1. DNA Structure

2. Video Clip: Virginia Commonwealth University Secrets of the Sequence

3. DNA Double Helix
Many scientists contributed to the discovery of DNA’s structure
Erwin Chargaff (1949)
In every organism, the amount of adenine always equals the amount of thymine (A = T)
In every organism, the amount of guanine always equals the amount of cytosine (G = C)

4. DNA Double Helix Maurice Wilkins and Rosalind Franklin (1952)
Used a method called x-ray diffraction to take pictures of DNA molecules.
Showed that DNA resembled a tightly coiled helix composed of chains of nucleotides.

5. DNA Double Helix Watson and Crick (1953)
Used Chargaff, Wilkins, and Franklin’s observations to develop their DNA model.
They proposed that DNA is made to two chains that wrap around each other in the shape of a double helix.

6. DNA Nucleotides DNA stands for Deoxyribonucleic Acid
Made up of building blocks (monomers) called nucleotides
Nucleotide Structure:
Phosphate group
5-carbon sugar (deoxyribose)
Nitrogenous base

7. Nitrogenous Bases
The sugar molecule and the phosphate group are the same for all DNA nucleotides, but nitrogenous bases can change.
There are 4 Nitrogenous Bases in DNA:
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)

8. Nitrogenous Bases

9. Shape of DNA
The shape of DNA is called a Double Helix.

10. Shape of DNA DNA is composed of two strands of nucleotides.
Structure is similar to a spiral staircase or twisted ladder.
Alternating sugar and phosphate molecules = side handrails of the staircase
Base pairs = steps of the staircase

11. The Double Helix
Nitrogenous bases face each other and are attracted by weak hydrogen bonds, which hold the double helix together.

12. Complementary Bases
Erwin Chargaff – determined that %A = %T and %G = %C in DNA
Base-Pairing Rules
Adenine always pairs with Thymine
A – T or T – A
Guanine always pairs with Cytosine
G – C or C – G
We call each pair a “base pair”
Remember: AT the Grand Canyon they fell in love

13. Complementary Bases
The two strands of nucleotides making up a DNA molecule are complementary.
The sequence of bases on one side determines the sequence on the other strand.

14. DNA Double Helix P O 1 2 3 4 5 G C T A
3 H-bonds
2 H-bonds

15. Write the missing bases on the strand of DNA!
Base-Pairing Practice
Write the missing bases on the strand of DNA! G C T A

16. Write the missing bases on the strand of DNA!
Base-Pairing Practice
Write the missing bases on the strand of DNA! G C T A C

17. Question: a. Thymine is present? Answer: 30% b. Guanine is present?
Remember that %A = %T and %G = %C . If there is 30% Adenine in a section of DNA, how much…
a. Thymine is present?
Answer: 30%
b. Guanine is present?
Answer: 20%
c. Cytosine is present?

18. Guanine (20%) = Cytosine (20%) (50%) = (50%)
Adenine (30%) = Thymine (30%)
Guanine (20%) = Cytosine (20%)
(50%) = (50%)