The Road to Discovering DNA as the Genetic Material

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• The Road to Discovering DNA as the Genetic
  Material

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Mendel published the results of his pea-breeding
experiments in an obscure horticultural journal
in 1866. His work is summarized follows A
character (now called a gene or allele) may be
dominant or recessive. A dominant allele is fully
expressed in an organisms appearance while the
recessive allele is has no observable effect on
the organism. Law of Segregation Characters
(alleles) occur in pairs, one inherited from the
maternal parent the other from the paternal
parent. These alleles separate during gamete
formation (meiosis). Law of Independent
Assortment Each character is inherited
separately (independently from the other).
Gregor Mendel 1822 - 1884
http//www.nndb.com/people/015/000083763/
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Friedrich Miescher a Swiss chemist working in
Germany was the first to isolate nucleic acids
around 1869. Miescher was working on the
physiology of leukocytes (white blood cells) and
devised a technique to harvest whole white blood
cells from pus laden bandages. He originally
called the substance that he isolated from the
nuclei of these cells nuclein.
Johann Friedrich Miescher 1844 - 1895
http//www.laskerfoundation.org/rprimers/gnn/timel
ine/1869a.html
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In the late 1800s Hugo de Vries, Carl Correns and
Erich Tschermak were each studying heredity and
were coming to the same conclusions that Mendel
had derived 35 years earlier. These three
workers are now credited with the re-discovery of
Mendels work.
Huge de Vries 1848 - 1935
http//www.nceas.ucsb.edu/alroy/lefa/deVries.jpg
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Sutton, W. S. 1902. On the morphology of the
chromosome group in Brachystola magna.Biological
Bulletin, 424-39.
W. S. Sutton 1877 - 1916
T. Boveri 1862 - 1915

• Sutton and Boveri are responsible for the
  Chromosome Theory of Heredity
• Each chromosome is unique.
• Chromosomes occur in pairs, one of each pair
  inherited from each parent.
• Chromosome pairs separate during meiosis
  (gamete formation).

http//www.ac-amiens.fr/pedagogie/svt/info/hist_ge
net/Theo_chrom_html_m7c6432de.gif
http//clendening.kumc.edu/dc/pc/boveri.jpg
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Experimental work in the early 1900s with fruit
flies (Drosophila melanogaster) in Morgans lab
established that genes were arranged linearly on
chromosomes. In time, experimental work became
so refined that the relative order and distances
between genes were established. Morgan
established that genes were the unit of heredity.
Thomas Hunt Morgan 1866 - 1945
http//campus.udayton.edu/hume/Chromosomes/chromo
24.gif
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Wilson is credited for discovery of sex
determination by the XY chromosome pairing. More
interesting is his vacillation over whether
protein or nucleic acids are the carriers of
genetic information. Subsequent editions of his
book 1898 The Cell in Development and
Inheritance 1915 The Cell in Development and
Inheritance 2nd ed. 1925 The Cell in Development
and Heredity 3rd ed. stated a preference for one
then the other compound.
Edmund B. Wilson 1856 - 1939
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Acetabularia sp.
www.accessexcellence.org/RC/VL/GG/hammerling_s.htm
l
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Frederick Griffiths worked with two strains of
pneumonia causing bacteria.
http//biology.kenyon.edu/courses/biol114/KH_lectu
re_images/How_DNA_works/how_DNA-works.html
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Griffiths Experiments 1928
biology.kenyon.edu/.../FG11_01a.JPG
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Griffiths Experiment contd
The material that transformed the bacteria from
the non-lethal R strain to the lethal is the
genetic material.
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In 1944, Avery, McLeod and McCarty identify
Griffiths transforming material as DNA
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In 1944, Avery, McLeod and McCarty identify
Griffiths transforming material as DNA but
their work did not gain universal acceptance.
Oswald Avery
Colin MacLeod
Maclyn McCarty
Titled "Studies on the Chemical Nature of the
Substance Inducing Transformation of Pneumococcal
Types," their paper was published in the Journal
of Experimental Medicine. Although their findings
were clear and revolutionary, their conclusions
were cautious, and they presented several
interpretations of their results. Consequently,
response to their report was ambivalent. Their
findings were accepted immediately by some, but
the remained a source of considerable debate for
others.
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Enter Bacterial viruses called bacteriophages.
http//www.bact.wisc.edu/themicrobialworld/Viruses
3.jpg
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Enter Bacterial viruses called bacteriophages
contd.
www.mie.utoronto.ca/.../biopic/fig/11.2.jpg
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An animation describing the lytic cycle of the
bacterial virus.
http//student.ccbcmd.edu/courses/bio141/lecguide/
unit3/viruses/images/lysosum.gif
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In 1952, Martha Chase and Alfred Hershey answered
the 50 year old question whether proteins or
nucleic acids carried genetic information with
their famous blender experiment. Viruses consist
of protein and nucleic acid only. Proteins can
be distinguished from nucleic acids because only
proteins contain the element sulphur. Similarly,
nucleic acids contain the element phosphorus
which is not found in protein. Thus proteins and
nucleic acids could be uniquely labelled and
identified by radioactive isotopes of sulphur
(35S) and phosphorus (32P) respectively.
Martha Chase Alfred Hershey
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The Hershey and Chase blender experiment
conclusively demonstrates that DNA is the carrier
of genetic information.
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An Animation Explaining the Hershey and Chase
Experiment

• http//highered.mcgraw-hill.com/olc/dl/120076/bio
  21.swf

Now one of the race was on to be first to
describe the structure of DNA.
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Text References Nelson Biology 12 Chapter 4
Introduction, p 204 Section 4.1, pp 205
209. Section 4.1 Questions, p 209. Campbell and
Reese Biology Chapter 16, pp 287 290.