Viruses are the smallest and simplest of the micro organisms, they are acellular - which means they are not made of cells! Since viruses are acellular and cannot reproduce (in fact, viruses don't actually fulfil any of the seven characteristics of life on their own), viruses are classified as non-living.

 

So, viruses are non-living (consider them more like complex chemical packages than simple living organisms), and think about their definition:

"Viruses are obligate parasites which are only able reproduce (replicate) inside host cells"

what does the term obligate mean?

Well the term obligate means that "something is obliged to - i.e. it has to, it just has no option" in this case a virus 'has to' reside inside a living host cell so that it can "highjack" the host cell in order to replicate. Therefore viruses are intracellular parasites.

 

...and what is a parasite?

Parasites are organisms that "live" inside or on other species and cause harm to the host species. 

Well as we know, viruses are not alive, but they do require a host cell to replicate - and when they do they cause damage to the host. Therefore a virus is a parasite.

A level biology Interactive Worksheets - Viruses PDF

A Level Biology: Virology The Basic Structure of a Virus

 

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A-Level Biology Lesson “The Basic Structure of a Virus"

The Basic Structure of  a Virus Revision Notes with Knowledge Check Questions and Answers PDF for A- Level Biology
The Basic Structure of  a Virus Revision Notes Poster A3 PDF for A Level Biology

Basic Virus Structure:

Viruses (Virions) are tiny! On average they range in size from about 10nm to 400nm so it stands to reason that viruses cannot be seen with a light microscope. So, viruses were first observed in the 1930s using more powerful electron microscopy. Viruses are defined as “obligate intracellular parasites” which exist either extracellularly or intracellularly. Extracellular, i.e. outside of a host cell viruses are “inactive” or “dormant” because they do not have the necessary cellular machinery (e.g. DNA or RNA, Enzymes, Ribosomes, Golgi, ER etc.) which would allow them to replicate, or multiply (outside of the host). Viruses are known to infect all cells, from all classifications of life. There are viruses that infect prokaryotes (bacteria), the so called bacteriophages, but mostly viruses infect eukaryotic organisms, such as plants, algae, fungi, protists and of course, viruses that infect animals.

A complete virus particle is known as a virion, and the terms virus and virion are often used synonymously. However, for clarity just be aware that a “virion” is the “dormant” yet, complete form of a virus which is transmitted between host cells. Whilst a “virus” is the “active” infectious agent. Once inside a host cell, the virus dismantles into its separate parts, and can now be reproduced (replicated) by using the host cells “machinery” (DNA or RNA, Ribosomes, Enzymes, etc).

There is always an exception!

There is always an exception! Biology is the study of life, all the fascinating forms of life, interactions, cycles and biochemistry that make life possible… Biology is vast, complicated and multidisciplinary - to say the least. And, there are always exceptions. Viruses for example are so tiny they cannot be seen without an electron microscope… that is unless the virus is the comparatively massive mimi virus! Mimi viruses are from the family of mimiviridae (commonly called the giant viruses) and these large viruses (average size 400nm) infect amoebas. Now 400nm is massive if you’re a virus, so 600nm would be considered gigantic! So when Megavirus chilensis was first discovered in 2010 it became the new big boy in town measuring in at (on average) 600nm and some measuring 750nm! large enough to be observed with a light microscope!

 

In 2013 another new ‘giant’ virus was discovered - the pandoravirus. This giant virus is not only impressive because of its size (500nm) but also its genome. Described as “evolutionary innovators” these giant viruses have muddied the waters of what was once the clear distinction between the viral world and that of the cellular world, due to their genomes being as complex as some simple eukaryotic cells. 

