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19.3 Phylogeny -- Part 5
Viruses were seen for the first time in the twentieth century.
By the 1950s, the study of viruses and prions has contributed to our understanding of disease, genetics and the characteristics of living organisms.
Viruses are not living organisms and are difficult to classify.
The classification system used for living organisms has been developed by the International Committee on Taxonomy of Viruses.
The higher levels of kingdom, class, and order are not included in viral classification.
When a new type of virus emerges within a single species of virus, additional classification levels are required for clear identification.
The type of two glycoprotein spikes in the envelope, hemagglutinin (H) and neuraminidase (N), are what determines the type of influenza A.
The size of a large macromolecule is 10 to 400 nm.
Viruses can be studied through electron microscopy.
Many viruses can be stored in the same way as chemicals are.
When viral particles are given the chance to invade a cell, they become infectious.
The outer capsid of all viruses is composed of only one type of DNA orRNA, but not both.
Some viruses have a membranous envelope.
Viruses can be threadlike or polyhedral.
All viruses have the same basic structure, consisting of an outer capsid and an inner core of nucleic acid.
The virus is said to be naked if the viral capsid is the outermost structure.
The viruses are surrounded by membranous glycoprotein spikes.
Each virus has a different set of molecules that allow it to bind to a new cell.
A viral particle's genome is not the only part of it's structure that is important to produce viral DNA and/orRNA.
Viruses have the same genetic material as prokaryotic and eukaryotic cells.
A virus cannot duplicate its genetic material or any of its other components on its own because a cell can.
A living cell is the only way a virus can be reproduced.
Once inside a living cell, the virus hijacks the cell's synthesis machinery to replicate the nucleusc acid and other parts of the virus, including the capsid, viral enzymes and the envelope.
Cells that are killed or damaged by a replicating virus can cause symptoms associated with viral infections.
When adenoviruses are complete, cells in the respiratory tract are lysed, which leads to conditions such as bronchitis and pneumonia.
Only plants in the tobacco family are affected by the tobacco mosaic virus.
Human viruses can be specific to a particular tissue.
The human immunodeficiency virus only enters certain blood cells and the other viruses only enter certain nerve cells.
Host specificity is determined by the structure of the naked capsid.
Attaching these in a lock-and-key manner with a receptor on the host cell's outer surface.
A cell that doesn't have a receptor to match a virus can't be infections because it won't be able to enter.
The lock-and-key attachment of viruses to host cells is interfered with by many antiviral medications.
Viruses hijack the metabolism of a host cell during their reproduction cycle, which consists of five steps.
A virus binding to a specific host cell is based on the host specific match between the virus surface molecule and the host cell receptors.
The virus injects its genome into the host cell.
The host's ribosomes, tRNA, and energy are used to synthesise new viral components.
New viruses are made from viral components.
New viruses exit the host cell through lysis or budding.
There are exceptions to the reproductive cycle of different types of viruses.
"To eat" are viruses that attackbacteria.
Scientists can use the model to study the reproductive cycle of viruses.
The cell is broken open in the lytic cycle.
In the lysogenic cycle, host and viral DNA are integrated.
The lytic cycle can be followed by the lysogenic cycle in the future.
The lytic cycle is named because the host is lysed at the end of the cycle.
Several hundred new viral particles are released after the cell dies.
When a virus enters the lysogenic cycle instead of the lytic cycle, reproduction may take place in the future.
Sometime in the future, certain environmental factors, such as ultraviolet radiation, can induce the prophage to re-enter the lytic stage of biosynthesis, followed by maturation and release.
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