1. DNA
Structure

2. Discovering the structure of DNA
DNA = Deoxyribose nucleic acid
Made out of sugars (deoxyribose), phosphates and nitrogen bases

3. DNA Structure
DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix.
In 1950’s, James Watson & Francis Crick determined model of DNA
A molecule of DNA is made up of millions of tiny subunits called Nucleotides.
Each nucleotide consists of:
1. Phosphate group
2. 5-carbon (Pentose) sugar
3. Nitrogenous base

4. Discovering the structure of DNA
Structure was discovered in 1953 by James
Watson and Francis Crick

5. Nucleotides
Phosphate
Nitrogenous
Base
Sugar

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DNA Nucleotide
O=P-O O
Phosphate
Group N
Nitrogenous base
(A, G, C, or T) O
CH2 O C1 C4 C3 C2 5
Sugar
(deoxyribose)
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7. Nucleotides
The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”.
There are four types of nitrogenous bases.

8. Nucleotides A
Adenine T
Thymine C
Cytosine G
Guanine

9. Nucleotides Each base will only bond with one other specific base.
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Form a base pair.
Form a base pair.

10. DNA Structure
Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

11. Chargaff’s Rule A == T C == G T == A G == C
Erwin Chargaff – Scientist: Discovered base-pairing rules
The bases form the “rungs” on the DNA ladder by complementary pairing
A == T
C == G
T == A
G == C
Sugar-phosphate backbone

12. Discovering the structure of DNA
Rosalind Franklin’s DNA image
“Chargoff’s rule”
A = T & C = G

13. A C T G G A T C

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Nitrogenous Bases
Double ring PURINES
Adenine (A)
Guanine (G)
Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
A or G
T or C
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15. Four Nitrogen Bases of DNA
Pyrimidines:
Nitrogeneous bases
That have a SINGLE
Ring of Carbon and
Nitrogen Atoms.
Ex. Cytosine and
Thyamine
Purines:
That have a Double
Ex. Adenine and
Guanine

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Base-Pairings
Purines only pair with Pyrimidines
Three hydrogen bonds required to bond Guanine & Cytosine C G
3 H-bonds
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17. How Chemical Bonds hold DNA together
Covalent bonds- between sugar & phosphates of 2 nucleotides
Hydrogen bonds- between complementary nitrogenous bases

18. DNA Structure
To crack the genetic code found in DNA we need to look at the sequence of bases.
The bases are arranged in triplets called codons.
A G G - C T C - A A G - T C C - T A G
T C C - G A G - T T C - A G G - A T C

19. DNA Structure A gene is a section of DNA that codes for a protein.
Each unique gene has a unique sequence of bases.
This unique sequence of bases will code for the production of a unique protein.
It is these proteins and combination of proteins that give us a unique phenotype.

20. DNA
Gene
Protein
Trait