1. Discovering DNA: Structure and Replication

2. A collaborative effort!
The discovery of DNA resulted from the combination of contributions from several scientists.
Each conducted experiments that provided different pieces of information needed to solve the puzzle of the role and structure of DNA.

3. Good things happen when you’re not looking…….
Griffith’s Experiment
1928 – Frederick Griffith conducted experiments with mice to study the effects of pneumonia bacteria.
Griffith isolated two strains of pneumonia bacteria – one caused the lung disease (pneumonia) and the other did not.

4. Griffith’s Experiment
Mice injected with the disease causing bacteria died.
Mice injected with the harmless bacteria lived.
Mice injected with the heat-killed disease causing bacteria also lived.
Griffith then mixed the live harmless bacteria with heat-killed disease causing bacteria.
Mice injected with this combination died.

5. Griffith’s Experiment

6. Based on his results, Griffith hypothesized that when the harmless and heat-killed bacteria where mixed, some factor was exchanged between them, making the live harmless bacteria deadly.
Transformation – process in which one strain of bacteria is changed by the gene(s) of another bacteria

7. Avery modifies Griffith’s experiment.
In 1943, Avery, with fellow scientists, conducted an experiment similar to Griffith’s, except they used enzymes to selectively destroy molecules one at a time.
When they injected harmless bacteria with only lipids, carbs, or proteins: transformation did not occur.
When they used the nucleic acids (DNA): transformation did occur, the bacteria became lethal.
This helped to determine that DNA stores and transmits genetic information.

8. Hershey and Chase
Hershey and Chase (1950) conducted experiments with bacteriophages (viruses that attack bacteria) to determine if genetic information is carried on proteins or DNA.
They used radioactive elements to ‘mark’ DNA and protein.
Only the radioactively-labeled DNA was found in bacteria cells.
These findings further supported the conclusions of Avery’s experiment & specified that genetic material is DNA and NOT protein.

9. Chargaff’s Rule Chargaff determined that in any sample of DNA:
The # of adenines (A) = the # of thymines (T)‏
The # of cytosines (C) = the # of guanines (G)‏
Thus in DNA, the bases A and T pair together, and C and G pair together.
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10. Rosalind Franklin
. Franklin used x-ray diffraction to create pictures of DNA’s molecular structure

11. Watson and Crick
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James Watson and Frances Crick determined the structure of DNA in 1953 using their data and the work of previous scientists.
Watson got a sneak peak at Franklin’s X-ray images and used them with other evidence to determine DNA’s structure.
The structure of DNA was determined to be shaped like a double helix, with strands held together by the weak hydrogen bonds formed between the bases A-T and C- G.

12. What is DNA? DNA = Deoxyribose Nucleic Acid
Code of Life – codes for proteins essential to the health of the organism.
Backbone made of alternating Phosphorus and Sugars
Made of matching Nitrogen bases
Adenine to Tyrosine
Cytosine to Guanine

13. What is the structure of DNA?
DNA is composed of Nitrogen bases, (ATGC) anchored to a deoxyribose sugar along a Phosphate backbone.
Nucleotides = 5 carbon sugar, phosphate and a Nitrogen base.
Each nitrogen base is bonded by weak Hydrogen Bonds.
Those bonds are weak for DNA to replicate or transcribe.

14. The Structure of DNA On the diagram: Circle and label a nucleotide.
Sugar /P
backbone
Sugar /P
backbone
Base pair
On the diagram:
Circle and label a nucleotide.
Label the sugar and phosphate molecules.
Label the bases that are not already labeled
Label a base pair.
Label the sugar- phosphate backbones.
Label the hydrogen bonds. A
Hydrogen bonds C G P A T S A T G C G
nucleotide

15. Base Pairing Practice
For each example below, please give the correct complementary strand of DNA.
T T G C T A G
A A C G A T C
T A G C G C T
A T C G C G A
A C C G T C A
T G G C A G T
G C T A T G T
C G A T A C A

16. DNA Replication
DNA must be replicated (copied) in order to insure that during cell division, each daughter cell receives a complete copy.
DNA replication occurs in the nucleus during S phase of the cell cycle, before chromatin (DNA wrapped around proteins) condenses into chromosomes.
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17. DNA Replication DNA Replication occurs in four basic steps:
Step 1 – Helicase unzips the strand of DNA by breaking the weak hydrogen bonds between base pairs.
Step 2 – DNA polymerase inserts the appropriate bases.
Step 3 – A new sugar-phosphate backbone is built.
Step 4 – The sequence is proofread by DNA polymerase.
DNA replication is semi- conservative because each side of the parent strand serves as a template for the 2 new DNA strands.