Content
⇒ Molecular Evolution
⇒ Early Beginnings
⇒ The RNA World
⇒ The DNA World
⇒ Phylogenetic Analysis
⇒ The Evolution of Eukaryotic Cells
Double-stranded DNA molecules are more stable than ssRNA. It would
thus be advantageous for living systems to store heritable information in
DNA molecules rather than RNA molecules. The 2' OH of RNA can attack
an adjacent phosphodiester bond, rendering RNAs much more labile
than DNA s. This autocatalytic process may have been accelerated by the
harsh conditions on the primitive earth. As cells became more complex,
their genome sizes had to increase. If early eugenotes had segmented
RNA genomes, at least one of each segment would have to be present in
each daughter cell for its survival. To enhance the probability that progeny
cells are provided with a full genome, natural
selection would favor production of polycistronic
genomes. But the larger the RNAgenomic segments
are, the less stable they would become because of autocatalysis.
Thus, it would be advantageous for more
stable polycistronic DNA molecules to take over genomic
functions of RNA, leaving the RNAs to carry
out functions that need not require long-lived molecules. The earliest anucleate
cells containing DNA genomes (and all subsequent such cells) are
known as prokaryotes. |
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At least four major processes were required to complete this transition:
(1) synthesis of DNA monomers by ribonucleoside diphosphate reductase;
(2) reverse transcription of DNA polymers from RNAgenomes;
(3) replication of DNA genomes by a DNA polymerase; and
(4) transcription of DNA genomes in functional (nongenomic) RNA molecules such as tRNA, mRNA, and rRNA.
The split genes of modern eukaryotic cells consist of coding regions
(exons) and noncoding regions (introns). The interruption of the gene
provided by introns offers an evolutionary advantage. Apparently, exons
from different genes can sometimes be recombined by natural mechanisms
to code for proteins of different functions but containing related
amino acid domains. Each of these domains may have a specific function
(e.g., binding to a receptor, forming an α-helix, etc.). This process, termed
exon shuffling, is inferred to have been used extensively in the DNA
world of early eukaryotes.
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