So, it’s obvious that you have heard the word virus. Many people think that these are living organisms, but the fact is that viruses are non-living things.
Viruses can’t be considered in the category of plants, animals, or even bacteria. We all know that bacteria are the smallest living organisms, right?
But, do you know that viruses are the smallest non-living organisms. These are so small that they can even enter the cell of bacteria and can infect them with ease.
While, we can see bacteria under an ordinary microscope but, it’s impossible to see viruses in such a condition.
Bacteria are giants when compared to viruses. The smallest bacteria are about 0.4 micron (one-millionth of a meter) in diameter while viruses range in size from 0.02 to 0.25 micron. This makes most viruses submicroscopic, unable to be seen in an ordinary light microscope.
These viruses are the quintessential parasites of the living kingdoms. Although they may seem like living organisms because of their impressive reproductive abilities, viruses are not living organisms in the strict sense of the word.
What are ribosomes? What are their functions?
Ribosomes are present in both Prokaryotic and Eukaryotic Cells. It consists of a large subunit and a small subunit.
Each subunit of ribosomes consists of one or more ribosomal RNA (rRNA) molecules and many ribosomal proteins (RPs). The ribosomes and associated molecules are also known as the translational apparatus.
A ribosome is also called a ribonucleoprotein complex because it is made from complexes of both RNAs and proteins.
Each ribosome is composed of a small 30S subunit and a large 50S subunit which are bound to each other:
The 30S subunit has mainly a decoding function and is also bound to the mRNA. And, the 50S subunit has mainly a catalytic function and is also bound to the aminoacylated tRNAs.
In bacteria, the small and large rRNAs contain about 1500 to 3000 nucleotides, respectively, whereas, in humans, they have about 1800 to 5000 nucleotides, respectively.
However, the structure and function of ribosomes is largely similar across all species.
Function of Ribosomes:
- Ribosomes facilitate RNA translation and then the synthesis of proteins respectively.
- RNA catalyses the protein synthesis mechanism of Gene expression. Within the ribosome, the rRNA molecules direct the catalytic steps of protein synthesis.
- Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to form polypeptide chains.
- rRNA of ribosomes acts as an intermediate between mRNA and tRNA to form amino acid chains.
Clarified: Viruses don’t have ribosomes
Viruses do not use ribosomes to encode proteins. Viruses don’t have a living cell and so they do not need the protein synthesis process to occur in them.
But, they do have the ability to create proteins if they enter the host cell using the host’s ribosomes.
Viruses have RNA as the genetic materials and in many rare cases has DNA as well without any ribosomes or rRNA.
In some cases, that is in some of the newly discovered viruses named Tupanvirus (a giant virus discovered in 2003) it is seen that they have developed a mechanism that helps them produce proteins. They have huge genomes that encode many proteins that can also participate in protein synthesis.
In the case of Tupanvirus, a hypothesis says that they may have developed a unique kind of a mechanism that allows them to take part in protein synthesis without the use of ribosomes. This statement is still a hypothesis and not an approved theory yet.
Viroids are those viruses that haven’t entered a host cell yet. And when viroids enter the host cell they become living viruses.
Viroids are actually disease-causing organisms that contain only nucleic acids and have no structural proteins that can bring about a genetic change.
Other virus-like particles called prions are composed primarily of a protein tightly integrated with a small nucleic acid molecule (RNA or DNA), but with no ribosomal contents.
Why would you never see ribosomes in a virus?
The job of proteins is to bring genetic changes and provide support for life to exist. Viruses are not living cells and so they don’t need any regular protein formation mechanism to perform any functions of life.
They are only like a capsule of protein with DNA or RNA inside without having the ability to translate more proteins. The proteins that are already present in the virus helps to attach with the receptors on the host’s cell wall to inject its viral particles inside the cell.
We know that viruses don’t create proteins but when the RNA/DNA of the virus enters host cell they use the host’s ribosomes to encode the viral proteins inside the host.
The DNA or RNA of the virus contains all the instructions necessary to borrow the host’s ribosomes to replicate new viruses, manufacturing all the proteins necessary.
Therefore they depend entirely on the host’s living cells into which they inject their viral particles, DNA, or RNA. Host cells are living, and they have ribosomes along with the proteins to translate the viral genetic material into complete copies of the original virus.
In such a way, the non-living viruses transform itself to become a living virus as so starts the process of protein synthesis with the host cell’s ribosomes.
So, you can consider that viruses are obligate parasites of the host cell translational machinery, the viral proteins are coded for by the viral genome but manufactured by the host cell.
So, that’s why you would never see ribosomes in a virus until and unless the virus enters the living body.
So, how do they make proteins?
It’s true that viruses themselves cannot create any proteins without a host cell. Inside the host cell, they become capable to carry out their life-sustaining functions or reproduce by creating proteins.
So, as the virus enters the host, it follows the model of Central Dogma or Reverse Central Dogma of biology to conduct its gene expression leading to protein synthesis.
Central Dogma Model of gene expression is seen if the virus injects its DNA particle in the host cell, whereas Reverse Central Dogma of gene expression is seen if the virus injects its RNA particles in the host cell.
Reverse Central Dogma of Biology is the model of gene expression seen in some RNA viruses (like the retrovirus, etc) where the flow of information is in reverse direction i.e from RNA to DNA. Then, the RNA of these viruses first synthesizes DNA in the presence of Reverse Transcriptase enzyme. DNA then transfers information to mRNA and the translation to protein occurs.
On the other hand, the Central Dogma of Biology is the model seen in some DNA viruses (like Adenovirus, Herpesvirus, etc) where the flow of genetic information is in the straight direction i.e. from DNA to mRNA to finally Proteins.
A virus can live in two different phases inside the host– the lytic phase (where the virus actively replicates in a host cell), and the lysogenic phase (where the viral DNA incorporate itself into the cell’s DNA and multiples whenever the cell multiplies).
In this way viruses make proteins not by themselves, but when entered into the host cell. Viruses reproduce by infecting their host cells and reprogramming them to become virus-making “factories.”
Are viruses alive? Let’s know!
Viruses can’t maintain their internal environment as they don’t have nuclei, organelles, cytoplasm, or even the cell itself. It is just made up of a protective protein ‘capsid’ shell having an extra lipid bilayer membrane surrounding the capsid. So, they can be considered non-living.
Inside the capsid, there can be either DNA or RNA but these nucleic acids are not able to express itself by making proteins, as ribosomes are also not present. All living organisms are able to make proteins. So, they can be considered non-living as well.
Living things have different levels of organizations that viruses don’t as these are made up of just simple and smaller building blocks like nucleic acids and some units of proteins only.
Viruses don’t replicate or reproduce as well of their own and so considered non-living in nature. But when they enter the host cell they start to reproduce and replicate thus becoming living organisms.
Viruses don’t grow as well. Meaning that they will neither increase in size nor in complexity throughout its existence. Whereas, all living things grow.
We know that all living things need energy as biochemical energy and is able to create, use, and even conserve that energy to perform its various metabolic activities. However, viruses don’t use, create, or conserve any such energy so are considered non-living. But, when they enter the host’s cell it uses the host’s energy to multiply itself.
These all above criterias show how viruses become alive and how they are not alive.