Reminder: All answers must be written in your own words and include complete and
ID: 176714 • Letter: R
Question
Reminder: All answers must be written in your own words and include complete and accurate citations for the sources you used or that support your answer. No points will be earned for answers without citations or with incorrect citations. Citations for the course modules must include the section topic.
6. Name the structural components common to viruses and describe the function of each. Describe at least three structural characteristics that contribute to the high diversity in types material of animal viruses and give examples to illustrate the differences. A virus is a parasite that is unable to reproduce by itself and it is also small. RNA and DNA is the genetic material that makes up most viruses. A single complete virus particle is called a virion. The components of a virion are viral proteins, capsid, envelope, and nucleic acid. Virial proteins it binds the host cell surface to the virus. Capsid is a shell made up of protein that keeps DNA and RNA genome enclosed. Envelope. Nucleic acid is either RNA or DNA. Animal virus diversity includes the envelope
7. Describe the steps from viral attachment to the host cells and translation of the HIV genome in the host cell and the role of the viral enzyme reverse transcriptase in the process. Include the names of the stages of the life (replication) cycle.
8. State the normal function of proto-oncogenes, and describe how some viruses may contribute to the development of tumors by altering proto-oncogenes. Name two oncogenic viruses and the type of cancer they cause.
9. What are the mechanisms by which mature enveloped and non-enveloped viruses enter and leave a host cell? Briefly describe the processes and name a virus for each.
10. How do viruses cause disease?
Explanation / Answer
6. Viruses consists of the following two components : 1) Viral genome 2) protein capsid , which encloses the genome. They are known as nucleocapsid. Some animal viruses may contain a lipid envelope. The entire intact virus is called virion.
Viral genomes:
Viral genomes can be comprised of single or double stranded DNA or RNA. they vary in size from 5-10 kb to 100-200 kb. The viral genomes can be :
DNA : Double Stranded - linear or circular, Single Stranded - linear or circular
RNA: Double Stranded - linear, Single Stranded - linear
The single stranded genomes can be either + sense, - sense, or ambisense. The sense strand is the one which serves directly as mRNA and code for protein, so for these viruses, the viral RNA is infectious. The viral mRNA from - strand viruses is not infectious, as it needs to be copied into the + strand before it can be translated. In an ambisense virus, part of the genome is the sense strand, and part is the antisense.
Protein capsid:
Viral genomes are covered by protein coats known as capsids. There is a "packaging" signal (sequence) on the viral genome that is recognized by the capsid proteins. The capsid is almost always made up of repeating structural subunits which are arranged in one of two symmetrical structures, a helix or an icosahedron.
Some viruses which contain the helical nucleocapsids, including the Orthomyxoviridae (influenza),and the Rhabdoviridae (rabies) etc.
In icosahedral structures the subunits are arranged in the form of a hollow, quasi spherical structure. An icosahedron is made up of 20 equilateral triangular faces arranged around the surface of a sphere. Some examples for viruses having an icosahedron are influenza virus , hepatitis C etc.
Viral envelope:
In viruses, the nucleocapsid is surrounded by a membrane, known as envelope. The envelope is made up of a lipid bilayer, and is comprised of host cell lipids. It also contains virally encoded proteins, often glycoproteins which are trans-membrane proteins. They help in binding to receptors on the host cell, playing a role in cell entry. Enveloped viruses also contain matrix proteins , which link the nucleocapsid to the envelope. examples for enveloped viruses inlcude Filoviridae( ebola virus) and Coronaviridae ( corona virus ) etc.
7 .Viral Attachment to host cell is mediated by virion protein binding to specific host surface molecule such as membrane proteins, lipids, or the carbohydrate moieties present either on glycoproteins or glycolipids. Binding to the target host molecule leads to viral genome entry into the host cell, which triggers signaling pathways, or allows the virion to be carried by host cells to a specific organ. Cell receptors to virus can be classified in two classes: adhesion receptors are attaching the virus in a reversible manner to target cells or organs. This adhesion is not mandatory for virus entry, and alone do not trigger entry. It enhances significantly the infectivity by concentrating the virus in the vicinity of it’s entry receptors. These receptors are triggering virus entry by endocytosis/pinocytosis or by inducing fusion/penetration, and the consequences of this binding are irreversible.
The integration process occurs in the cytoplasm of the host cell after the completion of reverse transcription of the HIV RNA into c-DNA. The enzyme reverse transcriptase plays an important role in this process.
The stages involved in the translation of the hiv genome are:
Binding of HIV integrase to the HIV DNA, Processing of the HIV DNA 3' ends, Translocation of the HIV integrase to the host cell nucleus , Binding of the preintegration complex to the host DNA , Transfer of the HIV DNA into the host DNA, Reparing of the gaps formed in the strand transfer process.
The stages of the life cycle of viruses includes : Exposure of host, Entry of the virus , Replication of the virus, Shedding of the virus and viral latency.
8. Protooncogenes delievers signals which lead to cell division, and they also regulate programmed cell death known as Apoptosis.
The two types of oncogenic retroviruses are : acute transforming viruses and non-acute transforming viruses.
Acute transforming viruses cause a rapid tumor growth as they carry viral oncogenes in their DNA/RNA to induce such growth. An example of an acute transforming virus is the Rous Sarcoma Virus (RSV) that carry the v-src oncogene. v-Src is part of the c-src, which is a cellular proto-oncogene that induces rapid cell growth and expansion. A non-acute transforming virus induces tumor growth by transcriptionally activating the proto-oncogenes particularly the long terminal repeat (LTR) in the proto-oncogenes.
Ex: papilloma viruses cause cervical cancer, Herpes viruses cause nasopharyngeal carcinoma.
9.
Enveloped viruses follow a two-step procedure to release their genetic material into the cell: 1) first they bind to specific surface receptors of the target cell membrane and then, 2) they fuse the viral and cell membranes. This occurs by endocytosis or by some other route (e.g., macropinocytosis). Enveloped animal viruses enter their host cells by a process of membrane fusion. Ex: Semliki Forest virus.
Non enveloped virus deliver a partially uncoated, but still large, particle with active transcriptases for viral mRNA synthesis directly into the cytoplasm.The process is started by a particle form that resembles infectious subvirion particles (ISVPs), disassembly intermediates produced from virions by proteolytic uncoating. The non enveloped virus are primed and trigger conformational changes, analogous to those in enveloped-virus fusion proteins, generate a hydrophobic 1 conformer capable of inserting into and disrupting cell membranes and (ii) activation of the viral particles for membrane interaction and mRNA synthesis are concurrent events. Ex : Reovirus
10.
The virus can cause infection by entering the host cell and they reproduce in sufficient numbers before it is overwhelmed by the body's immune response. Viruses can travel lightly as they reproduce themselves by breaching the wall of a healthy cell ans seizing the cell's biological equipment. This invasion and control permits the virus to replicate inside the host cell in huge quantity. The progeny viruses then leave the infected cell and move on to repeat the pattern in nearby healthy cells. Viruses are relatively harmless when they are in the bloodstream , but they are very dangerous when they are replicating inside a cell.