Viruses, amongst all entities, are unique which have the ability to ‘live’. Such a statement is required to be made as viruses are the only entities which are at the threshold of ‘life’ and ‘non – life’. What does this mean, let’s try to know?
The virus is an obligate parasite but unlike other parasites, when it is outside its host or host cell, it cannot perform the so-called functions of ‘life’. It is a non-living entity outside the host cell!
So how is life defined? What is living and what is non – living? The criteria are, something ‘living’ should have the ability to reproduce, to interact with the environment and have some kind of metabolism (biochemical reactions undergoing). Yes, that’s how the process of life is defined or can be defined. The viruses can definitely reproduce and interact with its environment but only when it has entered the host cell. These tiny creatures are mere ‘inanimate things’ outside the cell, as it has zero metabolism. They can neither interact with the environment nor reproduce when outside the host cell. Undoubtedly, they have this remarkable and unique ability. No wonder, these mysterious microbes are said to be on the threshold of ‘life’ and ‘non – life’.
Viruses can infect animals, plants and even bacteria! Yes, but different types. They are host specific. Different virus types can infect plant, animals and even bacteria. Based on their host types they are differentiated as animal, plant and bacterial viruses. So, you see, whether it is an animal virus or a plant virus or a bacterial virus, outside the host body or cell, it is only an object, a tiny microscopic inanimate object measured in nanometres (10– 6 or many more times smaller than the millimetre).
One such type of virus called the lambda phage (virus) has the ability to infect a particular type of bacterium. The lambda phage (bacterial virus) infects Escherichia coli, a bacterium, which normally exists in the human and animal gut (intestines). Being obligate intracellular parasites, thus, the viruses can only reproduce when inside the body of its host cell and the moment they enter the cell they become alive. Real monsters, but tiny.
What does this lambda phage look like? The lambda phage (virus) particle is nothing but a head and a tail with or without tail fibres. The entire particle consists of 12–14 different proteins with more than 1000 protein molecules total and one DNA molecule located inside the phage head. Thus, the genetic material of the lambda virus is DNA. But you will be surprised to know that it is only the viruses which can have even RNA as its genetic material and that is truly extraordinary.
Normally any virus enters the host cell through a receptor molecule. Yes, the virus recognises this receptor, these receptors (mostly a protein) are an open invitation to the virus to enter the host cell. So, you see if the host cell does not have or carry these receptors, this inanimate object cannot enter the host cell and multiply itself. This is the specificity which does not allow the plant virus to infect an animal and an animal virus to infect a bacterium and so on and so forth. So, it’s all about presence or absence of the specific receptor molecules or the ‘invitations’ on cell surface whether of plants, animals or bacteria.
The lambda virus, once it recognises its receptor i.e. the invitation, shoots its DNA inside the cell. Yes, again this is an intriguing event, the entire virus does not enter the host cell, but it only injects or shoots its DNA across the host cell membrane and enters the host cell. The entire ‘inanimate’ or ‘non – living’ virus structure lies outside the cell and the infection process is only about injecting its DNA into the host cell.
The lambda virus then starts multiplying i.e. it gets busy making its own copies, thousands of them. Here, it needs to be mentioned that the virus does not multiply, by itself, producing any protein or DNA independently but by using the same machinery which the host cell harbours. This act defines its parasitism. The virus DNA, to reproduce itself and the other viral parts, guides production of thousands of parts of the virus using the host cell machinery which actually exists for the sake of the production of host cell proteins and metabolites but now a slave of the virus, totally under the virus control.
What is unique and intriguing about their multiplication is that the different parts are made separately and simultaneously and voila automatically assemble themselves to form the new virus particles. Yes, they are also termed as viral particles. Its like a factory assembly line. No other living entity on this planet has this kind of a unique way of reproduction. It does not have to be born and wait to grow. No wasting time for this virus. The virus produces thousands of its ‘clones’ and bursts open the host cell and releases them free to infect thousands of other bacteria.
