in humans. Ebola Zaire was discovered first among five of these strains.
7. Typically, symptoms appear 8-10 days after exposure to the virus, but the incubation period can span two to 21 days.
8. Because of its high mortality rate, EBOV is also listed a select agent, World Health Organization Risk Group 4 Pathogen (requiring Biosafety Level 4-equivalent containment).

West Africa 2014 Outbreak

1. The outbreak began in Guinea in December 2013 but was not detected until March 2014.
2. The first suspect was a 2-year-old boy who died on Dec. 6, just a few days after falling ill in a village in Guéckédou, in southeastern Guinea.

Interesting facts about Ebola Virus

Signs and Symptoms of Ebola Virus Disease

Signs and Symptoms of Ebola Virus Disease

Symptoms may appear anywhere from 2 to 21 days after exposure to Ebola, but the average is 8 to 10 days.Ebola Zaire kills people quickly, typically 7 to 14 days after symptoms appear. A person can have the virus but not show any symptoms for as long as 3 weeks. People who survive can still have the virus in their system for weeks afterward. The virus has been detected in semen up to 7 weeks after recovery, according to the WHO. Humans are not infectious until they develop symptoms. A person infected with Ebola virus will typically develop a fever, headache, joint and muscle pain, a sore throat, and intense muscle weakness.

Signs and Symptoms of Ebola Virus Disease are as follows:

• High fever (usually higher than 38.3 °C (100.9 °F))
• Muscle and Joint aches
• Headache
• Sore throat  and Shortness of breath
• Chest pain and cough
• Red eyes
• Weakness
• Swelling
• Severe weight loss
• Chills
• Confusion
• Fatigue
• Nausea and Vomiting
• Diarrhea (may be bloody)
• Internal and External bleeding
• Bleeding, usually from the eyes
• Stomach Pain
• Hiccups
• Raised Rash
• Kidneys and Liver Failure

Recovery from Ebola depends on good supportive clinical care and the patient’s immune response. People who recover from Ebola infection develop antibodies that last for at least 10 years. Ebola virus disease is fatal in 50-90% of cases. The sooner a person is given care, the better the chances that they will survive.

Signs and Symptoms of Ebola Virus Disease

Replication of Ebola Virus

Replication of Ebola Virus

Ebola Virus do not replicate through any kind of cell division; rather, they use a combination of host and virally encoded enzymes, alongside host cell structures, to produce multiple copies of viruses. These then self-assemble into viral macromolecular structures in the host cell. The virus completes a set of steps when infecting each individual cell.

Following are the steps during the replication of Ebola Virus:

1. Attachment

First of all, there is attachment of virus to host receptors through GP glycoprotein which is endocytosed into vesicles in the host cell. Host DC-SIGN and DC-SIGNR play a role in virion attachment.

2. Viral Entry (Penetration)

The virion enters early endosomes by Macropinocytosis or clathrin-mediated endocytosis. 

A. Macropinocytosis

 In this process, ruffled segments of the host’s plasma membrane protrude outward from the cell and form invaginations where the virus utilizes glycoproteins in order to attach to the surface of the plasma membrane. Macropinocytosis is a process in which the Eukaryotic host cells form macropinosomes, segments of plasma membranes that extend out from the cell approximately 0.2-10 µm, in order to incorporate the virus into the cell. The formation of macropinosomes occurs spontaneously, as a result of the activation of various growth factors, or simultaneously with the intake of cellular molecules or extracellular fluid.

B. Clathrin-mediated endocytosis

Clathrin-mediated endocytosis is the other means by which Ebolavirus enters the host cell. This process is very similar to macropinocytosis in that the plasma membrane forms invaginations that engulf the cell. However, clathrin-mediated endocytosis is different in that proteins on the surface of the host’s surface, and in particular clathrin, facilitate the attachment of the virus to the host’s cell surface. Glycoproteins are still used to attach the virus to the cell surface, and the NP-C1 cholesterol transporter still facilitates the fusion of the virus with endosomes and lysosomes and still allows the virus to escape into the cytoplasm. Without the NPC1 cholesterol transporter, Ebolavirus cannot leave the vesicle in order to replicate and cause infection in other cells.

To penetrate the cell, the viral membrane fuses with vesicle membrane, and the nucleocapsid is released into the cytoplasm.

In some culture cells, GP glycoprotein can be processed by host Cathepsin L andCathepsin B into 19kDa GP1. But this processing is not happening in all cells or for all ebolavirus. 19kDA GP1 interacts with host NPC1, which is highly expressed in dendritic cells.

Fusion of virus membrane with the vesicle membrane is triggered by either low pH orNPC1 binding.

3. Sequential Transcription

During transcription, the RNA genome is transcribed into seven monocistronic mRNAs whose length is determined by highly conserved start and stop signals.

The transcription process begins with the binding of the polymerase complex to a single binding site located within the leader region of the genome. The complex then slides along the RNA template and sequentially transcribes the individual genes in their 3’ to 5’ order. Encapsidated, negative-sense genomic ssRNA is used as a template for the synthesis (3′-5′) of polyadenylated, monocistronic mRNAs and, using the host cell’s ribosomes, tRNA molecules, etc., the mRNA is translated into individual viral proteins.

