Biology Virus Essay
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Virus biology Essay
. The function of a capsid, in virus biology, is to serve as a protective outer layer that protects the genetic material of the virus from the host’s defensive cells. The capsid is made up of proteins. The nucleic acid is what is contained within the capsid, which is a group of long, linear macromolecules, consisting of either DNA or RNA. There are two different cycles in which the virus uses a host cell to replicate and reproduce itself. These two cycles are known as the lytic cycle, and the lysogenic cycle. In the lytic cycle, viral particles are made from the host bacteria cell’s DNA, and replicas of the virus are made and sent back into the environment. The steps that make up this cycle are: the virus latches to the bacteria cell, which then inserts its DNA into the bacteria, effectively taking over the cell’s machinery. It then reproduces itself and self-assembles. The replica viruses then burst from the host cell, destroying the host. In the second cycle, the lysogenic cycle, the virus begins as a prophage, the latent form of a bacteriophage, and binds to the bacteria. The viral DNA then gets incorporated into the cell’s chromosome, and is replicated along with the host cell’s chromosomal material. The basic differences between the two cycles is that the lytic cycle is exhibited by virulent phages (otherwise known as bacteriophages), where the lysogenic cycle is exhibited.
Biology Essay Succession
. Ecological Succession Script Andy Raskita The world around us is slowly and constantly changing. We might never see the small alterations happen, but many years later, changes will surely be visible. Over time small changes add up and soon the whole ecosystem is different. This slow change in the land and habitat is called ecological succession. Animals, plants, and weather are some factors that cause this type of change in an ecosystem. Ecological succession begins with a pioneer community. In many cases lichens are the "pioneers." Lichens break down rocks, making soil, so that plants can begin to grow. The process then continues on; trees grow, decay and then new trees grow back even bigger. Differences in the amount of sunlight, the amount of wind, and the type of soil all influence this changing community of organisms. The number and type of organisms change along with the habitat. Many, many years later the ecosystem becomes relatively stable. This stable condition can last for hundreds of years and is called a climax community. Sometimes nature’s power causes changes that happen more quickly, but not instantly. An example of this is repeated flooding. In some areas floods happen often, each time new sediment is carried by the water and the deposited. Over time a marsh habitat becomes present, next a grassland and then a forest. At other time nature works backwards by tearing down a climax community with a flood or fire. New kinds of organisms are able to move in.
Advanced Biology Virus Worksheet Essay
. Virus Multiple Choice Identify the choice that best completes the statement or answers the question. ____ 1. Which of the following statements is INCORRECT? a. Viruses are not able to move by themselves. b. Viruses are not able to reproduce by themselves. c. Viruses are not structurally organized. d. Viruses are nonliving particles. e. Viruses contain instructions to manufacture themselves. ____ 2. Which of the following is FALSE? a. The outer coats of all viruses are alike. b. The virus uses either DNA or RNA at its core but not both. c. Viruses can be replicated only after they enter a living cell. d. Most viruses have a protein coat or covering. e. A virus may not kill a host cell but may become inactive for a period of latency. ____ 3. Which of the following statements about viruses is TRUE? a. They were the first form of life to evolve. b. They do not attack plants. c. They are able to reproduce without using other organisms. d. They are made of protein only. e. They include some forms that are able to attack bacteria. ____ 4. When a virus takes over the machinery of a cell, it forces the cell to manufacture a. more mitochondria for energy for the virus. b. more liposome to isolate itself from water. c. more food particles. d. more viral particles. e. more Golgi bodies so that the cell will secrete the excess viruses. Examine the figure above and answer.
. maintain optimal internal environments while positive feedback mechanisms amplify responses. Changes in a biological system’s environment, particularly the availability of resources, influence an organism’s responses and activities. Organisms use various means to obtain nutrients and remove wastes. Homeostatic mechanisms across phyla reflect both continuity due to common ancestry and change due to evolution and natural selection. Examples of homeostatic mechanisms that have evolved in plants and animals include defense mechanisms as well as the timing and coordination of developmental, physiological and behavioral event regulation. These mechanisms increase the fitness of individuals and long-term survival of populations. Units in AP Biology Course Be sure to know the following: 01A – Viruses and Cells How organelles use and produce ATP. How organelles maintain homeostasis within cells. 01B – Homeostasis in Cells All the notes in the unit. How to use the water potential formula to calculate water potential values. 01C – Replication of Viruses and Cells The cell cycle and regulation of the cell cycle via check points (cyclins, CDKs, MPFs, and PDGFs) Three phases of interphase and alternation of interphase with mitosis The sequence of events in mitosis (replication, alignment, separation) How the reduction division of meiosis followed by fertilization ensures genetic diversity How viral replication differs from other reproductive strategies.
