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SITEMAP   School Biology revision notes: Cell structure: 7. Virus structure

Cell structure 7. The structure and function of a typical virus - reproduction and effects on cells

Doc Brown's GCSE level Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of biology notes on cell structure

(7a) Notes on the structure of viruses (which are NOT classified as living organisms)

Viruses are not living organisms.

They are particles smaller than bacteria; viruses are parasitic and can only reproduce inside living cells; they can infect every type of living organism.

Viruses have a wide variety of shapes and sizes and have no cellular structure but do have a protein coat and only contain one type of nucleic acid, either DNA or RNA.

Examples include the tobacco mosaic virus that causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts, the influenza virus that causes ‘flu’, the common cold virus and the HIV virus that causes AIDS

structure of a virus diagram gcse biology nucleic acid RNS DNA protein coat head tail fibres

The basic structure of a virus

Viruses are not considered to be a living organism such as a plant, animal, bacteria or archaea.

Viruses are not considered to alive because they do not fulfil the seven life processes, namely: movement, respiration, sensitivity, nutrition, excretion, reproduction and growth.

Biological science uses the phrase 'strains' of virus and not species.

Viruses are the smallest agents of infectious disease and are exceedingly small (about 20 - 500 nm diameter) and essentially round in shape.

Viruses are consist of a relatively short length of genetic material (DNA or RNA) which is enclosed in a thin protein coat, which is sometimes surrounded by an extra thin fatty coating or envelope.

A typical size of a virus is about 1/50th of a red blood cell, but they can vary in size from 20 to 500 nm).

Within the protein shell the DNA/RNA nucleic acid can be single- or double stranded.

The entire infectious virus particles are unable to grow or reproduce without a host.

They have non of the usual sub-cellular structures seen in most plant or animal cells.

The above diagram of the structure of typical virus includes the DNA or RNA, enclosed in a protein coat/sheath, which itself is surrounded by an envelope.

The virus has a head, tail fibres and spikes that can attach to protein receptor cells of the host, in which it induces the host cell molecular replication apparatus to reproduce the virus!

Viruses are different from all other infectious microorganisms because they are the only group of microorganisms that cannot replicate outside of a host cell.

Viruses do not consume food, but they obtain materials and energy from host cells by hijacking their host cell's cellular machinery.

Specific types of viruses only infect specific cells and persuades them to reproduce the invading virus.

Some scientists argue that they are more like complex molecules than living creatures.

Viruses are known to infect nearly every type of organism on Earth and some viruses, called bacteriophages, even infect bacteria - nothing is safe from some virus or other!

A virus is sometimes considered a non-cellular form of life, but it really isn't a living organism.

 Check on the characteristics of 'living organisms' in Part 1.

For more on virus infection mechanism see

 communicable diseases - pathogen infections

(7b) Viruses - reproduction and killing cells

structure of a virus diagram gcse biology nucleic acid RNS DNA protein coat head tail fibres

The basic structure of a virus

Viruses are NOT cells, but usually consist of a strand of genetic material enclosed in a protein coat.

The genetic material, nucleic acid can be either DNA or RNA.

Sometimes the virus has an outer coat or envelope.

Viruses come in all shapes and sizes e.g. spherical (on the left) or various other geometrical shapes with a tail and trailing fibres (on the right).

All viruses have spikes of protein that can attach to receptors of host cells - prior to invasion!


How do viruses reproduce?

They cannot reproduce on their and have to 'invade' i.e. infect a living cell that acts as a host.

Specific types of viruses only infect specific cells and persuades them to reproduce the invading virus.

The sequence of events is:

The virus particle attaches to a host cell when its protein spikes recognise the receptors.

The virus releases its genetic instructions into the host cell.

The injected genetic material takes over the host cell's enzymes.

These same enzymes make parts for more new virus particles.

The new particles assemble forming new viruses.

Specific types of virus will only infect specific cells, known as host cells.


The life cycle pathways of a virus

The life cycle of a virus begins when it gets through a cell membrane to infect a host cell.

Most viruses reproduce by the lytic pathway and but others involve the lysogenic pathway first.

Diagram of life cycle of virus gcse biology lytic pathway lysogenic pathway cell bursts killed RNS/DNA injected virus replicates

The lytic cycle pathway

In the lytic pathway the virus attaches itself to a specific host cell and injects its genetic material (viral DNA) into the cytoplasm through the host cell's membrane.

The host cell now contains the viral DNA.

The virus then uses the proteins and enzymes of the host cell to replicate its genetic material (DNA instructions) and so produce the material for the new viruses.

The virus components then assemble to form lots of new viruses.

Eventually so many viruses are produced that the cell bursts open, killing it - thus releasing lots more viruses to invade more host cells to create more viruses!

Therefore back to square and the cycles begin all over again, not good for the host cells!

The lysogenic cycle pathway

As with the lytic cycle, the virus attaches itself to a specific host cell and injects its genetic material (viral DNA) into the cytoplasm through the host cell's membrane.

and The host cell now contains the viral DNA, but unlike in the lytic cycle, the viral DNA becomes incorporated into the host cell's genome..

, and Therefore the viral genetic material gets replicated along with the host DNA every time the host cell divides, but for the time being, the virus stays dormant - doesn't do anything - so no new viruses are formed.

BUT, unfortunately, all the new cells produced in the lysogenic pathway are now carrying the viral genetic information and therefore, at a later time, when triggered, produce lots of viruses.

Eventually, some kind of trigger e.g. from a chemical stimulation, causes the viral genetic material to leave the genome and become separated in the host cell.

The virus then reproduces using the lytic pathway to described above.

Note: The lytic cycle can happen in a few minutes. but the viral DNA can stay in the lysogenic cycle for many years.

See also communicable diseases

Summary of learning objectives and key words or phrases

Know how a virus kills cells by multiplying in host cell. Be able to describe details of reproduction lytic cycle of viruses and the lysogenic cycle of virus reproduction. In the virus life cycles, the virus takes over the host DNA. From a diagram, understand the basic structure and function of a typical virus including features like strands of DNA and RNA, protein coat sheath, the head and tail fibres, the outer envelope of spikes of protein receptor cells.



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