 

To throw another spanner in the works, there are also the Acidionus Two-Tailed Viruses (from the family Bicaudaviridae. These hyperthermophilic archaeal viruses have shown extracellular activity - that is to say they have shown to be “active” outside of their host cells. Now, the Acidionus Two-Tailed Viruses still require the host cell to replicate, but following host cell lysis, two “protein tails” composed of 800 amino acids begin to project out and continue to assemble from each end of the virion until they reach a length at least equal to that of the capsid. But what is more perplexing is that the protein tails are only produced if the virions are exposed to high temperatures (hence the viruses being classified as hyperthermophilic). The natural environment of the Acidionus Two-Tailed Viruses are acidic hot springs with a pH of around 1.5, and temperatures of 85 - 93°C. 

 ALL viruses (virions) contain: -

  • Nucleic acid. The nucleic acid in a virus can be either DNA or RNA (never both) and may be single or double-stranded. Viruses are classified according to the type of nucleic acid they contain,(e.g. HIV is an RNA virus whilst the Epstein–Barr virus is a DNA virus).

 

  • A capsid (protein coat called made of subunits called capsomeres). If the capsid proteins are closely bound to the nucleic acid, then the combination is called a nucleocapsid. Capsids are composed of many repeating subunits and typically have simple geometrical shapes, i.e. helix viruses such as the tobacco mosaic virus or icosahedral viruses (An icosahedron is a regular polyhedron with 20 equilateral triangular faces). Examples of icosahedral viruses are adenovirus, poliovirus and rhinovirus.

Many viruses have simple structures containing nothing more than a capsid and nucleic acid, but many viruses have slightly more complex structures with additional features such as: -

  • A lipid envelope. Viral envelopes derive from the plasma (cell membrane) or nuclear membranes of host cells.

  • Proteins which attach the capsid to the envelope (called matrix proteins).

  • Glycoproteins which allow the virus to attach to host cells.

  • Enzymes, (e.g. Reverse transcriptase) which are required to replicate the viral nucleic acid or incorporate it into a host cell.

A Level Biology - Virology: The Lytic Cycle - The Steps Involved in Viral Replication.

 

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Viral replication - the lytic cycle Revision Notes Poster A3 PDF for A Level Biology

A Level Biology - HIV Virus Structure, Infection of T-cells and graph interpretation

 

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HIV Virus Structure, Infection of T-cells and graph interpretation A3 Poster PDF for A Level Biology

A Level Biology - HIV Virus Replication

 

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HIV Virus Replication cycle A3 Poster PDF for A Level Biology
Check Your Exam Specification
 

AQA Specification Reference: - 3.2.1.2 Structure of prokaryotic cells and of viruses. Viruses are acellular and non-living. The structure of virus particles to include genetic material, capsid and attachment protein. Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles.

 

CIE Specification Reference: - 1 Cell structure: Outline the key features of viruses as non-cellular structures (limited to protein coat and DNA/RNA).

 

★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - N/A

 

Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things. 2.2 Viruses: Understand that the classification of viruses is based on structure and nucleic acid. Know that viruses are not living cells. Know the lytic cycle of a virus and latency.

 

★ OCR (Biology A) Specification Reference: - N/A

★ OCR (Biology B) Specification Reference: -  N/A

 

WJEC Specification Reference: - Core Concepts 2. Cell structure and organisation: Learners should be able to demonstrate and apply their knowledge and understanding of: The structure of viruses. The relationship between the pathogenicity of viruses and their mode of

reproduction.

AQA Specification Reference: - 3.2.4 Cell recognition and the immune system. Structure of the human immunodeficiency virus (HIV) and its replication in helper T cells. 

 

CIE Specification Reference: - 10 Infectious disease: b) state the name and type of causative organism (pathogen) of each of the following diseases: cholera, malaria, tuberculosis (TB), HIV/AIDS, smallpox and measles (detailed knowledge of structure is not required. c) explain how cholera, measles, malaria, TB and HIV/AIDS are transmitted d) discuss the biological, social and economic factors that need to be considered in the prevention and control of cholera, measles, malaria, TB and HIV/AIDS (a detailed study of the life cycle of the malarial parasite is not required). 11 Immunity: Not HIV Specific  - useful for: c) describe and explain the significance of the increase in white blood cell count in humans with infectious diseases.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 6: Immunity, Infection and Forensics: 6.6 Understand how Mycobacterium tuberculosis (TB) and Human Immunodeficiency Virus (HIV) infect human cells, causing a sequence of symptoms that may result in death.

