Bio Sci 104
Overview of Cell Machinery, and Evolution
1. Life begins with cells. All cells are made up of amino acids, nucleotides, sugars, and fatty
is the minimal self-reproducing unit and is the vehicle for transmitting
genetic information to the next generation in all living species.
->Development of self-replicating systems permits evolution and natural selection to operate
(occurred more than 3 billion yrs ago).
2. Probably first life was RNA based.
RNA can both store information (encoded in nucleotide
sequence) and can function as catalysts
RNA is thus a central linkage
between genotype and phenotype. (catalyst: a substance that initiates or increases the
rate of a chemical reaction without itself being changed; enzyme: organic catalysts,
biomolecule that functions as a catalyst.)
3. Eventually, systems evolved to become more efficient.
Proteins are more versatile catalysts than RNA (20 types of amino acid, plus
modifications, compared to only 4 types of bases in RNA), therefore development of
allowing a sequence of RNA to be converted to a protein was
a major step
in evolution of phenotype
DNA is more stable than RNA (a major improvement for storage of genetic material-
significantly more resistant to degradation and to hydrolysis).
For example, RNA slowly
hydrolyzes itself using the 2’OH group (which is missing in DNA). Thus, most organisms
use DNA as genotype and RNA as intermediate leading to protein synthesis, catalysis,
structure, and phenotype.
4. The RNA world still exists: viruses that use RNA as genome (influenza, HIV), catalytic RNA
at the heart of ribosome function and mRNA splicing.
In fact, the actual catalytic
processes involved in the translation of proteins are virtually all RNA based (ex. t-RNA).
5. Early life probably evolved in complex organic soup, yet ultimately needed to create
barriers, to separate biological reactions (dependent on defined conditions) from the
Also, no selective advantage to a particular RNA able to encode a
useful enzyme if the enzyme diffuses and operates elsewhere.
Therefore, first cell is
defined by creation of cell membrane (or cell wall).
7. Even the simplest present day cells are much more complex than these cell-like entities.
an example- modern day mycoplasma- small bacteria-like organisms with cell membranes,
but no cell walls (ie. antibiotics that specifically target cell walls such as penicillin will
not kill them).
Their DNA encode about 500 proteins (enzymes, structural).
this is sufficient to encode all enzymes necessary for metabolism, generation and
storage of energy, DNA replication, repair, transcription, translation and all the
structural proteins needed to maintain the cell shape and integrity.