. Aim: To construct a field graph to study the local ecosystem at the Minnamurra estuary involving abiotic factors, transects and abundance. Method: Measuring abiotic factors 1. Measure the elevation along the transect line in centimetres using an inclinometer in each quadrat. 2. Measure the soil temperature in degrees using a thermometer by placing it in the ground in the quadrat. 3. Measure the air temperature in degrees using a thermometer by holding it up in the air just above the ground in the quadrat. 4. Measure the wind speed in knots using an anemometer held up in the air in the quadrat. 5. Measure the soil moisture in the quadrat using a soil moisture probe. Soil moisture is represented in a percentage 1%-100% with 100% being the maximum amount of moisture. 6. Measure the amount of sunlight in the quadrat using a lux meter. Calculating the abundance of animals across the mangroves and salt marsh 1. Place a quadrat somewhere in the shoreward edge of the intertidal zone 2. Align the quadrat with a compass so that one side faces north and the other faces south. 3. Calculate the area of the quadrat in metres squared. 4. Calculate the abundance by first, finding the area of the quadrat in m2. Total number of individuals in each quadrat and the total number of quadrats. Average number of individuals per quadrat = total no. Individuals ÷ total no. Quadrats. 5. Estimation of abundance = Average no. Individuals per quadrat x Area of quadrat.
. Aerobic Respiration Breaking down glucose to release energy Products are Carbon dioxide and water Diffusion is … the movement of molecules from a region of high concentration to a low concentration until the concentration is equal. Role of a human circulatory system: Diffuse In – oxygen (O2), dissolved food (glucose) and water (H2O) Diffuse Out – carbon dioxide (CO2),waste and water(H2O) Respiration – releasing energy in cells Breathing – getting air into and out of your body Exercise When you exercise your heart rate and breathing rate increases Heart rate measured – beats per min. Breathing rate – counting the number of breaths in a min. Muscle cells respire faster. Meaning- they need more oxygen and glucose and release more carbon dioxide. A faster breathing rate means that oxygen can be taken into the body at a faster rate and carbon dioxide can be released faster. Cardiac output = stroke volume x heart rate Effects of exercise: Ventilation – breathing Respire – release of energy from food/chemicals Anaerobic respiration … is release of glucose without using oxygen. This produces lactic acid. glucose = lactic acid + energy Advantage: useful for muscles because it can release energy to allow muscles to contract when the heart/lungs can’t deliver (O2) and glucose fast enough for aerobic respiration. Disadvantages: Lactic acid is not removed from the body. It builds up in the body and the blood.
biology Research Paper
. Activity 7.4.2: Challenge DHCP and NAT Configuration Topology Diagram Addressing Table Device Interface IP Address Subnet Mask R1 S0/0/0 172.16.0.1 255.255.255.252 Fa0/0 172.16.10.1 255.255.255.0 Fa0/1 172.16.11.1 255.255.255.0 R2 S0/0/0 172.16.0.2 255.255.255.252 S0/0/1 18.104.22.168 255.255.255.252 Fa0/0 172.16.20.1 255.255.255.0 ISP S0/0/1 22.214.171.124 255.255.255.252 Learning Objectives Upon completion of this lab, you will be able to: Prepare the network Perform basic router configurations Configure a Cisco IOS DHCP server Configure static and default routing Configure static NAT Configure dynamic NAT with a pool of addresses Configure NAT overload Scenario In this lab, configure the IP address services using the network shown in the topology diagram. If you need assistance, refer back to the basic DHCP and NAT configuration lab. However, try to do as much on your own as possible. Task 1: Perform Basic Router Configurations Configure the R1, R2, and ISP routers according to the following guidelines: Configure the device hostname. Disable DNS lookup. Configure a privileged EXEC mode password. Configure a message-of-the-day banner. Configure a password for the console connections. Configure a password for all vty connections. Configure IP addresses on all routers. The PCs receive IP addressing from DHCP later in the lab. Enable RIPv2 on R1 and R2. Do not advertise the 126.96.36.199/27 network. For all.
