The History of DNA
Early Work
Friedrich Miescher, 1869, first isolates a
substance from the nucleus of cells that he calls
“nuclein.” His student, Richard Altmann, calls the
substance “nucleic acid.”
Biochemists identify two types of nucleic acids,
later called RNA and DNA.
In 1929, Phoebus Levine at the Rockefeller center
identifies the four bases of DNA.
What Does DNA Do?
Though early researchers knew that DNA was
found in chromosomes, they doubted that it was
the hereditary material. There were only four
bases. How could for bases code for all sorts of
proteins?
Some researchers, including Linus Pauling,
thought that the protein also found in
chromosomes was probably the hereditary factor.
Frederick Griffith
In 1928, Frederick Griffith carried out
experiments on pneumonia bacteria in mice.
Discovery: something in heat-killed virulent
bacteria could be transferred to live, harmless
bacteria and make them virulent.
Griffith’s Experiment
Oswald Avery
Avery continued working with Griffith’s findings
in hope of discovering what factor in bacteria
carried the trait of virulence.
Isolated proteins, carbohydrates, nucleic acids
and applied them to non-virulent bacteria. Only
nucleic acids (DNA) caused a change.
Avery’s Work
Erwin Chargaff
Chargaff studied DNA itself, in hopes
of providing some clues about its
structure.
Discovered that there are always equal
amounts of the bases Adenine and
Thymine, and equal amounts of
Cytosine and Guanine.
Chargaff proposed that these bases
pair with one another in some way.
Wilkins and Franklin
Rosalind Franklin and Maurice Wilkins worked
with X-ray crystalography to find more clues
about the structure of DNA.
Franklin’s X-ray images suggested a helical
structure.
Franklin and Wilkins
Watson and Crick
James Watson and Francis Crick were also
working on discovering the structure of DNA.
Applied Chargaff’s rule, assumed that A always
pairs with T, C with G.
Watson was not entirely convinced of the helical
structure that Franklin had suggested, and his
critique of her work led her to doubt herself.
Watson and Crick
Wilkins consulted with Watson
and Crick. Without Franklin’s
knowledge, he handed them
the data that he and Franklin
had worked on.
Watson immediately
recognized the significance. He
and Crick went to work on a
model of DNA.
The First DNA Model
DNA structure
DNA is made up of four bases. RNA also has four bases,
but has uracil instead of thymine.
DNA structure
Across the DNA double-ladder, A always pairs with T, C always
pairs with G because of the number of hydrogen bonds the
bases form.
DNA structure
The DNA ladder forms a spiral, or helical, structure, with the
two sides held together with hydrogen bonds.
DNA Replication
Before cells divide, they must double their DNA so
that each cell gets identical copies of the DNA
strands.
DNA replication helps assure that the bases are
copied correctly.
Enzymes carry out the process.
Overview of DNA
replication. The
hydrogen bonds
break to “unzip” the
DNA strand.
Enzymes guide free
nucleotides to the
exposed single
strands and match
the nucleotides.
This diagram from
your textbook (page
157) shows how
enzymes carry out the
replication process.
DNA Helicase unzips
the DNA. DNA
Polymerase
synthesizes the new
strands, using the old
strands as templates.
DNA Replication
Build a DNA Model interactive feature (web)
DNA Replication animation (web)
Summary
DNA is a nucleic acid made up of nucleotides.
The order of the nucleotides is important, and is
maintained by matching of bases across the DNA
ladder (A-T, C-G), and by enzymes that patrol the
DNA
DNA replication occurs before cell division, and is
an orderly, enzyme-driven process.