Genetic information flows from DNA, in the nucleus of each cell, to RNA, which carries the information out of the nucleus into the body of the cell and uses the instructions encoded in it to produce proteins (which act as enzymes and also provide the structural framework of cells). However, the duplication of DNA requires numerous enzymes that catalyze those reactions. And enzymes are proteins themselves – the end product of the information coded in DNA. In other words, proteins are required for DNA synthesis and DNA is required for protein synthesis.

How then could the

first living cell with DNA-based molecular biology

have originated by spontaneous chemical processes

on the pre-biotic earth? This

has been the chicken and

egg problem of life’s evolution from chemicals – Which

came first, DNA or the protein molecule?

In the late 1960s, several

biologists – including Francis Crick, Carl Woese and

Leslie Orgel – suggested

that the ancestor molecule

was neither DNA nor protein, but RNA. RNA, they

suggested, might have catalyzed reactions necessary for

replication as well as provided the genetic information necessary to replicate

itself. Self-replicating RNA-based systems would have

arisen first, and DNA and

proteins would have been

added later. DNA could

have evolved from RNA

and, then, being more stable,

taken over RNA’s role as the

guardian of heredity.

This idea further got support in the early 1980s from

the independent discoveries

of Thomas Cech and Sidney

Altman of a kind of RNA

that catalyzes a reaction.

These catalytic RNA molecules have subsequently

been termed as “ribozymes.”

In 1986, Walter Gilbert, in an

article in Nature, portrayed

the primordial world as

“RNA World” where RNA

molecules catalyze their

own synthesis. Since then,

the term “RNA World” has

stuck to the general hypothesis – RNA first, DNA and

protein later.

Researchers continue to discover new func-

tions for existing RNA, illustrating repeatedly how versatile these molecules can be.

The recent determination of

the structure of the ribosome,

showing that it is a ribozyme,

gave further support to the

belief in the RNA World.

However, there are many difficulties and problems in the

RNA World.

Leslie Orgel,

one of the scientists who first

proposed it in the 1960s, himself concedes that researchers who have attempted to

illustrate the possibility of

spontaneous generation of

the chemical elements of RNA

itself have had only modest

success. Ribose, the sugar that

is part of the backbone of the

RNA molecule, is difficult to

create from hypothetical early

earth conditions, except in

very small quantities. Stanley

Miller and his colleagues have

also recently reported “ribose

and other sugars have surprisingly short half-lives for

decomposition at neutral pH,

making it very unlikely that

sugars were available as pre-biotic reagents.”

RNA World assumes that in

the primordial world, ribonucleotides spontaneously

condense into polymers to

form RNA molecules and

RNA molecules, once formed,

would have the catalytic

activity to replicate itself,

and a population of such

self-replicating molecules

would arise. However, it is

objected that even if RNA

could have formed spontaneously, it would have been

continuously degraded by

spontaneous hydrolysis and

other destructive processes

operating on the primitive

Earth.

Joyce and Orgel point

out many detailed problems

with these postulates of

RNA World.

They finally suggest not to

accept “the myth of a self-replicating RNA molecule

that arose de novo from a

soup of random polynucleotides. Not only is such a

notion unrealistic in light

of our current understanding of pre-biotic chemistry,

but it should strain the credulity of even an optimist’s

view of RNA’s catalytic

potential.”

Similarly, Crick

has expressed great doubt

about the RNA World. He

says, “At present, the gap

from the primal ‘soup’ to the

first RNA system capable of

natural selection looks for-

biddingly wide.”

This article is an excerpt from

the late Dr. T.D. Singh’s book Life,

Matter and Their Interactions.