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DNA
Discovery and Structure
What is DNA?
• Deoxyribonucleic acid
• DNA is the molecule of genes.
• It is found in the chromosomes.
• It holds the instructions to making
all the proteins in an organism.
• Proteins make up the structures of
cells.
• Proteins, in the form of enzymes,
also control all cell processes.
• DNA carries the instructions from
mother to daughter cells, from
parents to children. Therefore it is the
molecule of heredity.
Who discovered DNA?
• Deoxyribonucleic acid (DNA) was
first isolated in 1869 by the Swiss
scientist Friedrich Miescher. He
called the white, slightly acidic
chemical that he found in cells
"nuclein." He identified DNA as
part of a chromosome in 1871.
Did DNA or Protein makeup a
molecule of heredity?
By the 1920s it was thought that genes were
made of protein, the other main ingredient of
the chromosome.
Many scientists at the time thought DNA was
too simple to carry genetic information and
that proteins were molecules of heredity.
---They
were wrong!
How did scientists discover that
DNA was the hereditary molecule?
Scientists such as the Griffith’s
bacteria experiment in 1928, and
Avery’s bacteria experiment in
1944, pointed toward DNA.
The famous Hershey-Chase
experiment finally ended the debate.
Hershey-Chase: The “Waring Blender”
experiment – 1952.
Alfred Hershey and Martha Chase
• They experimented with
bacteriophages, viruses that infect
bacterial cells.
• These viruses are basically
composed of protein surrounding
DNA.
• Bacteriophage viruses force bacteria cells to
make new virus particles.
• The experiment would see if the virus’
proteins or the virus’ DNA caused the
bacteria to produce the new virus particles.
Conclusion: The genetic material of the
bacterophage viruses was not the protein but
DNA.
DNA is the molecule of heredity!
Rosalind Franklin and Maurice Wilkins
examined DNA in the laboratory to determine
its structure.
X-Ray Diffraction (X-Ray Crystallography)
photograph of DNA, taken by Rosalind Franklin
was very important to the scientists who eventually
discovered the structure of DNA.
“photo 51”
DNA structure was discovered
by James Watson
and Francis Crick in 1953.
What is DNA’s Structure?
• DNA is a biological polymer made up of
subunits called nucleotides.
• DNA Nucleotides are made up of:
a) 5-carbon deoxyribose
sugar
b) phosphate group
c) nitrogen-containing base
By the way ---the sugar and base alone make up a
nucleoside
There are two classes of nitrogen bases:
• Purines (double ring structure)
adenine (A) guanine (G)
• Pyrimidines (single ring structure)
cytosine (C) thymine (T)
Mnemonic Device
• All that is Good is Pure.
Adenine
Guanine
purines
• C (see) The pyramids.
Cytosine
Thymine
pyrimidines
Another discovery that helped Watson & Crick
Erwin Chargaff - demonstrated that
guanine and cytosine are always in equal
amounts (as well as thymine & adenine)
Rosalind Franklin’s X-Ray evidence
showed, among many other things, that
DNA has the shape of a double helix or
“twisted ladder.”
Summary of DNA’s Structure:
1) Alternating sugars & phosphates on the
sides of the ladder. (Covalently-bonded)
2) Nitrogen bases in the rungs
A-T , G-C (base-paring)
3) Weak bonds between bases (Hydrogen
bonds)