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Transcript of DNA Introduction
Back to the 20th Century
Frederick Griffith: Transformation
Avery, McCarty & MacLeod:
Hershey & Chase:
The "Blender" Experiment
Erwin Chargaff's "Rules"
Watson & Crick,
Franklin & Wilkins
The First Puzzle Solved!
1950 - 1952
By the middle of the 20th century, genetics was well established as a field of study.
It was known that traits were inherited, but it was not known how that process happened.
Mathematical analysis can only go so far. What were genes made of? How did they work?
The discovery of DNA's role in inheritance is arguably the most significant contribution to understanding how life works.
It was not the result of any one person, but the final result of decades of investigation by many different researchers.
Observation of chromosomes during cell division demonstrates that they act in a way consistent with molecules of heredity.
Chromosomes are made of 2 ingredients: DNA and protein.
This suggests that heritability is controlled by one of these two molecules.
Before 1940 (or so), most biologists thought that protein was probably the molecule responsible for inheritance.
No one had any idea about DNA's structure or function.
A Scottish Microbiologist
Discovered that bacteria could give other bacteria heritable traits, even after they were dead.
That's where he left it.
The two forms of Streptococcus pneumoniae
R (rough, on left) is harmless
S (smooth, on right) is pathogenic.
How do you explain this?!?
Refined Griffith's Experiment
Exposed R-strain Streptococcus to purified S-strain protein, and purified S-strain DNA
Only the bacteria exposed to the S-strain DNA were transformed
Not enough evidence for the haters
Conclusively demonstrated that DNA was the molecule of heredity by tagging phage DNA and protein with radioactive atoms and tracking the transmission of that radioactivity to infected bacteria
Nobel Prize: Hershey (1969)
A colorized electron micrograph showing bacteriophages infecting an E. coli cell by injecting DNA
An Austrian Biochemist
Demonstrated two major rules of DNA composition
1. All species have different amounts of adenine, thymine, cytosine and guanine in their DNA.
2. In every species:
the amount of adenine = the amount of thymine
the amount of cytosine = the amount of guanine
Why does this matter?
Two competing teams to determimne the structure of DNA
Watson and Crick used X-ray diffraction data developed by Rosalind Franklin to develop their "double helix" model of DNA
Nobel Prize: Watson, Crick & Wilkins (1962)
Photo 51: The crucial data used by Watson & Crick
The Double-Helix Model of DNA
Understanding DNA structure helps explain its role in heredity
Bases on one strand are covalently bonded to each other ("
Bases on opposite strands are hydrogen bonded to each other ("
Adenine = Thymine
Cytosine = Guanine
Chromosomes are densely packed double-stranded DNA molecules (with hundreds of millions of base pairs). Chromosomal proteins help mediate this packing.
Make Sure You Can:
Discovered nucleic acids (1869)
What does DNA look like?
How does DNA work?
How was the structure and function of DNA determined?
Explain how the structure of DNA is related to its function as the information storage molecule in living systems.
Explain the contributions of all scientists discussed in this presentation to understanding DNA structure.
Describe how the historical development of scientific understanding of the structure of DNA is an example of science as a collaborative, evidentiary, and technological process.
To put it another way...
The Helix Unwound
The information storage molecules for biological systems.
Made of C, H, O, N & P
2 kinds of nucleic acids:
All nucleic acids are polymers of
Nucleotides consist of a
, and a
. 4 different bases in DNA & RNA
While similar in structure, there are a few key differences which lead to major differences in function.
Adenine, Thymine, Guanine, Cytosine
Adenine, Uracil, Guanine, Cytosine
DNA = 2 strands
RNA = 1 strand
DNA vs. RNA
DNA serves 2 functions in all life on Earth:
1. Stores information about the primary structure of proteins, and the sequences of RNA molecules.
2 chains of covalently bonded nucleotides, from sugar to phosphate.
Chains are bonded to each other by
between N Bases.
A bonds to T, G bonds to C.
(A,G) always opposite
The most important biological discovery of the 20th century (and arguably, the 2nd most important ever).
Watson & Crick
- published the paper
Wilkins & Franklin
- did the X-Ray diffraction work
Some controversy about ethics of Watson & Crick.
Nobel Prize (1962)- Watson, Crick, & Wilkins (Franklin was dead)
Discovery of DNA Structure
RNA serves many functions for life:
1.* Transmits and translates DNA information into protein.
2. Many enzymatic and regulatory functions.
3. 1 kind of DNA, ~15 types of known RNA at current (3 main types)
Turns out it is MUCH more interesting than DNA is.
less stable than DNA.
1 strand, but base-paring can still occur (A bonds to U)
Biological systems are process matter, energy, & INFORMATION.
The information stored in DNA moves to RNA before some of that information finally directs the construction of proteins.
This is known as the "
" of molecular biology.
It will be the underpinning of the most important biological advances during your lifetime (it already is!)
Information in Biology
James Watson & Francis Crick
Photo 51: X marks the helix!