Table of contents
Learning Objective
By the end of this lecture you should be able to fully describe a virus, its structure and how it reproduces. Remember, you are to go nowhere if you are not able to score upto 70% in the quiz.
Some Facts about viruses
- Viruses infect all Living things.
- We eat and breathe billions of virions regularly.
- Amazingly, the vast majority of the viruses that infect us have little or no impact on our health or well being. This means that Not all viruses make you sick.
- While some are mild, many others cause the most deadly infections.
- There are more viruses in a liter of coastal seawater than people on Earth.
- We carry viral genomes as part of our own genetic material.
- When you sneeze, you fire an aerosol that contains enough viruses to infect thousands.
What is a Virus?
Virus is called an obligate intracellular parasite because it depends on host cells. It cannot live independently.
Viruses do NOT think! (or employ, ensure, exhibit, display, etc...). They do not achieve their goals in a human-centered manner.
Viruses are passive agents!
Viruses are very small.
Viruses are referred to as Acellular organisms (organisms without cell).
The Virus and the Virion
Earlier, we said a virus is an obligate intracellular parasite. Within a host cell, it is called a Virus, but outside a living cell, it is called a Virion.
Virions are extracellular (out of cell) Viruses.
All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid).
A virion can enter a living organism through different ways.
Structure of viruses
Virion size range is approximately 10–400 nm in diameter and most viruses must be viewed with an electron microscope.
All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid).
Some viruses consist only of a nucleocapsid, others have additional components.
Some viruses are enveloped (they have outer envelope) while others are non-enveloped.
Capsids
Capsids are large macromolecular structures which serve as protein coat of virus.
Protect viral genetic material and aids in its transfer between host cells.
Made of protein subunits called protomers.
Capsids are helical, icosahedral, or complex.
Helical capsids are shaped like hollow tubes with protein walls.
An icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices.
Some viruses do not fit into the category of having helical or icosahedral capsids E.g: poxviruses – largest animal virus
large bacteriophages – binal symmetry
Head resembles icosahedral, tail is helical.
Virus Envelope
Animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or nuclear membranes.
Virus Genome
A virus may have single or double stranded DNA or RNA and is referred to as virus Genome.
The length of the nucleic acid also varies from virus to virus.
Genomes can be segmented or circular.
How does a virus enter a cell?
Virus can be passed from host to host either through direct contact or through a vector, or carrier. Inside the organism, the virus can enter a cell in various ways.
Some (such as Bacteriophages) attach to the cell wall surface in specific places. Once attached, enzymes make a small hole in the cell wall, and the virus injects its DNA into the cell.
Other viruses (such as HIV) enter the host via endocytosis (the process whereby cells take in material from the external environment). After entering the cell, the virus's genetic material begins the destructive process of taking over the cell and forcing it to produce new viruses.
How does a Virus reproduce?
The mechanism used in Viral reproduction depends on the virus' structure and genome.
However, Steps are similar –
- Attachment to host cell.
- Entry and uncoating of genome.
- Synthesis.
- Assembly.
- Release.
Attachment to host cell (Adsorption)
The virus attaches to receptors on the host cell wall.
Entry and uncoating (penetration)
The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell.
Three methods used:
- Fusion of the viral envelope with host membrane; nucleocapsid enters.
- Endocytosis in vesicle; endosome aids in viral uncoating.
- Injection of nucleic acid.
Synthesis (Replication)
The viral genome (either DNA or RNA) contains all the information necessary to produce new viruses. Once inside the host cell, the virus induces the host cell to synthesize the necessary components for its replication.
Assembly
The newly synthesized viral components are assembled into new viruses.
Some are assembled in nucleus, some in cytoplasm.
Release
Assembled viruses are released from the cell and can now infect other cells, and the process begins again.
When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction.
The host produces three kinds of proteins:
- Early proteins: Enzymes used in nucleic acid replication.
- Late proteins: Proteins used to construct the virus coat.
- Lytic proteins: Enzymes used to break open the cell for viral exit.
How does a Virus leave a cell?
