1. DNA Introduction
Honors

2. A gene is a segment of DNA that encodes a protein
Genes vary in size, from just a few thousand pairs of nucleotides to over 2 million!
You have about 6 billion nucleotides in each of your cells. This means in your DNA there is roughly 30,000 genes!

3. Hershey-Chase Experiment (1952)
Back in the day, the question was: is the genetic material of organisms made of DNA or proteins?
Hershey-Chase Experiment (1952)
Proved that the genetic material that infects bacteria is found in the viral DNA
Bacteriophage “phage”: virus that infects bacteria.
Composed of DNA and protein
Use of Radioactive Trackers
Injected radioactive markers in the virus…markers in phosphorus (not in protein) and sulfur (not in DNA)
If 32 P were found in bacteria (DNA infects the bacteria)
If 35 S were found in bacteria (protein infects the bacteria

4. Animation Radioactivity in liquid Radioactivity in pellet 1
Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. 2 2
Agitate in a blender to separate phages outside the bacteria from the cells and their contents. 3 3
Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4
Measure the radioactivity in the pellet and liquid.
Radioactive protein
Empty protein shell
Radioactivity in liquid
Phage
Bacterium
Phage DNA
DNA
Batch 1 Radioactive protein
Centrifuge
Pellet
Hershey Chase wanted to discover which part of the virus, the protein coat or the DNA core infects cells (were genes made of protein or DNA)
Injected radioactive markers in the virus…markers in phosphorus (not in protein) and sulfur (not in DNA)
If 32 P were found in bacteria (DNA infects the bacteria)
If 35 S were found in bacteria (protein infects the bacteria)
Measured the radioactivity in the PELLETS (which were the bacteria when centrifuged and blended)
Radioactive DNA
Batch 2 Radioactive DNA
Radioactivity in pellet
Pellet

5. Overall Conclusion:
Genetic information is stored in and passed to subsequent generations through DNA molecules

6. Structure of DNA

7. Rosalind Franklin: Photo 51
The X in the center of photo 51 was caused by the helical shape of the DNA molecules in the sample.

8. DNA Structure History
James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

9. DNA is a Nucleic Acid
Made up of long chains (polymers) of nucleotides (monomers)

10. Basic Structure of a Nucleotide
3 components to all nucleotides
Pentose sugar
Deoxyribose or ribose
Phosphate group
Nitrogenous Bases
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Uracil (U)
Nucleic acid: nuclei (within the nucleus of cells)
Deoxyribose- sugar that is different from ribose because it lacks an oxygen (deoxy)
Phosphate group is acidic component. Nucleic ACID

11. Back Bone of DNA
Sugar and Phosphate groups make up the backbone of DNA. They are covalently held together via
Phosphodiester bonds
The nitrogenous bases are on the inside and hold the two strands of the double helix together via
Hydrogen bonds
Each phosphate group links the 3’ carbon of one sugar to the 5’ carbon of the next sugar along the backbone
Pairs of complementary bases from hydrogen bonds that hold the two strands of the DNA double helix together

12. Nucleotides 4 nucleotides differ only in their nitrogenous bases
Adenine (A) and Guanine (G) are purines
Thymine (T) and Cytosine (C) are pyrimidines
Nucleotides are joined together y covalent bonds between sugar of one and the phosphate of the next = sugar-phosphate backbone.
Thymine (T)
Cytosine (C)
Adenine (A)
Guanine (G)
Pyrimidines
Purines

13. Hydrogen bonds hold together the nitrogenous bases in the DNA double helix
Always pyrimidine with purine
If not, the diameter of double helix would not be equal
An A-A pair would be twice as wide as a C-C pair
Each base has chemical side groups that can best form hydrogen bonds with one appropriate partner
Which pair do you think is stronger?
C-G have 3 hydrogen bonds and are therefore stronger than A-T

14. Chargaff’s Rules Adenine and Guanine are purines = 2 ringed
Thymine and Cytosine are pyrimidines = 1 ring
Chargaff’s Rules

15. Amount of adenine always equal to the amount of thymine
If 30% of the nucleotide bases are thymine. How many nucleotide bases are guanine?
If there are 30% T then there are 30% A = 60%
100%-60% = 40%
40%= C + G
G = 20%

16. DNA Strand Orientation
Each carbon in the nucleotide is numbered
Nitrogenous base is on 1’
Phosphate group is on 5’
3’ binds to phosphate group of another nucleotide
Complementary strands are antiparallel
Phosphate end = 5’ end
Sugar (-OH) end = 3’ end
One strand runs 5’3’, the other 3’5’

17. Structure of DNA Nucleotide Hydrogen bonds Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)