Question #d8 | Socratic
It is proposed that the inner capsid of the virion has icosahedral symmetry and the I2 spaces at the apices being surrounded by 5 capsomeres and the other 8o quently made difficult because the image represents a superimposition of top and . Morphological and antigenic relationships between viruses (rotaviruses). The capsomere is a subunit of the capsid, an outer covering of protein that protects the genetic Edit links. This page was last edited on 5 November , at (UTC). Text is available under the Creative Commons Attribution- ShareAlike. distribution of capsid protein subunits within the capsomeres. With future A schematic diagram of an atomic force microscope used for imaging viruses in liquids. AFM fluid cells .. crystalline, the constraints introduced by association into an.
Such an assembly with repeating subunits then greatly reduces the amount of genetic information required. In some viruses, the capsid formation involves a single gene product, whereas in other viruses which are more complex it involves multiple gene products. Such an assembly involving repeating subunits raises several interesting questions.
How do these subunits interact with one another with high fidelity and specificity to form the capsid architecture? This question becomes even more interesting in complex viruses in which the capsid formation involves multiple gene products. Are there any specific structural properties that these proteins should have for the capsid formation? How is capsid assembly directed and controlled?
How is the genome encapsidated? In addition to containing and protecting the genome, the capsid architecture must also be conducive for interactions with the host cell for entry; how is this process coordinated? Given that capsid has to disassemble to make the genome available for replication, what are the cues for disassembly? How does the capsid organization respond to and evade the antiviral response mounted by the host?
In the last half century, structural studies on a variety of viruses have provided a wealth of information regarding some of the questions listed above. In addition to providing insight into the fundamental principles underlying various aspects of capsid assembly, more importantly, such studies have had practical impact in providing a rational basis for the design and development of antiviral strategies.
Several excellent reviews on virus structures and principles underlying capsid formation have been published periodically over years Klug and Caspar ; Caspar and Klug ; Rossmann and Johnson ; Johnson and Speir ; Harrisonwe will emphasize here the recent developments along with the some historical perspective.
Structural techniques Two principal techniques used in the structural studies on viruses are electron microscopy and X-ray crystallography. Contributions from other elegant studies using a variety of biochemical and biophysical techniques and theoretical modeling have been crucial in providing a more complete understanding of the capsid construction and assembly pathways.
Electron microscopy of negatively stained virus specimens provided the first glimpse of viruses and led to early classification of viruses based on shape and form Green et al. Even today this technique is used as a diagnostic tool in identifying clinical virus samples.
Subsequently, the discovery that EM images of virus particles, which are essentially projection images, can be used to reconstruct the three dimensional structure of the virus using computer image analysis protocols Crowther et al.
In the last two decades, this exciting new technology called three-dimensional electron cryomicroscopy cryo-EM has revolutionized the structure analysis of variety of viruses Baker et al. Much of our understanding of subunit interactions in a viral capsid at the atomic level has come from X-ray crystallographic structure of spherical viruses. Beginning with the structures of three small spherical plant viruses in early s Harrison et al.
The closely related technique of X-ray fiber diffraction has been used to study viruses that have helical symmetry Namba and Stubbs ; Namba et al. In recent years, cryo-EM technique has allowed visualization of variety of spherical viruses at subnanometer Bottcher et al. For some viruses that are not amenable for high resolution structural analysis by these techniques, complementarity between cryo-EM and X-ray crystallography has been exploited in deriving the pseudo atomic models of the capsid Grimes et al.
In these studies, when the virus capsid could not be crystallized, but a lower resolution structure could be determined by cryo-EM, as this technique does not require the specimen in a crystalline form, independently determined X-ray crystallographic structures of the capsid components are fitted into lower-resolution cryo-EM map of the capsid.
Such a hybrid technique has been most useful in studying capsid-receptor, capsid antibody interactions and in studying capsid associated structural dynamics Rossmann et al.Entry of non-enveloped virions
Structure determination of spherical viruses either by X-ray crystallography or cryo-EM techniques rely implicitly on the symmetry of the capsid. As a result, the structural organization of the encapsidated genome is amenable to these structural techniques only when the genome follows the capsid symmetry.
However, in recent years, there are several examples in which the entire genome or a significant portion of it is observed to follow the capsid symmetry and visualized in the structural analysis Chen et al. A detailed discussion on the genome organization in viruses is provided in a review by Prasad and Prevelige In this review we mainly focus on the capsid organization. These enveloped viruses do not necessarily kill the host cell.
Some viruses have envelopes that are not derived from plasma membrane. The envelope of the herpesvirus is derived from the nuclear envelope of the host. These double-stranded DNA viruses reproduce within the cell nucleus using viral and cellular enzymes to replicate and transcribe their DNA. Herpesvirus DNA may become integrated into the cell's genome as a provirus, which remains latent within the nucleus until triggered by physical or emotional stress to leave the genome and initiate active viral production.
This complementary strand is the template for the synthesis of additional copies of genome RNA. Retroviruses class VI have the most complicated life cycles. The newly made DNA is inserted as a provirus into a chromosome in the animal cell.
These can function both as mRNA for the synthesis of viral proteins and as genomes for new virus particles released from the cell. The viral particle includes an envelope with glycoproteins for binding to specific types of white blood cells, a capsid containing two identical RNA strands as its genome and two copies of reverse transcriptase.
Some viruses damage or kill cells by triggering the release of hydrolytic enzymes from lysosomes. Some viruses cause the infected cell to produce toxins that lead to disease symptoms.
Others have molecular components, such as envelope proteins, that are toxic. In some cases, viral damage is easily repaired respiratory epithelium after a coldbut in others, infection causes permanent damage nerve cells after polio.
