Re: the evolution of DNA

Hi Dear Becky

Here is your answer.

A living system would qualify as a living system if it at least capable of 
both replication and evolution. Replication enables the parental copy of 
the system to give rise to two identical daughter copies of itself.  
Evolution implies that the process of replication is always slightly 
imperfect, so that variant daughter copies may have the potential to 
replicate faster that the parent or to adapt better to changing 
environmental conditions.

The living molecule was probably a Nucleic Acid.  The self-complementary 
structure of a nucleic acid double helix is perfectly suited for the task 
of replication. One strand serves as the template surface for 
polymerization of a new daughter strand. Polypeptide chains, on the other 
hand, are not self-complementary and cannot serve as templates for their 
own replication. Also, the universality of nucleic acid genomes in 
contemporary cells and viruses makes it difficult to believe that the 
ancestral living systems did not have either an RNA or DNA.

It was discovered in 1981 that ribosomal RNA of the ciliate Tetrahymena 
can excise itself from the precursor rRNA without the help of a protein 
catalyst.  This implied that the very first molecule might have been an 
RNA replicase that catalyzed its own replication witout the help of a 
protein. Many new RNA molecules have now been shown to possess the catalytic 
properties.  Enzymatic activity has been attributed to DNA, but only under 
extremely controlled conditions.  This is perhaps the most powerful argument
that the first replicating nucleic acid was RNA rather than DNA.

Other reasons include:
-The difference between RNA and DNA is that the deoxyribose rings of DNA 
lack 2’–hydroxyl groups.
-Lacking 2’-hydroxyl groups, DNA may be less adept than RNA at folding 
into the complex three-dimensional shapes required for catalytic activity.
-Although all cells have today have DNA genomes, DNA precursors are always 
synthesized by reduction of RNA precursors using the highly conserved 
enzyme ribonucleoside diphosphate reductase. This suggests that the 
biochemical pathway for ribonuclotide synthesis evolved first, when cells 
had RNA genomes.
-Ribose is much more readily synthesized than deoxyribose under simulated 
prebiotic conditions.
 http://exobio.ucsd.edu/Space_Sciences/rna-dna.htm

Please feel free to ask.

Man Mohan