What are viruses and how they infect cells?

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These tiny packets of genetic code are the most successful parasites in the world

Viruses are the tiniest biological replicators on the planet, roughly 100-times smaller than bacteria. Made from a small strand of genetic code and covered with a tiny protein shell, they can’t ‘live’ on their own. In fact, scientists aren’t sure whether they’re even alive at all.

The cells of living organisms have their own molecular production lines. They make temporary copies of their genes and pump them through molecular machines called ribosomes. These read the genetic code and use it as a template to assemble proteins. The simplest living organisms need between 150 and 300 genes to make all the proteins they need to survive, but viruses get by on as few as four. They simply hijack other cells and turn them into virus factories.

Viruses are clever; they make up for their genetic shortfall by borrowing from the cells they infect. Viruses don’t have their own ribosomes, so they feed their code into the machines of other organisms, taking over the production line. The infected cell stops making its own proteins and starts reading virus code and assembling virus proteins.

The core of a virus is its genetic code, which is stored in the same strings of biological letters used by living organisms. Some viruses have two strands of DNA like us, others get by with just one strand, and some carry their genes as RNA. This molecule is like DNA but with a different chemical letter, and it’s used by living cells to make temporary copies of genes. Some viruses also carry the code to make an enzyme called reverse transcriptase, which allows them to convert RNA into DNA inside a living cell.

Genetic information is fragile, so to move from one cell to the next viruses need a way to protect their code. Some of their most important genes provide the instructions to build proteins that make a protective coat called a capsid. The capsid proteins form repeating structures that lock together to make a 3D shape. This crystal-like patterning means that viruses only need a few genes to make a complete shield. Icosahedral capsids, for example, often contain small triangles made from just three proteins. These triangles slot together to make a 20-sided ball that covers the viral genome.

The infectious packages of capsid and genetic code can survive outside of cells, but they can’t replicate on their own. Known as virions, these virus particles need to get back into cells to continue their lifecycle. They do this by attaching to molecules on the cell surface.

Proteins on the outside of the capsid interact with proteins on the outside of the cell. This attachment may change the shape of the virion itself, allowing the particle to fuse with the cell membrane. Alternatively, it might trick the cell into pulling the virus into a membrane-covered sphere known as an endosome. Once inside, enzymes carried by the virion – or from the cell itself – break down what’s left of the capsid, releasing the genetic code into the cell. The viral genome then enters the cell’s production line and quickly begins manufacturing three main types of protein.

The first are enzymes that enable the virus to construct more copies of its own genes. The second are proteins that interfere with the cell’s normal manufacturing processes. The third type are the structural proteins that work to build new virus particles.

When the new virus particles are complete, the virus needs a way to release them to infect more cells. ‘Lytic’ viruses simply burst out, releasing all their virions in one huge pop and killing the cell in the process. ‘Lysogenic’ viruses release new virions one by one, allowing the host cell to survive and reproduce. Some viruses even stitch their genetic code into the code of their host, so that every time the cell divides the new cells also get a copy of the viral genes. This allows viruses to remain inside cells for a long time, staying dormant and then reactivating later, a property known as latency.

Cells do attempt to defend themselves from this type of attack. They destroy loose genetic code and send signals to the immune system to let it know about the infection. But, viruses have evolved ways to evade these defences. In the process, some have gained characteristics that harm their hosts, a property known as virulence.

Many viruses cause disease, diverting healthy cells away from their normal activities. The type of damage a virus does depends on the cells it infects, the way it interferes with molecular machines and the way it releases new virions. Some of the most serious problems arise when viruses infect immune cells, preventing the body from fighting back. Ebola, Marburg and HIV all harm the immune system.

However, viruses aren’t all bad; infections help to shape the way our bodies work. Studies of the human genome have revealed that around eight per cent of our genetic code actually came from viruses. Known as ‘human endogenous retroviruses’, or HERVs, they are easy to spot because they still carry the remnants of three viral genes: gag, pol and env. These genes belong to retroviruses, which stitch their genetic code into the genome of their host.

Retroviruses leave a permanent mark on DNA, and the results of ancient infections have been passed from parent to child for thousands of years. Evolution has gradually changed the sequence of these leftover viral genes, making them unable to produce new virions. Our bodies have found new uses for the code left behind.

One HERV, HERV-W, codes for proteins that would once have sat in the outer envelope of a virus, helping it to fuse with cells. We have adapted the code to make new proteins that help to fuse cell membranes together to form the placenta. Without the leftovers of ancient viral infections we wouldn’t be here today.