Many of the temporary symptoms associated with a viral infection results from the body's own efforts at defending itself against infection. The first vaccine, using cowpox, was developed in the late s by Edward Jenner to prevent smallpox.
Many others have since been developed. Antibiotics don't work against viruses.
Antivirals - recently developed drugs to combat some viruses, mostly by interfering with viral nucleic acid synthesis. Acyclovir inhibits herpesvirus DNA synthesis. The deadly Ebola virus has caused hemorrhagic fevers in central Africa periodically since The emergence of these new viral diseases is due to three processes: Influenza strains spread of existing viruses from one species to another It is estimated that about three-quarters of new human diseases have originated in other animals.
For example, hantavirus, which killed dozens of people innormally infects rodents, especially deer mice. Dissemination of a viral disease from a small, isolated population. AIDS, present only in small populations in Africa, went unnamed and unnoticed for decades before spreading around the world.
Principles of Virus Structural Organization
Affordable international travel, blood transfusion technology, sexual promiscuity, and the abuse of intravenous drugs, allowed a previously rare disease to become a global scourge. Tumor viruses include retrovirus, papilloma virus, adenovirus, and herpesvirus types. The hepatitis B virus is associated with liver cancer. The Epstein-Barr virus, which causes infectious mononucleosis, has been linked to several types of cancer in parts of Africa, notably Burkitt's lymphoma.
Papilloma viruses are associated with cervical cancers. The HTLV-1 retrovirus causes a type of adult leukemia. All tumor viruses transform cells into cancer cells after integration of viral nucleic acid into host DNA. Viruses may carry oncogenes that trigger cancerous characteristics in cells. These oncogenes are often versions of proto-oncogenes that generally code for growth factors or proteins involved in growth factor function.
In other cases, a tumor virus transforms a cell by turning on or increasing the expression of proto-oncogenes. It is likely that most tumor viruses cause cancer only in combination with other mutagenic events. Most are RNA viruses with rod-shaped capsids produced by a spiral of capsomeres. Plant viral diseases are spread by two major routes. In horizontal transmission, a plant is infected with the virus by an external source.
In vertical transmission, a plant inherits a viral infection from a parent. This may occurs by asexual propagation or in sexual reproduction via infected seeds. Viroids and prions Viroids, smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants. Their several hundred nucleotides do not encode for proteins but can be replicated by the host's cellular enzymes. Prions are infectious proteins that spread a disease.
They are thought to cause several degenerative brain diseases including scrapie in sheep, "mad cow disease," and Creutzfeldt-Jacob disease in humans. According to the leading hypothesis, a prion is a misfolded form of a normal brain protein, which then converts normal proteins into the prion version.
Viruses living or nonliving? An isolated virus is biologically inert and yet it has a genetic program written in the universal language of life. Because viruses depend on cells for their own propagation, they evolved after the first cells appeared, probably fragments of cellular nucleic acids that could move from one cell to another. A viral genome usually has more in common with the genome of its host than with those of viruses infecting other hosts.
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The evolution of capsid genes may have facilitated the infection of undamaged cells. Candidates for the original sources of viral genomes include plasmids, small, circular DNA molecules that are separate from chromosomes, and transposons, DNA segments that can move from one location to another within a cell's genome. Both are mobile genetic elements. The Genetics of Bacteria Bacteria are very adaptable, both in the evolutionary sense of adaptation via natural selection and the physiological sense of adjustment to changes in the environment by individual bacteria.
The major component of the bacterial genome is one double-stranded, circular DNA molecule. Many bacteria have plasmids, much smaller circles of DNA. Bacterial cells divide by binary fission. New mutations, though individually rare, can have a significant impact on genetic diversity when reproductive rates are very high because of short generation spans. Genetic recombination In bacteria, the combining of DNA from two individuals into a single genome.
Recombination occurs through three processes: Transformation Transduction Conjugation Transformation is the alteration of a bacterial cell's genotype by the uptake of naked, foreign DNA from the surrounding environment. For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells, when a live nonpathogenic cell takes up a piece of DNA that happens to include the allele for pathogenicity from dead, broken-open pathogenic cells.
Many bacterial species have surface proteins that are specialized for the uptake of naked DNA. Transduction occurs when a phage carries bacterial genes from one host cell to another. In generalized transduction, a small piece of the host cell's degraded DNA is packaged within a capsid, rather than the phage genome.
Capsomere - Wikipedia
When this phage attaches to another bacterium, it will inject this foreign DNA into its new host. This type of transduction transfers bacterial genes at random. Specialized transduction occurs via a temperate phage. When the prophage viral genome is excised from the chromosome, it sometimes takes with it a small region of adjacent bacterial DNA. These bacterial genes are injected along with the phage's genome into the next host cell.
Specialized transduction only transfers those genes near the prophage site on the bacterial chromosome. Both generalized and specialized transduction use phage as a vector to transfer genes between bacteria.
Conjugation transfers genetic material between two bacterial cells that are temporarily joined. One cell "male" donates DNA and its "mate" "female" receives the genes. A sex pilus Fig Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules.
Episomes, are a genetic element that can exist as a plasmid or as part of a chromosome. Episomes, like the F plasmid, can undergo reversible incorporation into the cell's chromosome. Temperate viruses also qualify as episomes.
Polyoma virus capsid structure at 22.5 Å resolution
Plasmid genes are advantageous in stressful conditions. Recombination exchanges segments of DNA. This recombinant bacteria has genes from two different cells. The genes conferring resistance are carried by plasmids, specifically the R plasmid R for resistance.
Principles of Virus Structural Organization
Some of these genes code for enzymes that specifically destroy certain antibiotics, like tetracycline or ampicillin. When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population.
Because R plasmids also have genes that encode for sex pili, they can be transferred from one cell to another by conjugation. A transposon is a piece of DNA that can move from one location to another in a cell's genome.