. Cell Shape and Diffusion-Practical Report (Re-Written) 23-04-2012 Year 11 IB Biology Title: Cell Shape and Diffusion Diffusion is a type of passive transport which allows particles to move from a region of high concentration to a region of low concentration. In a living system, diffusion involves a cell membrane. Cells also need to excrete waste which also occurs at the cell membrane. Aim: The aim is to use model cells made from agar which contains phenolphthalein and sodium hydroxide (cut in different sizes) to investigate the effect of shape on the time taken for sulfuric acid to diffuse into the centre of the ‘cells’. This will be measured by the time it takes for the agar to become colourless. Research Question: How will changing the shape or size of the agar prisms affect the time taken for the sulfuric acid to diffuse into the centre? Hypothesis: If the surface area to volume ratio of the agar prism is large (eg: 1 x 1 cm), then the time taken for the sulfuric acid to diffuse it will be shorter. The smaller the size of the agar block, less time will be needed to diffuse the agar block compared to a larger agar block with a lower SA: V ratio. Variables: The independent variable was the shapes of the agar prisms and its sizes. The surface area to volume ratio (cm 2 : cm 3 ). The sizes and shapes used were: Dimensions of each agar block: Table 1 Shape Dimensions (cm) Cube 1 1 x 1 x 1cm Cube 2 1.5 x 1.5 x 1.5cm Cube 3 2.
A computer virus is a program that can copy itself and infect a computer without the user knowing. Some viruses are made to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but replicate themselves and perhaps make their presence known by displaying text, video, or audio messages. Even these benign viruses can create problems for the computer. They usually take up computer memory used by other programs. As a result, they often cause crazy behavior and can result in system crashes. Also many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
Antivirus software is a computer program that trys to identify, stop and remove computer viruses and other malicious software like trojan horses.
Trojan horse is a program that unlike a virus, contains or installs a malicious program (sometimes called the payload or ‘trojan’) while disguising itself as being something else. Trojan horses may appear to be useful or interesting programs to an unsuspecting user, but are harmful when executed.
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. How worried are you about the Influenza Virus? The Influenza virus, commonly known as the flu, is a viral disease that can be infectious in mammals and birds, it´s caused by RNA viruses. This disease is most common between the months of October and May, it can last from three days to five days; it´s an acute disease, because manifests quickly and last a short time. There are three types of Influenza according to how severe they are; Type A: The most severe type with the most acute symptoms. Type B: Similar symptoms to type A, but not as serious. Type C: The mildest one and with symptoms similar to a cold. The Influenza virus can be transmitted mainly by two different ways: by direct transmission (when an infected person sneezes the mucus goes directly into the person that is closest); by contaminated air (when someone inhales the “aerosols” produced by an infected person coughing or sneezing). The disease mainly affect the nose, the throat and the lungs, often spread out as an epidemic, which quickly spreads from town to town and country to country, it affects lots of people in a short period of time. It´s most common symptoms are: headache, fever (38º to 40º), dry cough, vomiting or diarrhoea, fatigue and weakness, chest pains, nasal congestion and sore throat, irrigating and watering eyes, pneumonia. All this symptoms have to be recognized early to cure the infection. The droplets that are small enough for people.
