Before
the twentieth century, if you asked a biologist what life was, you might have
gotten several answers. Some biologists
believed that is something only God could answer because he created life and no
science would be able to duplicate this.
Life, for those scientists, involved vitalism in which a non-material
essence, soul, or spirit-like supernatural component was introduced to make
living matter. Most biologists in the
1890s would have rejected this. They would
argue that the organisms we see on earth, plant and animal, are composed of
cells and that cells contain a nucleus with chromosomes and a surrounding
cytoplasm that contained organelles. They
would argue that studying the cell’s organelles would reveal a lot about how
life worked. They would also argue that events in the nucleus suggest a mechanism for
cell division and for the formation of reproductive cells—sperm and eggs.
In the first half of the twentieth century biologists studying
heredity identified genes as units of inheritance found in chromosomes in the
nuclei of cells and mapped them. They
knew some of the properties of genes and the mutation process. What they did not know was the way genes functioned
at a biochemical or molecular level nor did they know the chemical composition
of genes and chromosomes. That changed
in the last half of the twentieth century.
Genes were shown to be composed of nucleic acids, especially DNA in
chromosomal genes. They worked out the structure
of DNA and worked out the way nucleotide sequences in DNA specified
corresponding sequences of nucleotides in RNA and in the proteins that the
genes made. The making of proteins took
place in organelles of the cytoplasm.
Science became very specialized for biochemists and molecular biologists
so most of the public has little understanding of how genes work.
But
understanding molecular genetics was not enough. Additional findings showed how genes were turned
on or off. They showed how RNA could
enhance or diminish the activity of genes.
They showed there was a category of genes that led to body plan symmetry
or to the shape and location of organs.
In addition to DNA activity governed by these genes and by mutations,
there were RNA molecules that enhanced or diminished the activity of
genes. New fields of epigenetics and
genomics opened as the century came to an end. For epigenetics genes could be
silenced or activated by coating genes with methyl groups. This was often reversible. As the twenty first century began, epigenetics
was supplemented with a variety of small RNA molecules acting as regulators of
gene activity especially for timing when genes go on or off in the cell and
how much product a given gene puts out. The genomics started in the late twentieth
century has created evolutionary histories of the complete sequence of all
genes in a species and comparative genomics allows biologists to study evolution
at a molecular level.
What is
not known is the composition, organization, and function of the cytoplasm and
nuclear fluid that are not associated with membranous organelles in the cells.
In the nineteenth century this would be called protoplasm. How it works and how it differs from species
to species is not yet worked out. In all
likelihood it will be worked out first in bacteria which have very few
cellular organelles. The history of biology has been a retreat for vitalists
who moved from the whole organism, to the organs, to the cells, and to the organelles
as the bastions of vitalistic life. They
are now embedded in the non-organelle protoplasm hoping science will not work
out the complex and dynamic system that surrounds the organelles and which is
essential for the functioning of these cellular components. If you are a holist but not a vitalist, you
will accept a material basis for protoplasm but you will argue the complexity
of life is beyond human capacity for analysis.
If you are a reductionist you will believe it is a matter of time, very
likely in this century, when this last bastion of ignorance will fall.
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