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BIO.2
The student will investigate and understand the history of biological concepts. Key concepts include
a) evidence supporting cell theory
There are three components to cell theory: (1) all life consists of one or more cells; (2) cells carry
out the basic functions and comprise the basic structure of organisms; and (3) cells can only come from
pre-existing cells. In the latter half of the 17th century, Robert Hooke observed cork using a crude
microscope and coined he work cell to describe the small boxes he observed. Leeuwenhoek examined
pond water under magnification and observed a world of tiny living creatures. Throughout the next two
centuries, as microscopes were refined, more and more tissues and organisms were examined
microscopically and all were observed to consist of small, identical units, i.e. cells. Specifically, Schleiden
observed cells in the leaves, stems and roots of plants and Schwann extended these observations to
animals. Each of these men independently developed the cell theory. Virchow, in considering Pasteur’s
experiments, summarized the concept that cells can only come from other cells, thus denying the idea of
spontaneous generation. Please see the following web site for a summary of these individual’s
contributions:
http://kentsimmons.uwinnipeg.ca/cm1504/celltheory.htm
or you can view the power point on Cell Theory.
Throughout the 19th and 20th centuries, especially after the development of the electron microscope,
organelles within cells were described and specific functions were demonstrated to be associated with each
organelle. Cells were observed to be differentiated to specific functions in different tissues.
b) scientific explanations of the development of organisms through time (biological evolution)
Prior to the publication of Charles Darwin’s landmark work, On the Origin of Species, in 1859, there
was a long and varied history of evolutionary thinking. Please see the power point on Evolution. At the
time Darwin published his work, he did not know the source of variation in populations or how these
variants were inherited. Alfred Russell Wallace was on target with his concept of sexual selection (see
BIO.8), but Darwin vehemently denied this as a means of change! Furthermore, Darwin did not repudiate
Lamarckian theories of evolution. Had Darwin known of Mendel’s work (published about 5 years after
Origin, but in an obscure publication) a lot of Darwin’s problems with his own work would have been solved!
Curiously enough, it was not evolution that got people upset during the Victorian era, but his means of
evolution by natural selection. The opposite thinking is prevalent today. Interestingly enough, acceptance
of cell theory and rejection of spontaneous generation requires a common ancestor for all life.
c) evidence supporting germ theory of infectious disease
Redi, in 1668, clearly demonstrated with meat in 3 jars that spontaneous generation could not
produce maggots. One jar was left open with meat, another was covered with gauze and another was
tightly sealed. Neither the one that was tightly sealed, nor the one covered with gauze developed maggots.
Maggots were found on the gauze outside the jar. Maggots appeared in the open jar, hence maggots
came from flies, not the air. In 1835, Bassi associated a disease of silkworms to the occurrence of a
fungus and is credited with being the first to express a germ theory of disease. By the mid-19th century, the
popular theory of disease was that it was related to miasma in air, signified by bad odors. When Snow
developed mapping and detailed interviews as a means of identifying the source of cholera as a local water
supply in London (1854), it was still a difficult sell to the educated public. Even during this time microscopic
analysis of water had revealed the presence of Vibrio cholerae, but it was not linked to the disease Cholera.
Koch in the late 19th century was able to associated specific organisms with specific disease. His
postulates to associate a microorganism with a disease are as follows: (1) microorganism must be present
in abundance in those suffering from the disease; (2) the microorganism must be isolated from the carrier
and grown in culture; (3) when introduced to another subject, the cultured microorganism should cause the
disease; (4) the microorganism must then be re-isolated and grown in culture and be identical to the
original organism. In the 19th century, hospitals often contributed to the spread of disease. Pasteur’s
experiment showed that decay was caused by living organisms present in air. Joseph Lister connected this
with sepsis of wounds. He pioneered the use of antiseptics in surgery, thus reducing the incidence of
hospital infections.
d) development of the structural model of DNA
James Watson and Francis Crick are credited with figuring out the structure of DNA. However,
their work was the culmination of the work of many who had gone before. Through the work of Griffiths,
Avery, Chase & Hershey, DNA was conclusively identified as the genetic material. DNA was known to be a
polymer of nucleotides, and that each nucleotide consisted of a phosphate group, a sugar (deoxyribose)
and a nitrogenous base. There were four different bases found in DNA: adenine, thymine, cytosine and
guanine. Chargaff, a biochemist, in 1949 found that in DNA, the amount of adenine always equaled the
amount of thymine and that the amount of cytosine always equaled the amount of guanine. Wilkins and
Franklin had purified and crystallized DNA and were able to observe its structure using x-ray diffraction.
Their work showed that DNA had a helical structure, and was composed of two or three chains of
nucleotides. Their work, together with Chargaff’s ratios of the bases suggested to Watson and Crick that
DNA was a double helix held together by pairs of nitrogenous bases. A purine (adenine or guanine) would
always be bonded to a pyrimidine (thymine or cytosine) to hold the chains together. Because guanine and
cytosine required 3 hydrogen bonds and adenine and thymine only required two hydrogen bonds,
Chargaff’s ratios of A = T and G = C were explained. Watson and Crick were excited by this possibility
because there were certain requirements of the genetic material: there had to be a way that it was exactly
copied and it had to have the molecular possibility of encrypting enformation. This sequence of base pairs
was later identified as the genetic code making up the molecular basis of genes.
e) the collaborative efforts of scientists, past and present.
See the above discussions and the web site below for interactive materials on modern biological
concepts, scientists and their research.
http://www.dnai.org/a/index.html
After reviewing material in your textbook, go to the file labeled BIO.2 Review Response and open it in
Word. Type your answers below each question and make them a distinctive readable color or font. E-mail
this file as an attachment