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. can prevent this disease. If death occurs, it is within few days of onset of symptoms. Cowpox Cowpox is a skin disease caused by a virus known as the cowpox virus. Cowpox is transmitted by touch from infected animals to humans. Cowpox is similar to but much milder than the highly contagious and sometimes deadly smallpox disease. Cowpox resembles mild smallpox. Cowpoxes symptom is red skin blisters. Roseola Roseola is a viral infection that commonly affects infants and young children. It involves a pinkish-red skin rash and high fever. Roseola is common in children ages 3 months to 4 years, and most common in those ages 6 months to 1 year. Roseola is caused by a virus called human herpes virus. Symptoms include Irritability, runny nose, sore throat and high fever. There is no specific treatment for roseola. The disease usually gets better on its own without complications. Bolivian hemorrhagic fever Bolivian hemorrhagic fever (BHF), also known as black typhus or Ordog Fever. Has a slow onset with fever, malaise, headache and muscular pains. In February and March 2007, at least 20 suspected BHF cases (3 fatal) were reported to the El Service Departmental de Salud (SEDES) in Beni, Bolivia. In February 2008, at least 200 suspected new BHF cases (12 fatal) of BHF were reported to SEDES. Ebola Ebola is a rare and deadly disease. Ebola virus disease (EVD) or Ebola hemorrhagic fever (EHF) is the.
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. Ebola Virus— What type of virus is it? How it spreads? What is being done to contain its spread? What treatments are available? Ebola genome is a single-stranded RNA approximately 19,000 nucleotides long. It encodes seven structural proteins such as nucleoprotein, polymerase cofactor, transcription activator, RNA- dependent RNA polymerase. The Ebola virus is a Filovirus. These virus types cause fever or cause bleeding inside and outside the body when having a very high fever. Ebola can be further divided into subtypes that are named for the location they were identified. Ebola carries a RNA genome that are cylindrical, contains a viral envelope, matrix and nucleocapsid components. The overall cylinders are generally approximately 80 nm in diameter, and have a virally encoded glycoprotein (GP) projecting as 7-10 nm long spikes from its lipid bilayer surface. Ebola virus will enter two kinds of protein.The first is a cholesterol transporter protein, the host-encoded Niemann–Pick C1, which is essential for entry of Ebola virus into the host cell and for its ultimate replication. Being acellular, Ebola do not replicate through any type of cell division; rather, they use a host- and virally encoded enzymes, alongside host cell structures, to replicate themselves. These then self-assemble into viral Macromolecular structure.The virus completes a set of steps.
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Viruses: Essay for Students on Viruses | Biology (556 Words)
Here is your essay for students on Viruses!
Viruses are so small that they cannot be seen even with the highest magnification of the microscope using visible light. They are recognizable only by their biological behaviour, such as, by the disease they cause. Their exact nature was a mystery for long. They were variously regarded as invisible form of bacteria, protozoa, enzymes, toxins or as unusual products of metabolism of the cells in which they were found.
Image Courtesy : biowan2509.edublogs.org/files/2009/10/Viruses_and_Bacteria.jpg
An American biochemist, Stanley in 1935, however isolated by chemical means from the diseased tobacco leaves a material which appeared to be a protein of high molecular weight. He studied this material in detail, and found that it possessed all the properties of a tobacco mosaic virus, which has already been discovered.
There are a number of direct, indirect and circumstantial, evidences to show that the material itself is the virus. Stanley, on the basis of the chemical properties, identified the material as autocatalytic proteins which can multiply within a living cell only. Bawden purified the tobacco mosaic virus, and found it to be a crystalline nucleo-protein of very high molecular weight, retaining its infectivity even when diluted to a concentration of 1/1,000,000.
This virus when examined with the electron microscope using X-rays, is found to be in the form of bundles of rod-like protein. Such nucleoprotein cannot be obtained from healthy plants and is the virus itself. In 1938 the study of potato virus confirmed this. Since then, proteins of high molecular weight possessing all the properties of the respective viruses have been isolated and studied.
Most of the viruses are crystalline and rod-like, but different in size. Tobacco mosaic virus is rod-like in shape, 280 mμ in length and 18 mμ in breadth. ‘Bushy stunt’ virus of tomato is 274 mμ in diameter and its molecular weight is 8,800,000-12,800,000. Stanley studied the nucleo- proteins of the ‘ring spot’ virus of tobacco. The virus is rounded with 19 mµ diameter and 3,400,000 molecular weight. Tobacco ‘necrosis’ virus is smallest of all the plant viruses known so far. It is rounded with 13-20 mμ diameter.
