Viruses can infect all types of organisms, from prokaryotic bacteria to Eukarytoic human cells. They are refereed to as obligate intracellular parasites (can’t replicate outside of a host cell). They enter a cell and take over its metabolic pathways, using the cells resources to create many virus babies.

Structure

A single virus (virion) is composed of:

• Genetic material (DNA or RNA)
• Single stranded DNA or RNA
• Double stranded DNA or RNA
• Protein coat

They do not contain:

• Cytoplasm
• Organelles

Scroll over the words in the animation to the left

How do they work?

Depending on the type of virus, they have different methods of entering the cell. However most viruses are specific, meaning they can only affect specific cell types.

The virus uses specialised proteins on its surface, to attaches itself to a specific receptor on the cells surface.

This causes a change in the cells behaviour.

Once attached the virus will will instruct the cell to open up and absorb it.

This can happen through endocytosis, accepting only the its viral Nucleic acid or merging the virus with the cell membrane, releasing viral nucleic acid into the cell via a capsule.

The animation to the left shows a virus being taken in through endocytosis. Once inside the viral protein shell merges with the vesicle membrane and releases viral single stranded DNA

Once the viral DNA has entered the cell it must avoid any defensive enzymes. Some virus have altered their DNA to avoid destruction from host cell enzymes.

The DNA makes its way into the nucleuses of the cell. Once inside, enzymes assist in merging viral DNA with healthy human DNA.

This is shown in the animation to the right.

Once integrated into the DNA of the host cell, the viral DNA orders the host cell to begin making baby viruses.

This done through the basic metabolic pathways of protein synthesis.

The viral particles will begin taking up space in the cell and eventually start budding out of the cell.

As the cell fills up with viruses, and the resources begin to fade, the cell will eventually burst. Releasing viral particles into the body.

The new viruses will find a host and repeat the cycle.

Knowing this, its not hard to understand why a single virus can cause so much damage to the body.

Types of viruses

There are many different types of viruses in the world. An important distinction is their nucleic acid. Some Viruses will contain DNA, while other RNA. Those that contain RNA are called retro viruses.

Retro viruses contain only RNA instead of DNA. This means when they enter a cell, they must transcribe their RNA back into DNA.

In order to do this, they carry a specific enzyme called, reverse transcriptase in their protein coat. Once inside a host cell, the enzyme uses the cells nutrients to convert RNA back into DNA.

HIV and influenza is an examples of Viruses containing RNA.

The image to the right and left are of Bacteriophages.

These viruses infect bacteria, this gives you a hint at how small viruses can actually be.

They can contain either DNA or RNA, but only affect bacteria.

The video to the left shows different methods of entering a cell by 3 different types viruses.

Influenza, Bacteriophage and HIV.