User:AshLin/Spoken script Introduction to Viruses
This spoken script has been prepared for making a spoken article from the wiki article Introduction to viruses. The specific version of the article being used for recording is this one (permalink 949226889).
Script starts below this line.
Introduction to viruses, from Wikipedia, the free encyclopedia, at E N dot wikipedia dot org.
This is a spoken article recorded by Ashwin Baindur on 15th of April 2020 at Pune, India. The duration is approximately 24 minutes. The content is current as on the 15th of April 2020.
The table of contents of this spoken article as as follows:
Section 1 - Introduction
Section 2 - Discovery
Section 3 - Origins
Section 4 - Structure
Subsection - 4.1 Size
Subsection - 4.2 Genes
Subsection - 4.3 Protein synthesis
Section 5 - Life-cycle
Section 6 - Effects on the host cell
Section 7 - Viruses and diseases
Subsection 7.1 - In humans
Subsection 7.2 - In plants
Subsection 7.3 - Bacteriophages
Subsection 7.4 - Host resistance
Subsection 7.5 - Prevention and treatment of viral disease
Section 8 - Role in ecology
You are listening to the first part of the sound recording which contains the introduction, contents and sections 1 to 4. The remaining sections wil be covered in part two of the spoken article.
Section 1 - Introduction
[edit]An image accompanied this section of the article, with the caption "Illustration of a SARS-CoV-2 virion".
A virus is a tiny infectious agent that reproduces inside the cells of living hosts. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most living things, viruses do not have cells that divide. New viruses assemble in the infected host cell. But unlike simpler infectious agents like prions, viruses contain genes, which allow them to mutate and evolve. Over 4,800 species of viruses have been described in detail out of the millions in the environment. Their origin is unclear. Some viruses may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
Viruses are made of either two or three parts. All viruses contain genes. These genes contain the encoded biological information of the virus and are built from either DNA or RNA. All viruses are also covered with a protein coat to protect the genes. Some viruses may also have an envelope of fat-like substance that covers the protein coat, and this envelope makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific receptors—to enter a new host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses range in size from 20 to 300 nanometres. It would take 33,000 to 500,000 of them, side by side, to stretch to 1 centimetre (0.4 in).
Viruses spread in many ways. Although many are very specific about which host species or tissue they attack, each species of virus relies on a particular method to copy itself. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of humans and other animals are spread by exposure to infected bodily fluids. Viruses such as influenza are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as norovirus are transmitted by the faecal–oral route, which involves the contamination of hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. Others, such as the dengue virus, are spread by blood-sucking insects.
Viruses, especially those made of RNA, can mutate rapidly to give rise to new types. Hosts may have little protection against such new forms. Influenza virus, for example, changes often, so a new vaccine is needed each year. Major changes can cause pandemics, as in the 2009 swine influenza that spread to most countries. Often, these mutations take place when the virus has first infected other animal hosts. Some examples of such "zoonotic" diseases include coronavirus in bats, and influenza in pigs and birds, before those viruses were transferred to humans.
Viral infections can cause disease in humans, animals and plants. In healthy humans and animals, infections are usually eliminated by the immune system, which can provide lifetime immunity to the host for that virus. Antibiotics, which work against bacteria, have no impact, but antiviral drugs can treat life-threatening infections. Those vaccines that produce lifelong immunity can prevent some infections.
Section 2 - Discovery
[edit]The material for this section has been taken from the wikipedia article 'History of virology'.
An image accompanied this section of the article, with the caption "Scanning electron micrograph of HIV-1 viruses, coloured green, budding from a lymphocyte".
In 1884, French microbiologist Charles Chamberland invented the Chamberland filter (also referred to as the Chamberland–Pasteur filter), that contains pores smaller than bacteria. He could then pass a solution containing bacteria through the filter, and completely remove them. In the early 1890s, Russian biologist Dmitri Ivanovsky used this method to study what became known as the tobacco mosaic virus. His experiments showed that extracts from the crushed leaves of infected tobacco plants remain infectious after filtration.
At the same time, several other scientists showed that, although these agents (later called viruses) were different from bacteria and about one hundred times smaller, they could still cause disease. In 1899, Dutch microbiologist Martinus Beijerinck observed that the agent only multiplied when inside dividing cells. He called it a "contagious living fluid", or a "soluble living germ", because he could not find any germ-like particles. In the early 20th century, English bacteriologist Frederick Twort discovered viruses that infect bacteria. French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria growing on agar, would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension.
The invention of the electron microscope in 1931 brought the first images of viruses. In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it to be mainly made from protein. A short time later, this virus was shown to be made from protein and RNA. A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when American pathologists Ernest William Goodpasture and Alice Miles Woodruff grew influenza, and several other viruses, in fertilised chickens' eggs. Some viruses could not be grown in chickens' eggs. This problem was solved in 1949, when John Franklin Enders, Thomas Huckle Weller, and Frederick Chapman Robbins grew polio virus in cultures of living animal cells. Over 4,800 species of viruses have been described in detail.