Before 1935 they were believed to be living on the basis of few following properties:
1. That they can live only in a living cell.
2. They can infect healthy plants just like bacteria and fungi.
3. They multiply in number and grow in size, as the living organisms reproduce and grow.
4. They have physiologic specialization in relation to the insect vectors and the plants.
5. They respond to stimuli, such as acids, alkalies, light and temperature.
The supporters of the non-living nature of viruses based their views on the following few properties:
1. They are too small to be observed under visible light.
2. They retain infectivity even in very low concentrations.
3. They can be crystallized like a chemical substance.
4. They can be sedimented like proteins.
5. They can be precipitated by a number of chemical substances.
6. They retain the power of infection even after 31 years in non-living tobacco leaves.
Some workers did not agree with any of the above extreme views and adopted a middle course. They regarded viruses, representing a stage in between living and non-living with the acquired property of multiplication.
At present most of the virologists have agreed that viruses are nucleo-proteins of high molecular weight and have the power of multiplication.
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Essay on Viruses
In this essay we will discuss about Viruses. After reading this essay you will learn about: 1. General Characters of Viruses 2. Size of Viruses 3. Shapes 4. Chemistry and Microstructure 5. Symmetry 6. Classification 7. Biological Status or Nature 8. Economic Importance.
- Essay on the General Characters of Viruses
- Essay on the Size of Viruses
- Essay on the Shapes of Viruses
- Essay on the Chemistry and Microstructure of Virus
- Essay on the Symmetry among Viruses
- Essay on the Classification of Viruses
- Essay on the Biological Status or Nature of Viruses
- Essay on the Economic Importance of Viruses
Essay # 1. General Characters of Viruses:
A large number of viruses are known. They exhibit diversity of form and infect a number of organisms.
Despite diversity of form and structure they show the following important characteristics common to all viruses:
1. They are ultramicroscopic disease producing entities and smaller than bacteria.
2. They have no cellular organization and also no metabolic machinery of their own.
3. Unit of structure of virus is virion which is a-cellular and lacks protoplasm.
4. Virion is simple in structure, basically composed of nucleic acid wrapped in a protein coat.
5. Nucleic acid is only of one type, either DNA or RNA but never both.
6. They are highly physiologically specialized i.e., host specific.
7. They are obligatory inter-cellular parasites as they are completely inactive outside the host.
8. They multiply within the host by taking over the metabolic machinery of the host cell.
9. They are transmissible from diseased to healthy hosts.
10. They are incapable of growth and division.
11. They are effective in very small doses.
12. They are resistant to antimicrobial antibiotics and extreme physical conditions.
13. They can be crystallized and even in crystalline form, they retain their infectivity.
14. They can be inactivated by chemotherapy and thermotherapy.
15. They may undergo mutation.
Essay # 2. Size of Viruses:
Viruses are very small ultra-filterable and ultramicroscopic structures. The average size of viruses ranges between the smaller bacterium and the largest protein molecule. The foot and mouth disease virus is the smallest animal virus approximately 10 nm in size (Fig. 1). The largest animal virus is parrot fever virus, approximately 400 nm in size. The bacterial and animal viruses are larger than the plant viruses. The plant viruses vary in size from 17 nm to 300 nm.
Essay # 3. Shapes of Viruses:
Viruses may be classified in relation to their shape into several groups:
(i) Spherical Form:
Viruses are spherical in shape e.g., cucumber mosaic virus (CMV), viruses of influenza, measles and fever etc. (Fig. 2A).
(ii) Rod Shaped Form:
These are rod shaped forms e.g., Tobacco mosaic virus (Fig. 2B).
(iii) Cuboidal Form:
These are cube like e.g., cow pox (vaccinia) virus, small pox virus (Fig. 2C).
(iv) Spermatozoid or Complex Form:
Characteristic for viruses of the lower plants for example, Bacteriophages (Fig. 2D).
Rabies or Rhodoxin are bullet shaped (Fig. 2E) and pox viruses are brick shaped (Fig. 2F).
Essay # 4. Chemistry and Microstructure of Virus:
Chemistry of TMV was studied for the first time by Bawden and Pirie in 1937. According to them virus particles are micro-proteins of high molecular weight.
