I see my backyard chickens pecking for food all the time. What could possibly be their purpose in life, I ask myself. Just feed their guts to process food and absorb the valuable and reject the unwanted? And produce eggs to make offspring who in turn will do the same? Is that all?
But why just chickens? Why on earth do we humans exist? At some point we ask ourselves: Do we eat to live or live to eat? Does the God or nature or whoever the supreme power is, if you believe in one, want us to protect our ‘selves’ or contribute something to the nature?
Let’s state this conundrum of life’s purpose in biological terms. All the work/activities that we do boils down to metabolism inside the body. We eat food which fuels metabolism. Of course it also helps us survive. But survival goes beyond that. Our genes want us to reproduce so that they get passed on to the next generation. It’s more about survival of our genes. Our fundamental question of activity-vs-survival thus translates to metabolism-vs-reproduction. What is the cause and effect relationship here? Does reproduction facilitate metabolism or benefit from it? Such chicken-and-egg problems refuse to go away whether you look at life at organism level or cell level or molecule level.
Inside the cell, genes containing information as special set of DNAs reside in the nucleus. However, all the action is outside of the nucleus – in the cytoplasm. That’s where protein synthesis happens and proteins make us do everything. The manufacture of protein however is directed by the code written in the DNAs.
Superfast reactions inside the cell materialise metabolic actions. The extraordinary operation of these reactions require a special type of proteins – enzymes – to speed things up. This process of catalysis is one of the fundamental requirements for life to exist. Even the DNAs require enzymes for replication.
Let’s go one level deeper. Life stores information as molecules. One may argue that inanimate objects also contain information. So what’s special about life? Where life differs is that it works on the information stored inside. For instance, an organism’s unique information is stored in the DNAs as code. Life then uses this information and produces proteins for various functions. One ‘special’ function replicates the information so that it can be passed on.
DNAs and proteins have strong mutual dependence. The proteins need DNAs to get created. The DNAs need proteins to get replicated. Another chicken-and-egg!
In spite of this mutualism, metabolic proteins and DNAs do not coexist in the same physical space. Proteins are chains of amino acids manufactured in the cytoplasm of the cell which faces the external environment. DNAs made of nucleotides, on the other hand, are tucked safely as single source of truth inside the nucleus. They are too precious – not to be messed up with – and hence are far removed from the external environment. Yet the DNAs must communicate with the cytoplasmic world to be able to guide the mechanism of protein synthesis. Enter RNA.
RNA, in fact a special type called messenger RNA or mRNA, is a working copy of the DNA. The copy gets made inside the nucleus and it crosses the membrane and enters cytoplasm with the code handy. The mRNA hands over the code to ribosomes in the cell cytoplasm. That is the building site where using the instructions in the code protein is synthesised.
RNA is capable of storing information and it can act as enzyme for catalysis. It can self-replicate too. Voila! You’ve got it. Here is the thing that can do it all. And a new theory of ancient life gets proposed – the RNA World. So, ladies and gentlemen, the first real ‘life’ was when it was all RNAs around! The specialised worlds of DNAs and proteins came later. The most fundamental of the chicken-and-egg problems is resolved.
You wish. If RNAs came first, how did they get converted to DNAs? Was reverse transcriptase available then? How? And also making RNAs out of inorganic raw materials is extremely difficult if not impossible.
If you are still hungry for more, here is another chicken-and-egg food for thought. What came first – virus or cell?