 

  Topic 6: Microbiology and Pathogens:  6.6 Understand how Mycobacterium tuberculosis (TB) and Human Immunodeficiency Virus (HIV) infect human cells, causing a sequence of symptoms that may result in death. Also - Not HIV Specific but useful for: 6.7 Response to infection.

 

OCR (Biology A) Specification Reference: - 4.1.1 Communicable diseases, disease prevention and the immune system: Learners should be able to demonstrate and apply their knowledge and understanding of: (a) the different types of pathogen that can cause communicable diseases in plants and animals. To include, viruses – HIV/AIDS (human). 

 

OCR (Biology B) Specification Reference: - 3.2.1 Pathogenic microorganisms: Learners should be able to demonstrate and apply their knowledge and understanding of: how pathogens (including bacteria, viruses and fungi) cause communicable disease. The causes, means of transmission, symptoms and the principal treatment of tuberculosis (TB) and HIV/AIDS. To include an outline of the general mechanisms of pathogenicity by bacteria (toxin production), viruses (taking over cell metabolism) and fungi (enzyme secretion). the structure of the Human Immunodeficiency Virus (HIV). To include the use of diagrams showing the location of enzymes and the nature of the genetic material.

 

WJEC Specification Reference: - Immunology and Disease: but useful for:

1. Disease: - Learners should be able to demonstrate and apply their knowledge and understanding of: The structure of viruses. Learners should be able to demonstrate and apply their knowledge and understanding of: (d) the relationship between the pathogenicity of viruses and their mode of reproduction.

AQA Specification Reference: - 3.2.4 Cell recognition and the immune system. Structure of the human immunodeficiency virus (HIV) and its replication in helper T cells. 

 

CIE Specification Reference: - 10 Infectious disease: b) state the name and type of causative organism (pathogen) of each of the following diseases: cholera, malaria, tuberculosis (TB), HIV/AIDS, smallpox and measles (detailed knowledge of structure is not required. c) explain how cholera, measles, malaria, TB and HIV/AIDS are transmitted d) discuss the biological, social and economic factors that need to be considered in the prevention and control of cholera, measles, malaria, TB and HIV/AIDS (a detailed study of the life cycle of the malarial parasite is not required). 11 Immunity: Not HIV Specific  - useful for: c) describe and explain the significance of the increase in white blood cell count in humans with infectious diseases.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 6: Immunity, Infection and Forensics: 6.6 Understand how Mycobacterium tuberculosis (TB) and Human Immunodeficiency Virus (HIV) infect human cells, causing a sequence of symptoms that may result in death.

 

Edexcel (Biology B) Specification Reference: - Topic 6: Microbiology and Pathogens: Response to infection.

 

OCR (Biology A) Specification Reference: - 4.1.1 Communicable diseases, disease prevention and the immune system: Learners should be able to demonstrate and apply their knowledge and understanding of: (a) the different types of pathogen that can cause communicable diseases in plants and animals. To include, viruses – HIV/AIDS (human). 

 

OCR (Biology B) Specification Reference: - 3.2.1 Pathogenic microorganisms: Learners should be able to demonstrate and apply their knowledge and understanding of: how pathogens (including bacteria, viruses and fungi) cause communicable disease. The causes, means of transmission, symptoms and the principal treatment of tuberculosis (TB) and HIV/AIDS. To include an outline of the general mechanisms of pathogenicity by bacteria (toxin production), viruses (taking over cell metabolism) and fungi (enzyme secretion). the structure of the Human Immunodeficiency Virus (HIV). To include the use of diagrams showing the location of enzymes and the nature of the genetic material.

 

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