Each virus particle (technically called virion) consists of two parts:
(1) Nucleic acid or Nucleoid and
(2) Protein coat or capsid; and enzyme systems with the help of which the virus penetrates into tissue cells.
It is present within the protein coat (Fig. 3). Each virus particle has only one type of nucleic acid either RNA or DNA. The viruses containing DNA are called Deoxyviruses and those having RNA are known as Riboviruses.
All the plant viruses had been reported to contain single stranded RNA while animal viruses either single or double stranded DNA. Bacterial viruses contain mostly double stranded DNA. Most of the insect viruses contain RNA and only a few have DNA.
The DNA of several animal and bacterial viruses is circular whereas in others it is like RNA. Shephered et al (1968) reported that cauliflower mosaic plant virus contains double stranded DNA. Double stranded RNA is also reported in Rice dwarf viruses and wound tumor viruses (Miura et al 1968).
The type of nucleic acid and the number of strands in different viruses are given ahead (Table 1):
It is the protein coat surrounding the internal nucleoid. It is made up of repeating protein subunits called capsomeres. The capsomeres are composed of either one or several types of protein. Host specification of viruses is due to protein of capsid.
The number of capsomeres in the capsid of a given virus is constant (32 in the poliomyelitis virus, 252 in the Adenovirus, 2130 in the Tobacco Mosaic Virus etc.) and they are arranged in a very symmetrical manner and give a specific shape to a particular virus.
Usually they are arranged in two geometric forms:
The nucleic acid is coiled like a spring and capsomeres are helically arranged around their strings, e.g., Tobacco mosaic virus.
The nucleic acid is packed in unknown manner within a hollow polyhedral head. The capsid is antigenic and protects nucleic acid from un-favourable extra-cellular environment. It also makes easy the entry of nucleic acid into the host cells. The core within capsid is called the nucleocapsid core.
Some animal viruses e.g., Herpes virus and pox virus develop a 10-15 nm thick lipoprotein envelope around their protein coat while it is absent in plant viruses. The lipid is derived from the host while the protein is of viral origin. This envelope is also known as mantle or limiting membrane and is covered with projections or spikes. It is made up of several subunits called peplomers.
Enzymes and Other Contents:
In addition to the nucleoprotein, viruses have enzymes, water and carbohydrates. The enzymes are collectively called “transcriptase’s”. These enzymes are essential for infection and growth of viruses. However, the function of carbohydrates is not known.
Essay # 5. Symmetry among Viruses:
Symmetry (the property of bodies to repeat their parts) is an important feature in the structure of virion. The electron microscopic studies have revealed that the arrangement of the capsomeres determines the shape of the virus particles.
There are three different types of symmetry among viruses:
(i) Viruses with Cubic or Icosahedral Symmetry:
This type of symmetry is found in bacteriophage ф (phi) x 174, Turnip yellow mosaic virus, Adenovirus, Tipula iridescent virus etc. In it the viruses are spherical and capsomeres are closely arranged in polyhedral manner.
An icosahedral capsid comprises of two types of capsomeres:
(a) Pentameres. Containing 5 structural units or monomers.
(b) Hexameres. Containing 6 structural units or monomers.
Each monomer is connected with the neighbouring monomers on either side with the bonds. In the same manner the capsomeres are also connected with weaker bonds. The minimum number of capsomeres in an icosahedral virus is 12 or its multiple e.g., Bacteriophage ф (Phi) x 174= 12 capsomeres, Turnip yellow mosaic virus (TYMV) = 32 capsomeres, Polyoma virus and papilloma virus = 72 capsomeres, Reovirus = 92 capsomeres, Herpes virus = 162 capsomeres. Adenovirus = 252 capsomeres, Tipula iridescent virus = 812 capsomeres.
(ii) Viruses with Helical Symmetry:
The common example of this type of symmetry is Tobacco Mosaic virus (TMV), Bacteriophage M 13, Influenza virus. In it the viruses are cylindrical or rod like and the capsomeres are arranged in spiral manner around the long axis.
(iii) Viruses with Complex Symmetry:
The common example of this type of symmetry is Bacteriophages, vaccinia virus, and small pox virus.