4. Replication

As viral protein levels rise, a switch occurs from translation to replication. Using the negative-sense genomic RNA as a template, a complementary +ssRNA is synthesized; this is then used as a template for the synthesis of new genomic (-)ssRNA, which is rapidly encapsidated

Replication presumably starts when enough nucleoprotein is present to encapsidate neo-synthetized antigenomes and genomes.

5. Budding

The newly formed nucleocapsids and envelope proteins associate at the host cell’s plasma membrane; budding occurs, destroying the cell.

These viruses recruit components of the cellular ESCRT (endosomal sorting complex required for transport) system to mediate host-assisted viral budding. SCRT complexes are normally used by the cell for biological functions involving membrane remodeling, such as intraluminal vesicle formation, autophagy or terminal stages of cytokinesis. The ESCRT family consists of ESCRT-0, ESCRT-I, ESCRT-II which are primarily involved in cargo sorting and membrane deformation, and ESCRT-III which cleaves the bud neck from its cytosolic face . In the last step, vps4 disassembles the complex. The budding reaction catalyzed by the ESCRT machinery has reversed topology when compared with most other budding processes in the cell, such as endocytosis and formation of transport vesicles.

6. Release

Finally, the virion is released.

Replication of Ebola Virus

Genome of Ebola Virus

Genome of Ebola Virus

1. Ebola Virus have a negative-sense, non-segmented single stranded linear RNA genome about 18-19 kb in size.
2. The 3′ terminus is not polyadenylated and the 5′ end is not capped.
3. It contains approximately 19,000 base pairs.
4. It encodes seven structural proteins: nucleoprotein (NP), polymerase cofactor (VP35), (VP40), GP, transcription activator (VP30), VP24, and RNA polymerase (L).

Genome of Ebola Virus

Structure of Ebola Virus

Structure of Ebola Virus

1. Ebola Virus are generally approximately 80 nm in diameter, 970 nm long.
2. They are cylindrical/tubular, and contain viral envelope, matrix, and nucleocapsid components.
3. The virus generally appears in a long, filamentous form, but it can also be “U-shaped,” in the shape of a “6” (the “shepherd’s crook” appearance), or even circular.
4. They have a virally encoded glycoprotein (GP) projecting as 7-10 nm long spikes from its lipid bilayer surface.
5. Glycoproteins are proteins that contain carbohydrate chains (glycans) covalently attached to their polypeptide side chains, a process known as glycosylation.
6. The glycoprotein GP is the sole resident of the Ebolavirus surface and is responsible for attaching to and entering new host cells.
7. The outer viral envelope of the virion is derived by budding from domains of host cell membrane into which the GP spikes have been inserted during their biosynthesis.
8. This virus belongs to the Filovirus family, and structurally it resembles a length of thread.

Structure of Ebola Virus

MECHANISM OF TRANSPOSITION

Mechanism of Transposition-Explainattion/Animation(3D)

Mechanism of transposition in prokaryotes

Several different mechanisms of transposition are employed by prokaryotic transposable elements. And, as we shall see later, eukaryotic elements exhibit still additional mechanisms of transposition.

In E. coli, we can identify replicative and conservative (nonreplicative) modes of transposition. In the replicative pathway, a new copy of the transposable element is generated in the transposition event. The results of the transposition are that one copy appears at the new site and one copy remains at the old site. In the conservative pathway, there is no replication. Instead, the element is excised from the chromosome or plasmid and is integrated into the new site.

Go to:

Replicative transposition

When transposition is from one locus to a second locus for certain transposons, a copy of the transposable element is left behind at the first locus. An analysis of transposon mutants revealed an interesting fact about the mechanism of transposition. Using the transposon Tn3 (Figure 20-22), researchers grouped the mutations that prevent transposition into two categories. A trans-recessive class maps in the gene that encodes the transposase enzyme, a catalyst of transposition. A second class of cis-dominant mutations results in the buildup of an intermediate in the transposition process.Figure 20-23 diagrams the transposition pathway in the Tn3 transposition from one plasmid to another. The intermediate is a double plasmid, with both donor and recipient plasmid being fused together. The combined circle resulting from the fusion of two circular elements is termed acointegrate. Apparently, the mutations in this second class delete a region on the transposon at which a recombination event takes place that resolves cointegrates into two smaller circles. This region, called the internal resolution site (IRS), appears in Figure 20-22.

Figure 20-22

The structure of Tn3. Tn3 contains 4957 base pairs and encodes three polypeptides: the transposase is required for transposition, the repressor is a protein that regulates the transposase gene (see Chapter 11), and β-lactamase confers ampicillin (more...)

Figure 20-23

Transposition of Tn3 takes place through a cointegrate intermediate. Cointegrates in Tn3 transposition are observed for some internal deletions in the transposon. The correct explanation for this observation is that the cointegrates are intermediates (more...)

