1. DNA and its Structure

2. Study Quizlet!
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3. Vocabulary Nucleotide Deoxyribose Nitrogenous base Purine Pyrimidine
Base-pairing rules
Complementary Base pair
Base Sequence

4. (Deoxyribonucleic acid)
The Structure of DNA
(Deoxyribonucleic acid)
Where is DNA found?
Eukaryotes: nucleus, chromosomes
Prokaryotes: cytoplasm
DNA is a polymer made up of many repeating single units (monomers) These monomers are called nucleotides
Each nucleotide is made up of three things:
5-carbon sugar (deoxyribose)
Phosphate group
Nitrogenous base
The “backbone” of DNA molecules is made of phosphate and sugar

5. Four Nitrogenous Bases
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Chargaff’s Rule: Organisms should have 1:1 ratio purines and pyrimidines(base pair rule)
A=T and G=C

6. The Double Helix
1950’s Rosalind Franklin used X-ray diffraction to study DNA
Credit for the double helix was given to James Watson and Francis Crick
Only Watson and Crick received the Nobel Prize in 1962.

7. The DNA molecule appears as a “twisted ladder”
This shape is referred to as a double helix
The phosphates and the sugars form the “handrails”
of the helix with covalent bonds, and the hydrogen
bonds are in the “steps” of the DNA helix.

9. Why is complementary base pairing important?
Hydrogen bonds between the base pairs help
hold the two strands together.
2) The complementary nature of DNA helps
explain how DNA replicates before a cell
divides.

10. Anti-parallel strands
Video
Anti-parallel strands
We refer to DNA as being anti-parallel because the sides of the DNA are going in opposite directions.
One side is travelling 5’ to 3’ 5 3
While the other side is travelling 3’ to 5’. 5 3

11. Bottom strand is opposite. It moves 3’ 5’
Looking within the sugars you can see there is an intrinsic difference between the two strands.
Top strand is a 5’ carbon of each sugar is on the left and the 3’ carbon the right. Thus, it moves left to right 5’ to 3’
Bottom strand is opposite. It moves 3’ 5’

12. Nitrogenous Bases

14. DNA vs. RNA DNA RNA Sugar = deoxyribose Sugar = ribose
DRAW
DNA vs. RNA
DNA
RNA
Sugar = deoxyribose
Sugar = ribose
Double-stranded molecule
Single-stranded molecule
Thymine bonds with adenine
Uracil instead of thymine

15. Elbow Partner Group Work
1. Use the base-pairing rules to determine the base sequence that is complementary to the sequence below C-G-A-T-T-G 2. Distinguish between purines and pyrimidines. 3. A plant’s DNA has nucleotides that are 20% thymine. What percentage of guanine would be present?

16. Homework:
1. What piece of information did Rosalind Franklin have that helped Watson
and Crick determine the double helix structure of DNA?
2. Name the three parts of a nucleotide.
3. Summarize the locations of covalent bonds and hydrogen bonds in a DNA
molecule.
4. Describe why the two strands of the double helix are considered to be
complementary.
5. State the base-paring rule in DNA.
6. How do the base-pairing rules relate to the structure of DNA?