Section 3 - Origins
[edit]Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. Their origin remains unclear because they do not fossilize, so molecular techniques have been the best way to hypothesise about how they arose. These techniques rely on the availability of ancient viral DNA or RNA, but most viruses that have been preserved and stored in laboratories are less than 90 years old. Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century. New groups of viruses might have repeatedly emerged at all stages of the evolution of life. There are three major theories about the origins of viruses.
Regressive theory
[edit]Viruses may have once been small cells that parasitised larger cells. Eventually, the genes they no longer needed for a parasitic way of life were lost. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can reproduce only inside host cells. This lends credence to this theory, as their dependence on being parasites may have led to the loss of the genes that once allowed them to live on their own.
Cellular origin theory
[edit]Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
Coevolution theory
[edit]Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth, and would have depended on cellular life for many millions of years.
There are problems with all of these theories. The regressive hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape or the cellular origin hypothesis does not explain the presence of unique structures in viruses that do not appear in cells. The coevolution, or "virus-first" hypothesis, conflicts with the definition of viruses, because viruses depend on host cells. Also, viruses are recognised as ancient, and to have origins that pre-date the divergence of life into the three domains.[10] This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses.
Section 4 - Structure
[edit]An image accompanied this section of the article, with the caption "Simplified diagram of the structure of a virus".
A virus particle, also called a virion, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a capsid. The capsid is made of many smaller, identical protein molecules called capsomers. The arrangement of the capsomers can either be icosahedral (20-sided), helical, or more complex. There is an inner shell around the DNA or RNA called the nucleocapsid, made out of proteins. Some viruses are surrounded by a bubble of lipid (fat) called an envelope, which makes them vulnerable to soap and alcohol.
Subsection - 4.1 Size
[edit]Viruses are among the smallest infectious agents, and are too small to be seen by light microscopy, most of them can only be seen by electron microscopy. Their sizes range from 20 to 300 nanometres; it would take 30,000 to 500,000 of them, side by side, to stretch to one centimetre (0.4 in). In comparison, bacteria are typically around 1000 nanometres (1 micrometer) in diameter, and host cells of higher organisms are typically a few tens of micrometers. Some viruses such as megaviruses and pandoraviruses are relatively large viruses. At around 1000 nanometres, these viruses, which infect amoebae, were discovered in 2003 and 2013. They are around ten times wider (and thus a thousand times larger in volume) than influenza viruses, and the discovery of these "giant" viruses astonished scientists.
Subsection - 4.2 Genes
[edit]The genes of viruses are made from DNA (deoxyribonucleic acid) and, in many viruses, RNA (ribonucleic acid). The biological information contained in an organism is encoded in its DNA or RNA. Most organisms use DNA, but many viruses have RNA as their genetic material. The DNA or RNA of viruses consists of either a single strand or a double helix.
Viruses can reproduce rapidly because they have relatively few genes. For example, influenza virus has only eight genes and rotavirus has eleven. In comparison, humans have 20,000–25,000. Some viral genes contain the code to make the structural proteins that form the virus particle. Other genes make non-structural proteins found only in the cells the virus infects.
All cells, and many viruses, produce proteins that are enzymes that drive chemical reactions. Some of these enzymes, called DNA polymerase and RNA polymerase, make new copies of DNA and RNA. A virus's polymerase enzymes are often much more efficient at making DNA and RNA than the equivalent enzymes of the host cells, but viral RNA polymerase enzymes are error-prone, causing RNA viruses to mutate and form new strains.
In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called reassortment.
For further information on the topic of genes, please refer to the wikipedia article: Introduction to genetics.
Subsection - 4.3 Protein synthesis
[edit]An image accompanied this section of the article, with the caption "Diagram of a typical eukaryotic cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles within centrosome (14) a virus shown to approximate scale".
Proteins are essential to life. Cells produce new protein molecules from amino acid building blocks based on information coded in DNA. Each type of protein is a specialist that usually only performs one function, so if a cell needs to do something new, it must make a new protein. Viruses force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce. Protein synthesis consists of two major steps: transcription and translation.
Transcription is the process where information in DNA, called the genetic code, is used to produce RNA copies called messenger RNA (mRNA). These migrate through the cell and carry the code to ribosomes where it is used to make proteins. This is called translation because the protein's amino acid structure is determined by the mRNA's code. Information is hence translated from the language of nucleic acids to the language of amino acids.
Some nucleic acids of RNA viruses function directly as mRNA without further modification. For this reason, these viruses are called positive-sense RNA viruses. In other RNA viruses, the RNA is a complementary copy of mRNA and these viruses rely on the cell's or their own enzyme to make mRNA. These are called negative-sense RNA viruses. In viruses made from DNA, the method of mRNA production is similar to that of the cell. The species of viruses called retroviruses behave completely differently: they have RNA, but inside the host cell a DNA copy of their RNA is made with the help of the enzyme reverse transcriptase. This DNA is then incorporated into the host's own DNA, and copied into mRNA by the cell's normal pathways.
We now come to the end of the first part, that is part one, of the spoken article 'Introduction to viruses'.
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