The finding of a cointegrate structure as an intermediate in transposition helped establish a replicativemode of transposition for certain elements. In Figure 20-23, note how the transposable element is duplicated in the fusion event and how the recombination event that resolves the cointegrate into two smaller circles leaves one copy of the transposable element in each plasmid.

Go to:

Conservative transposition

Some transposons, such as Tn10, excise from the chromosome and integrate into the target DNA. In these cases, DNA replication of the element does not occur, and the element is lost from the site of the original chromosome. Researchers demonstrated this lack of replication by constructing heteroduplexes of λTn10 derivatives containing the lac region of E. coli. The researchers used DNA from Tn10-lacZ+ and Tn10-lacZ− derivatives. The heteroduplexes, therefore, contain one strand with the wild-type lac region and a second strand with the mutated (Z−) lac region. Figure 20-24 diagrams this part of the experiment. The heteroduplex DNA is used to infect cells that have no lac genes, and transpositions of the TetR Tn10 are selected. Different types of colonies arise from the transpositionof a heteroduplex Z−/Z+ carrying transposon (Figure 20-25). If replication takes place (the replicativemode of transposition), all colonies are either completely Lac+ or completely Lac−, because the replication will convert the heteroduplex DNA into two homoduplex daughter molecules. The mechanism by which this conversion takes place will be examined in detail in the next section. However, if the transposition is conservative and does not include replication, each colony arises from a lacZ+/lacZ− heteroduplex. Such colonies are partly Lac+ and partly Lac−. By using media that stain Lac+ and Lac− cells different colors, researchers can observe the Lac+ and Lac− sectors in colonies.

Figure 20-24

Generation of heteroduplex and homoduplex Tn10 elements. The denaturation and reannealing of a mixture of two parental λ phages carrying Tn10 elements that differ only at three single bases in the transposon yields a mixture of heteroduplex and(more...)

Figure 20-25

Consequences of conservative and replicative transposition. (a) The heteroduplex or homoduplex nature of DNA (see Figure 20-24) is transposed into a target gene. If the starting DNA is heteroduplex, then the resulting DNA will still be heteroduplex only (more...)

Therefore, the determination of whether Tn10 undergoes replicative or conservative transposition can be made by observing whether differently colored sectors exist within the same colony resulting from the transposition. Sectored colonies are observed in a majority of cases (Figure 20-26). Thus, Tn10—and perhaps other transposable elements in E. coli—transpose by excising themselves from the donor DNA and integrating directly into the recipient DNA.

Figure 20-26

Colonies resulting from the experiment illustrated in Figures 20-24 and 20-25. A dye is used that stains Z+ cells blue. One-half of this colony is Z+ (dark area), and the other is Z− (white). (Photograph courtesy of N. Kleckner.)

Go to:

Molecular consequences of transposition

The molecular consequences of transposition reveal an additional piece of evidence concerning the mechanism of transposition: on integration into a new target site, transposable elements generate a repeated sequence of the target DNA in both replicative and conservative transposition. Figure 20-27depicts the integration of IS1 into a gene. In the example shown, the integration event results in the repetition of a 9-bp target sequence. Analysis of many integration events reveals that the repeated sequence does not result from reciprocal site-specific recombination (as is the case in phage λ integration; see page 229); rather, it is generated in the process of integration itself. The number of base pairs is a characteristic of each element. In bacteria, 9-bp and 5-bp repeats are most common.

Figure 20-27

Duplication of a short sequence of nucleotides in the recipient DNA is associated with the insertion of a transposable element; the two copies bracket the inserted element. Here the duplication that attends the insertion of IS1 is illustrated in a way (more...)

The preceding observations have been incorporated into somewhat complicated models oftransposition. Most models postulate that staggered cleavages are made at the target site and at the ends of the transposable element by a transposase enzyme that is encoded by the element. One end of the transposable element is then attached by a single strand to each protruding end of the staggered cut. Subsequent steps depend on the mode of transposition (replicative or conservative).

Go to:

Rearrangements mediated by transposable elements

Transposable elements generate a high incidence of deletions in their vicinity. These deletions emanate from one end of the element into the surrounding DNA (Figure 20-28). Such events, as well as element-induced inversions, can be viewed as aberrant transposition events. Transposons also give rise to readily detectable deletions in which part of the element is deleted together with varying lengths of the surrounding DNA. This process of imprecise excision is now recognized as deletions or inversions emanating from the internal ends of the IR segments of the transposon. The process ofprecise excision—the loss of the transposable element and the restoration of the gene that was disrupted by the insertion—also occurs, although at very low rates compared with the frequencies of the events just described.

Figure 20-28

Deletion formation mediated by a transposable element. In this example, the transposable element IS1 is shown at a point in theE. coli chromosome near the gal genes. Deletions can be generated from each end of the IS1 element, extending into the neighboring (more...)

MESSAGE

Some DNA sequences in bacteria and phages act as mobile genetic elements. They are capable of joining different pieces of DNA and are thus capable of splicing DNA fragments into or out of the middle of a DNA molecule. Some naturally occurring mobile or transposable elements carry antibiotic-resistance genes.