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Watson and Crick- Modeled DNA’s structure.- Using Franklin’s work, created a new model in which two strands of
nucleotides wound about each other, forming a double helix
• Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases
What does DNA look like?
Double Helix: structure of DNA – 2 strands, twisted into a helical shape
Make an analogy for the structure of DNA.
Nucleotides• Subunits (monomers) of DNA (polymer)
Made up of:
1.Sugar
2.Phosphate group
3.Nitrogenous Base
Nucleotides join together• Joined together by covalent bonds that connect sugar of
one nucleotide to the phosphate group of the next• The repeating pattern of sugar-phosphate is called a
sugar-phosphate “backbone”
Base-Pair Rule
• DNA is made up of 4 nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
The Base Pairing Rule
(A) Adenine = (T) Thymine
(C) Cytosine = (G) Guanine
Bases joined together by hydrogen bonds
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width consistent with X-ray data
Practice• One DNA strand is provided, write the
bases of the complimentary strandC
G
T
T
A
C
G
T
C
G
G
C
A
A
T
G
C
A
G
C
http://www.youtube.com/watch?v=qy8dk5iS1f0