In complex symmetry the capsids are of two shapes:
(a) Without identifiable capsids e.g., pox viruses etc.
(b) Tad pole shaped—Combined mixture of both helical and cubical symmetry e.g., Bacteriophage.
Essay # 6. Classification of Viruses:
Earlier, the viruses, on the basis of their hosts were classified into following groups:
(a) Plant viruses. Viruses infecting plants.
(b)Invertebrate viruses. Viruses infecting invertebrates.
(c) Vertebrate viruses. Viruses infecting vertebrates.
(d) Dual host viruses. Viruses parasitic on two different hosts.
(e) Bacterial viruses. Bacteriophage.
Later, in 1923 in Bergey’s Manual of Determinative Bacteriology all the viruses are placed in order Virales of class Schizomycetes, phylum Protophyta.
Holmes (1948) divided order Virales into following 3 sub-orders:
Sub-order 1. Phytophagineae:
Viruses causing plant diseases e.g., Tobacco Mosaic Virus (TMV), Best Yellow Virus (BYV).
Sub-order 2. Phagineae:
Viruses infecting bacteria, e.g., T2, T4, T6 bacteriophages.
Sub-order 3. Zoophagineae:
Viruses causing diseases to men and animals. With the advancement of knowledge of the physiochemical characteristics, a new classification had been proposed by A. Lwoff, R. W. Horne and P. Tournier in 1966. This classification has been accepted by the Provisional Committee of Nomenclature of Viruses (PCNV), especially constituted for this purpose.
This system is based on classical monothetic hierarchical system applied by Linneaus to plants and animals. This is a logical system known as LHT system, in which divisions are made as to relative importance of different properties which are then used to place a taxon in a particular phylum, order, family, genus etc.
This classification is based on the following major criteria of the viruses:
(1) Kind of nucleic acid in the particle-DNA or RNA.
(2) The architecture and symmetry of the capsid-helical-cubical or both.
(3) Presence or absence of the envelope around the capsid.
(4) Diameter of the helical nucleocapsid.
(5) Number of the capsomeres in cubical forms.
In this classification viruses are placed under phylum VIRA as follows:
Divided into the subphylums—
Subphylum 1. Deoxyvira (containing DNA)
Subphylum 2. Ribiovira (containing RNA)
Subphylum Deoxyvira dhided into 3 classes:
Subphylum RIBOVIRA is divided into two 2 classes:
An alternative system of classification was proposed by Adanson (1763). He considered that taxa were best derived by considering all available characters and equal weight to each. The method is laborious and has not been much used recently. The availability of computer has renewed interest in this kind of classification.
Nomenclature or Cryptogram of Viruses:
Latin binomial system of nomenclature of viruses is inappropriate at present. The main reason for the is that such names attempt to provide both a label and some information about the virus being labeled. In the present stage of knowledge or lack of it for most viruses, such name would be found to require frequent changes in future.
In an attempt to overcome this problem, Gibbs et. al. (1966), Gibbs and Harrison (1968), proposed a new scheme of classification popularly known as cryptogram of viruses. It is also known as Adansonian classification. It is a two part system, in which the first part of virus name is an unchanging label (the current vernacular name) and the other is a codified information store, which can be readily adopted and changed as facts and information accumulate about the virus.
Each cryptogram consists of four pairs of symbols as follows:
I. Pair: Type of nucleic acid/strandedness of nucleic acid.
(Symbol: R=RNA and D=DNA, 1 represents single stranded, 2 for double stranded).
II. Pair: Molecular weight of nucleic acid (in millions)/percentage of nucleic acid present in infect particle.
(Symbol: l-one million and one percent. 2-two million and two percent and so on)
III. Pair: Shape and the outline of virus particle/shape of capsid.
(Symbol: S-spherical, E-elongated with parallel sides, ends not rounded; U-elongated with parallel sides, ends rounded; X-complex or none of above).
IV. Pair: Kinds of hosts infected/kinds of vectors used.
Symbol for kinds of hosts: A-Actinomycete, B-Bacterium, F-fungus, I-Invertebrate, P-Pteridophyte, S-seed plant, V-Vertebrate.