The final viral product is assembled spontaneously, that is, the parts are made separately by the host and are joined together by chance.
This self-assembly is often aided by molecular chaperones (proteins made by the host that help the capsid parts come together).
The new viruses then leave the cell either by exocytosis or by lysis.
Envelope-bound animal viruses instruct the host's endoplasmic reticulum to make certain proteins, called glycoproteins, which then collect in clumps along the cell membrane.
The virus is then discharged from the cell at these exit sites, referred to as EXOCYTOSIS.
On the other hand, bacteriophages must break open, or LYSE the cell to exit.
To do this, the phages have a gene that codes for an enzyme called LYSOZYME. This enzyme breaks down the cell wall, causing the cell to swell and burst. The new viruses are released into the environment, killing the host cell in the process.
Summary
- A virus is an infectious obligate intracellular parasite.
- Virus are acellular organisms (without cell).
- When a virus is outside a living cell, it is called a virion.
- Virion size range is ~10–400 nm in diameter and most viruses must be viewed with an electron microscope.
- All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid).
- Capsids are large macromolecular structures which serve as protein coat of virus.
- A virus may have single or double stranded DNA or RNA and is referred to as virus Genome.
- Virus can be passed from host to host either through direct contact or through a vector, or carrier.
- Mechanism used in Viral reproduction depends on the virus' structure and genome.
- Steps are similar:
- Attachment to host cell.
- Entry and uncoating of genome.
- Synthesis.
- Assembly.
- Release.
- When a virus takes over a cell, it directs the cell to produce three proteins:
- Early proteins: Enzymes used in nucleic acid replication.
- Late proteins: Proteins used to construct the virus coat.
- Lytic proteins: Enzymes used to break open the cell for viral exit.
- The various parts of a virus are made separately.
- Chaperones are proteins made by the host that help the capsid parts come together.
- New viruses leave the cell either by exocytosis or by lysis and repeat the process of Adsorption and taking over.
Recommended Videos
Test Questions
TEST: VIRUS
please fill above required data
TEST: VIRUS
Total Questions: 20
you'll have 30 second to answer each question.
Quiz Result
Total Questions:
Attempt:
Correct:
Wrong:
Percentage:
Quiz Answers
1. A type of virus that exist outside a cell is known as
Virion
2. Capsids serve as _ coat of virus
Protein
3. The outer, flexible, membraneous layer which encloses some viruses is known as
Envelope
4. Viral attachment to a host cell is also called
Adsorption
5. Chaperones are proteins that aid
Self-assembly of viruses
6. Phages code for an enzyme that break down the cell wall of its host causing the cell to swell and burst. This enzyme is...
Lysozyme
7. A virus is an obligate extracellular parasite
False
8. Viruses are
Acellular
9. Viruses are _ agents
Passive
10. Capsids are made of protein subunits called
Protomers
11. A virus' DNA or RNA is referred to as the Virus'
Genome
12. Capsids are
3 dimensional
13. The length of the nucleic acid of all viruses are the same
False
14. A virus replicates itself where?
Inside a host cell
15. Lytic proteins are enzymes used for
Viral exit
16. Glycoproteins are synthesized by the
Endoplasmic reticulum
17. HIV is transferred via
Direct contact
18. The various parts of a new virus are produced together
False
19. Virus reproduce via Binary Fission
False
20. When a virus attacks its host cells and replicates, it stops attacking other cells
False
![GENERAL QUESTIONS ON ANP 101 [MEDIUM]](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh3Jdd42xekrYhc5OIwQ7vWl-_Qy9jM6WrMuuU8dhKeBulL5_YFhTnSfYfV1e6sTp2-TKsqVDbs4IJGIg1MgmOLSLqP5VfTbZ37VWa2cvRFXzezWRXRJkNnLuMpNDBKDBLbVjOG-F7uq8d-Yst4T4tZ8_fEvLG2bLenjuL0NuEbtrQuiJ_nznG3eDu0XQ/w74-h74-p-k-no-nu/20220704_063500.png)
