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Lecture 3

PHI 31 Lecture Notes - Lecture 3: Linus Pauling, Physical Chemistry, Scale Model


Department
Philosophy
Course Code
PHI 31
Professor
Griesemer James
Lecture
3

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Watson and Crick (1651-1953): Episode in Science
Physical chemistry applied to biology
X-ray pictures of crystallized DNA → allowed DNA structure to be studied
Making models → Theoretical, analog, and scale models
3-chain model of DNA
Backbones inside, bases outside - “dry” molecule; not “good fit” to reality
2-chain model of DNA
Backbones outside, bases inside - “wet” molecule; “good fit”
Theoretical model (or structure of DNA) - two polynucleotide chains w/ backbones
twisted around each other
Analog model
DNA is like a spiral staircase
Why spirals?
Linus Pauling found protein w/ σ-helix spiral structure
Watson: perhaps DNA has spiral structure too!
Models serve as tools for thinking about structures
A way of thinking about a structure → may or may not be good
way to think about details/may not be “good fit” to reality
Rosalind Franklin’s X-Ray Picture
B form-DNA → pointed to helix pattern
Picture served as model that didn’t immediately resemble
spiral staircase
Crick’s mathematical model
X-ray scattering by atoms regularly spaces on infinite helix
Helps understand the experimental set-up of x-ray scattering
experiments w/ helical molecules
Able to use his expertise in x-ray scattering to interpret Franklin’s
picture
Scale model
Corresponds to what is believed to be the actual structure → scaled up
DNA = billion:1 scale
Models ≠ the things they model!
Understanding theoretical hypotheses
Claim about the “fit” of model and world
Problem → model → is it a good fit?
“Real world” phenomenon = some aspect of physical world & its behavior
As opposed to abstract realm: numbers, symbols, concepts
Model = something physical (Watson/Crick DNA model) or abstract (structure
described by math eq)
Theoretical model = abstract structure that phys model shares/indicates
Key concept of model-based view
Theoretical models are representations that are:
Incomplete
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