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12.1 Identifying the Substance of Genes

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1 12.1 Identifying the Substance of Genes
Lesson Overview 12.1 Identifying the Substance of Genes

2 Griffith’s Experiments
The discovery of the chemical nature of the gene began in 1928 with British scientist Frederick Griffith, who was trying to figure out how certain types of bacteria produce pneumonia. He used two strains or types of  Streptococcus pneumonia bacteria R-strain rough strain; harmless or avirulent S-strain smooth strain; harmful or virulent

3 Griffith’s Experiments
R-strain + mouse = mouse lives S-strain + mouse = mouse dies Heat killed S-strain + mouse = mouse lives R- strain + heat killed S-strain + mouse = mouse dies

4 Summary of Griffith’s Experiment
He concluded that the harmless R-strain of bacteria was changed into a disease causing bacteria by the DNA of the dead harmful S-strain. He called this process transformation, because one type of bacteria had been changed permanently into another. Because the ability to cause disease was inherited by the offspring of the transformed bacteria, Griffith concluded that the transforming factor had to be a gene.

5 Transformation

6 The Molecular Cause of Transformation
Canadian biologist Oswald Avery & other scientists at the Rockefeller Institute in New York, wanted to determine which molecule in the heat-killed bacteria was most important for transformation. AVERY’S EXPERIMENT Extracted a mixture of various molecules from the heat-killed bacteria. Treated this mixture with enzymes that destroyed proteins, lipids, carbohydrates, and some other molecules, including the nucleic acid RNA. Transformation still occurred.

4. Avery and his team repeated the experiment one more time and used enzymes that would break down DNA RESULT: Transformation did not occur when the DNA was destroyed CONCLUSION: DNA was the transforming factor.

What is bacterial transformation? What conclusion did Frederick Griffith draw from his experimental results? What conclusion did Oswald Avery draw from his experimental results?

9 Bacterial Viruses Alfred Hershey and Martha Chase performed the most important of the experiments relating to Avery’s discovery. Hershey and Chase studied viruses—nonliving particles that can infect living cells. Hershey and Chase Experiment 1min. 50 secs.

10 Bacteriophages-bacteria eater
Virus that infects bacteria bacteriophage. which means “bacteria eater.” composed of a DNA core and a protein coat

11 How do bacteriophages infect bacteria?
When a bacteriophage enters a bacterium, it attaches the bacterial cell and injects its genetic information into it. The viral genes act to produce many new bacteriophages, which gradually destroy the bacterium. When the cell splits open, hundreds of new viruses burst out. BACTERIOPHAGE VIDEO

12 The Hershey-Chase Experiment
They wanted to determine which part of the virus entered the bacterial cell—was it the protein coat or the DNA core? THE EXPERIMENT: The pair grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S). 32P attaches to the DNA while 35S attaches to the protein coat. The two scientists mixed the marked viruses with bacterial cells. They waited a few minutes for the viruses to inject their genetic material.

13 Hershey and Chase Experiment cont.
5. Next, they separated the viruses from the bacteria and tested the bacteria for radioactivity (32P or 35S) 6. If they found radioactivity from 35S in the bacteria, it would mean that the virus’s protein coat had been injected into the bacteria. 7. If they found 32P, then the DNA core had been injected. RESULTS: Nearly all the radioactivity in the bacteria was from phosphorus (32P), the marker found in DNA. CONCLUSION: They concluded that the genetic material of the bacteriophage was DNA, not protein.


15 The Role of DNA The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell. The foremost job of DNA, as the molecule of heredity, is to store information. Before a cell divides, it must make a complete copy of every one of its genes, similar to the way that a book is copied.

16 Transmitting Information
Each complete copy is given to each daughter cell. The loss of any DNA during meiosis might mean a loss of valuable genetic information from one generation to the next.

17 Lesson Overview 12.2 The Structure of DNA

18 The Components of DNA DNA is a nucleic acid

19 Nucleic Acids and Nucleotides
Nucleic acids are long, slightly acidic molecules originally identified in cell nuclei. Nucleic acids are made up of nucleotides, linked together by covalent bonds to form long chains. DNA’s nucleotides are made up of three basic components: a nitrogenous base a 5-carbon sugar called deoxyribose, a phosphate group

20 Nitrogenous Bases and Covalent Bonds
DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), thymine (T).

21 Franklin’s X-Rays In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

22 Erwin Chargaff Erwin Chargaff discovered that
[A] = [T] and [G] = [C] This is known as one of “Chargaff’s rules.”

23 Watson & Crick James Watson, an American biologist, and Francis Crick, a British physicist, used Franklin’s X-ray pattern to build a 3D model of DNA. The work of Franklin and Chargaff helped Watson and Crick build the 3D model of DNA

24 The Double-Helix Model : Antiparallel Strands
In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions.

25 Hydrogen Bonding & Base Pairing
Hydrogen bonds would form only between certain base pairs— adenine with thymine, guanine with cytosine.

26 Structure of DNA

27 Lesson Overview 12.3 DNA Replication

28 The Replication Process
DNA replication is a process where DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. A=T & C=G Each strand of the double helix of DNA serves as a template, or model, for the new strand.

29 The Replication Process
The result of replication is two DNA molecules identical to each other and to the original molecule.

30 The Role of Enzymes The principal enzyme involved in DNA replication is called DNA polymerase. It joins individual nucleotides to produce a new strand of DNA. It “proofreads” each new DNA strand, ensuring that each molecule is a perfect copy of the original.

31 DNA Replication Video

32 Telomeres The tips of chromosomes are known as telomeres.
Telomeres are difficult to copy. Over time, DNA may be lost from telomeres each time a chromosome is replicated. An enzyme called telomerase compensates for this problem by adding short, repeated DNA sequences to telomeres, this Lengthens the chromosomes slightly and making it less likely that important gene sequences will be lost from the telomeres during replication.

33 Prokaryotic DNA Replication
Replication in most prokaryotic cells starts from a single point and proceeds in two opposite directions until the entire chromosome is copied. Prokaryotic DNA Replication Animation

34 Eukaryotic DNA Replication
Eukaryotic chromosomes are much bigger than those of prokaryotes. In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

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