Symbols for Kinds of Vector:
Ac-mites and ticks, Al-white fly, Ap-Aphid, Au-Leafy plant or tree hopper, Cl-Beetle, Di-fly) and mosquito; Fu-Fungus, Ne-Nematode, Si-Flea, Th-Thrips, Ve-Vector known but none of above, O-Spreads without a vector via a contaminated environment.
Here in all cases, Asterisk (*) indicates property of virus is unknown and ‘parenthesis’ ( ) indicates enclosed information is doubtful.
The cryptogram of some important viruses can be represented as:
1. Tobacco mosaic virus: R/1: 2/5: E/E: S/O.
2. Human influenza virus: R/1: (2 – 3)/10: S/E: V/O.
3. Coli phage ф x 174: D/l: 17/25: S/S :B/O.
The cryptogram of Tobacco Mosaic Virus may be explained as:
I. Pair. Single stranded RNA i.e., R/l.
II. Pair. 2 million molecular weight with percentage of nucleic acid in infective particles = 5 i.e., 2/5.
III. Pair. Nucleocapsid is elongated with parallel sides, ends not rounded i.e., E/E.
IV. Pair. Host is seed plant and spreads without a vector via contaminated environment i.e., S/0.
Essay # 7. Biological Status or Nature of Viruses:
With the discovery of viruses, the question regarding the nature of the virus has been a very interesting and controversial topic. Some virologists are of the view that they are animate (living) objects, while in the opinion of other virologists they are inanimate (non-living) objects.
The following points support the different views:
Viruses are Living Particles:
(1) They can grow and reproduce only inside specific hosts.
(2) They show mutation.
(3) They can infect healthy plants.
(4) They contain genetic material either DNA or RNA.
(5) They show sensitivity to heat stimuli and radiation.
(6) They have antigenic properties and stimulate the production of antibodies in the body of vertebrates.
(7) They show physiological specialization.
Viruses are Non-Living Particles:
1. They can be crystallized into fine crystals like a sugar molecule.
2. They do not respire or show any sign of metabolism.
3. They are characterized by high specific gravity.
4. Out-side the host they behave like complex organic molecules.
5. Membrane or cell wall is absent.
6. They can be sedimented like proteins.
Viruses differ from other living organisms:
According to Lwoff and Tournier (1966) the following characteristics separate viruses from other living agents:
(1) One type of nucleic acid either DNA or RNA is present in virion (other agents possess both types nucleic acid).
(2) Viruses are reproduced from their sole nucleic acid either from DNA or RNA whereas other agents reproduce from the integrated sum of their constituents and by division.
(3) Viruses make use of the ribosomes of their host cells.
(4) The genetic information for the synthesis of Lipman system (the system responsible for the production of energy with high potential) is absent in viruses.
(5) Viruses are incapable of growing or undergoing binary fission.
So, we find that supporters of both the views are correct at their own places. So, the question is where to place the viruses? Can we call virus a cell? The cell may be defined as “the smallest unit of life capable of independent existence and should be able to reproduce itself”. Second important difference is that the enzymes, ribosomes and cytoplasmic environment is not possessed by viruses. Because of the absence of ability to grow independently, the viruses cannot be regarded as ‘organism’. They are a-cellular or non-cellular.
Viruses can be crystallized like chemicals. Since they are able to reproduce inside the host cells they can be said as living. They are neither living nor non-living. A. Lwoff (1966), a French scientist, once said, ‘A virus is a virus’. It is neither a living organism nor a non-living chemical but in the transition stage and represents the threshold of life.
Essay # 8. Economic Importance of Viruses:
(i) Viruses are used in preparation of sera and vaccines to be used against diseases like rabies, polio etc.
(ii) Multiplication of viruses in bacterial cell is also utilized in the production of antibiotics.
(iii) Due to simplicity of structure and rapid multiplication, they are widely used in research, in the fields of molecular biology, medicine and genetic engineering.
(iv) Viruses are used by humans in eradicating the harmful pests insects and to control the population of organisms. Thus, they are used as a form of biological control.
(v) Viruses cause dreadful diseases in crop plants, domesticated animals and man.
(vi) Bacteriophages attack the nitrogen fixing bacteria of soil and the responsible for reducing fertility of the soil.
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