1. 12–1 DNA
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.

2. Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia.
He isolated two different strains of pneumonia bacteria from mice and grew them in his lab.

3. Griffith and Transformation
Griffith made two observations:
(1) The disease-causing strain of bacteria grew into smooth colonies on culture plates.
(2) The harmless strain grew into colonies with rough edges.

4. Griffith and Transformation
Griffith's Experiments
Griffith set up four individual experiments.
Experiment 1: Mice were injected with the disease-causing strain of bacteria. The mice developed pneumonia and died.
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.

5. Griffith and Transformation
Experiment 2: Mice were injected with the harmless strain of bacteria. These mice didn’t get sick.
Harmless bacteria (rough colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Lives

6. Griffith and Transformation
Experiment 3: Griffith heated the disease-causing bacteria. He then injected the heat-killed bacteria into the mice. The mice survived.
Heat-killed disease-causing bacteria (smooth colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Lives

7. Griffith and Transformation
Heat-killed disease-causing bacteria (smooth colonies)
Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died.
Harmless bacteria (rough colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Live disease-causing bacteria (smooth colonies)
Dies of pneumonia

8. Griffith and Transformation
Heat-killed disease-causing bacteria (smooth colonies)
Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.
Harmless bacteria (rough colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Live disease-causing bacteria (smooth colonies)
Dies of pneumonia

9. Griffith and Transformation
Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain).
Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones.

10. Avery and DNA Avery and DNA
Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation.
Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes.

11. Avery and DNA
The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA.
Transformation still occurred.

12. Avery and DNA
Avery and other scientists repeated the experiment using enzymes that would break down DNA.
When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor.

13. Avery and DNA
What did scientists discover about the relationship between genes and DNA?

14. Avery and DNA
Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

15. The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied viruses—nonliving particles smaller than a cell that can infect living organisms.

16. The Hershey-Chase Experiment
Bacteriophages 
A virus that infects bacteria is known as a bacteriophage.
Bacteriophages are composed of a DNA or RNA core and a protein coat.

17. The Hershey-Chase Experiment
When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.
The viral genes produce many new bacteriophages, which eventually destroy the bacterium.
When the cell splits open, hundreds of new viruses burst out.

18. The Hershey-Chase Experiment
If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA.
They grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

19. The Hershey-Chase Experiment
If 35S was found in the bacteria, it would mean that the viruses’ protein had been injected into the bacteria.
Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA into the bacteria, not proteins. From these results, Hershey and Chase concluded that the genetic material of the bacteriophage was DNA.
Phage infects bacterium
Radioactivity inside bacterium
Bacteriophage with
phosphorus-32 in DNA

20. The Hershey-Chase Experiment
If 32P was found in the bacteria, then it was the DNA that had been injected.
Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA into the bacteria, not proteins. From these results, Hershey and Chase concluded that the genetic material of the bacteriophage was DNA.
Phage infects bacterium
Radioactivity inside bacterium
Bacteriophage with
phosphorus-32 in DNA

21. The Hershey-Chase Experiment
Nearly all the radioactivity in the bacteria was from phosphorus (32P).
Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

22. The Components and Structure of DNA
DNA is made up of nucleotides.
nucleotide = a five-carbon sugar [deoxyribose]
a phosphate group
a nitrogenous base.

23. The Components and Structure of DNA
There are four kinds of bases in in DNA:
adenine
guanine
cytosine
thymine
The Components and Structure of DNA
DNA is made up of nucleotides. Each nucleotide has three parts: a deoxyribose molecule, a phosphate group, and a nitrogenous base. There are four different bases in DNA: adenine, guanine, cytosine, and thymine.

24. The Components and Structure of DNA
The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.
The nucleotides can be joined together in any order.

25. The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:
If you have G always Binding to C, then the amount of G should Equal C (or darn near close)
Same goes for A and T

26. The Components and Structure of DNA
X-Ray Evidence 
Rosalind Franklin
- Used X-ray Diffraction to study DNA Structure
This X-ray diffraction photograph of DNA was taken by Rosalind Franklin in the early 1950s. The X-shaped pattern in the center indicates that the structure of DNA is helical.
Photo credit: ©Cold Spring Harbor Laboratory Archives/Peter Arnold, Inc.

27. The Components and Structure of DNA
The Double Helix 
Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied.
Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.

28. DNA Double Helix
DNA is a double helix in which two strands are wound around each other. Each strand is made up of a chain of nucleotides. The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.

29. The Components and Structure of DNA
Watson and Crick discovered that hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine.
This principle is called base pairing.

30. 12-2 Chromosomes and DNA Replication
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.
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DNA and Chromosomes
DNA and Chromosomes
In prokaryotic cells, DNA is located in the cytoplasm.
Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information.
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DNA and Chromosomes
Chromosome
Most prokaryotes, such as this E. coli bacterium, have only a single circular chromosome. This chromosome holds most of the organism’s DNA.
E. Coli Bacterium
Bases on the Chromosomes
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DNA and Chromosomes
Many eukaryotes have 1000 times the amount of DNA as prokaryotes.
Eukaryotic DNA is located in the cell nucleus inside chromosomes.
The number of chromosomes varies widely from one species to the next.
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DNA and Chromosomes
Chromosome Structure
Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin.
Chromatin consists of DNA tightly coiled around proteins called histones.
DNA and histone molecules form nucleosomes.
Nucleosomes pack together, forming a thick fiber.
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DNA and Chromosomes
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA double helix
Coils
Supercoils
Eukaryotic chromosomes contain DNA wrapped around proteins called histones. The strands of nucleosomes are tightly coiled and supercoiled to form chromosomes.
Histones
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DNA Replication
DNA Replication
Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a single point and continues in two directions.
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DNA Replication
In eukaryotic chromosomes, DNA replication occurs at hundreds of places. Replication proceeds in both directions until each chromosome is completely copied.
The sites where separation and replication occur are called replication forks.
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DNA Replication
Duplicating DNA 
Before a cell divides, it duplicates its DNA in a process called replication.
Replication ensures that each resulting cell will have a complete set of DNA.
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DNA Replication
During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template for the new strand.
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New Strand
Original strand
Nitrogen Bases
Growth
Growth
During DNA replication, the DNA molecule produces two new complementary strands. Each strand of the double helix of DNA serves as a template for the new strand.
Replication Fork
Replication Fork
DNA Polymerase
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How Replication Occurs
DNA replication is carried out by enzymes that “unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken and the two strands of DNA unwind.
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DNA Replication
The principal enzyme involved in DNA replication is DNA polymerase.
DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand.
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