Now comes another interesting aspect of the lambda virus life cycle, this lambda phage belongs to a class of viruses, which can either remain dormant inside the body of the bacteria after entering it or can multiply if it wishes too. Now the interesting question is what makes it decide which route to take? To multiply or not to multiply? What happens to the phage when it is not multiplying? it smartly integrates itself into the DNA of the host cell and remains dormant. It becomes latent. Smart, isn’t it?
The mystery is that this lambda phage (virus) takes this decision based on certain circumstances and what are these? Yes, this is the virus thinking process albeit without any sensory system or brain. One of the vital factors is if the virus ‘feels’ (without a sensory system) that the bacterium, which it has infected is not surrounded by enough bacteria, it may not waste its time reproducing more viral particles and instead remain latent without producing more viral particles. It may also not reproduce if it finds that the host cell is starved. Doesn’t that leave you in wonderment, a miniscule entity with no known sensory system, as seen in highly evolved creatures, has a decision-making process which is fine tuned and flawless. All this despite having the capacity of the virus to reproduce is tremendous, it can produce thousands of viral particles within minutes using the host material. Truly a total obligate parasite.
Let’s come back to the interesting question as to how the virus arrives at this decision? to be or not to be. Of course, the biochemical events are known (i.e. certain proteins in the host cell are involved which interact with viral DNA, but what is not clearly known is how this decision is reached? Now again comes an important fact. What is known is, if the host cell (the bacterium) is ‘healthy’ and enough nutrients are available to the bacterium, the virus goes into the reproductive mode (lytic mode) and if it finds that the cell lacks nutrients or basic biochemical infrastructure to produce more viral particles, it decides to remain dormant, smart chap. Well don’t jump to conclusions, different viruses have different mechanisms of reproduction and remaining dormant.
Viruses are host specific i.e. they are able to infect only a particular host type. It recognises its chance to infect through the receptors present on the host cell body. Once it finds this receptor, rest is easy for this tiny creature to complete its life cycle.
What made me write this article is to highlight the fact that viruses are mysterious creatures. They are said to be at the threshold of ‘life’ and ‘non – non life’ and is the only category existing. How they evolve, or how they ‘sense’ the need to evolve is an enigma still to be understood. Also, they evolve very fast and this being the reason that we have to face influenza every season. The evolutionary process is nothing but signals from the environment sensed by the virus, guiding changes in its DNA resulting in slightly different metabolic capacities.
The fast-evolving virus is direct evidence of the ongoing evolutionary processes in nature and endorses the theory of evolution. The receptors on the host cell do not evolve at the rate at which the virus evolves its ability to recognise the receptor. Whether the virus type with a particular receptor identifying ability are already existing in nature or they evolve only after they come in contact with the host is a difficult question to answer. One thing is for sure. Evolution is a fact. Yes Darwin, you were right.
If we are a bit imaginative, we can realise that the ‘decision’ of the lambda virus to select its life cycle is truly complex and involves mere presence or absence of certain molecules and that’s what we can term as the ‘thinking and decision-making process’.
Now one more extraordinary thing that we should not miss is that when inside the host cell, the virus is only a DNA molecule. Sounds simple but actually is not, its super extraordinary. The implications are that the element called ‘life’ is existing in merely a DNA molecule. It is this DNA which represents life and which culminates into making of a full-blown virus particle.
So, the interesting conclusion is, life is all about the DNA or the genetic material of any life form, trying to preserve and multiply itself and the rest of the body or form of the cell is merely a tool which the DNA feels would be the appropriate shield or transport mechanism to propagate itself. The DNA of the lambda phage ‘feels’ that the head and tail structure is sound enough for itself to be safe and propagate itself and if need be, evolve a bit.
Nature, you are truly amazing. Thus, if we say, what we think and feel are mere biochemical events and is what our DNA wants, may not be totally vague. Life is pure chemistry and that provides me the material for another essay.