1. DNA: The Genetic Material
Biology
Ch. 9
Ms. Haut

2. Identifying the Genetic Material
Mendel’s experiments—inherit chromosomes that contain genes
The Question now:
What are genes made of?
Scientists searching for the answer:
Griffith and Avery
Hershey and Chase

3. Introduction
Basics of molecular biology began with the study of bacteriophages (viruses that infect bacteria)
DNA—deoxyribonucleic acid and RNA—ribonucleic acids were identified through various experiments
Polymers of nucleotides
Actual structure of these molecules not discovered until 1953

4. Griffith-Avery Experiment: Transformation of Bacteria
2 strains of Streptococcus pneumonia
--S strain—pathogenic (disease-causing)
--R strain—non-pathogenic
-Griffith found that if he mixed heat-killed S-strain bacteria with the harmless strain, some of the living cells were converted to the pathogenic form (TRANSFORMATION—assimilation of external DNA)
-Avery purified various chemicals from the heat-killed pathogenic bacteria, only DNA worked to transform the cells
-Discovery met with criticism—most still believe protein is the hereditary material
Controls

5. Hershey-Chase Experiment: DNA, the Hereditary Material in Viruses
Bacteriophages—viruses that infect bacteria
--attach to host cell and inject their DNA into the cell
--experiment demonstrated that it was DNA, not protein, that functioned as the phage’s genetic material
Radioactively-labeled (sulfur) viral proteins remained outside the host cell during infection
Radioactively-labeled (phosphorus) viral DNA traveled inside the host cell
-the injected DNA molecules cause the cells to to produce new viral DNA and proteins

6. Nucleotides of DNA
Nucleotides are the monomeric units that make up DNA
3 main parts:
5 carbon sugar—deoxyribose
Phosphate group
Nitrogenous base

7. Nitrogenous bases Pyrimidines: single-ring structures
Thymine (T)
Cytosine (C)
Purines: larger, double-ring structures
Adenine (A)
Guanine (G)

8. Discovery of the Double Helix
1953—James Watson and Francis Crick determined the structure of the DNA molecule to be a double helix
2 strands of nucleotides twisted around each other

9. Rosalind Franklin contributed to this discovery by producing an X-ray crystallographic picture of DNA
Determined helix was a uniform diameter and composed of 2 strands of stacked nucleotides

11. Sugar-Phosphate backbones of DNA oriented in opposite direction
Base-Pairing Rule: a purine forms a hydrogen bond with a pyrimidine adenine=thymine
guaninecytosine

13. Structure of DNA is related to 2 primary functions:
1. Copy itself exactly for new cells that are created
2. Store and use information to direct cell activities

14. Complementary Strands of DNA
If one strand is known, the other strand can be
determined 3’
= T 5’ A C G T
 G
 C
 C
= A
= T
= A
 G
 G 3’ 5’

15. DNA Replication
Replication occurs simultaneously at many sites (replication bubbles) on a double helix
Allows DNA replication to occur in a shorter period of time

17. DNA Replication DNA helicases unwind the double helix
DNA polymerases can only attach nucleotides to the 3’ end of a growing daughter strand
Thus, replication always proceeds in the 5’ to 3’ direction

18. DNA Replication
Within the replication bubbles, one daughter strand is made continuously while the other daughter strand must be made in short pieces which are then joined together by DNA ligase
These short pieces of DNA are called Okazaki fragments
Overall Direction of Replication-5’ to 3’

19. DNA Replication
This replication process assures that daughter cells will carry the same genetic information as each other and as the parent cell.
Each daughter DNA has one old strand of DNA and one new strand of DNA

20. DNA Replication

22. Checking for Errors
1/1,000,000,000 chance of an error in DNA replication
Can lead to mutations
DNA polymerases have a “proofreading” role
Can only add nucleotide to a growing strand if the previous nucleotide is correctly paired to its complementary base
If mistake happens, DNA polymerase backtracks, removes the incorrect nucleotide, and replaces it with the correct base