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Lesson Overview 12.1 Identifying the Substance of Genes.

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1 Lesson Overview 12.1 Identifying the Substance of Genes

2 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 bacteriaStreptococcus pneumonia –R-strain  rough strain; harmless or avirulent –S-strain  smooth strain; harmful or virulent

3 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes Transformation

6 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 1.Extracted a mixture of various molecules from the heat- killed bacteria. 2.Treated this mixture with enzymes that destroyed proteins, lipids, carbohydrates, and some other molecules, including the nucleic acid RNA. 3.Transformation still occurred.

7 Lesson Overview Lesson Overview Identifying the Substance of Genes AVERY’S EXPERIMENT cont… 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.

8 Lesson Overview Lesson Overview Identifying the Substance of Genes CHECKING FOR UNDERSTANDING 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. YkVdM Hershey and Chase Experiment 1min. 50 secs.

10 Lesson Overview Lesson Overview Identifying the Substance of Genes Bacteriophages-bacteria eater –Virus that infects bacteria  bacteriophage. which means “bacteria eater.” –composed of a DNA core and a protein coat

11 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 uxGuT7H8

12 Lesson Overview Lesson Overview Identifying the Substance of Genes 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: 1.The pair grew viruses in cultures containing radioactive isotopes of phosphorus-32 ( 32 P) and sulfur-35 ( 35 S) P attaches to the DNA while 35 S attaches to the protein coat. 3.The two scientists mixed the marked viruses with bacterial cells. 4.They waited a few minutes for the viruses to inject their genetic material.

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

14 Lesson Overview Lesson Overview Identifying the Substance of Genes

15 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Identifying the Substance of Genes Lesson Overview 12.2 The Structure of DNA

18 Lesson Overview Lesson Overview Identifying the Substance of Genes The Components of DNA –DNA is a nucleic acid

19 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes Nitrogenous Bases and Covalent Bonds –DNA has four kinds of nitrogenous bases: –adenine (A), –guanine (G), –cytosine (C), –thymine (T).

21 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes Erwin Chargaff –Erwin Chargaff discovered that [A] = [T] and [G] = [C]  This is known as one of “Chargaff’s rules.”

23 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes The Double-Helix Model : Antiparallel Strands –In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions.

25 Lesson Overview Lesson Overview Identifying the Substance of Genes Hydrogen Bonding & Base Pairing –Hydrogen bonds would form only between certain base pairs— adenine with thymine, guanine with cytosine.

26 Lesson Overview Lesson Overview Identifying the Substance of Genes Structure of DNA

27 Lesson Overview Lesson Overview Identifying the Substance of Genes Lesson Overview 12.3 DNA Replication

28 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes –The result of replication is two DNA molecules identical to each other and to the original molecule. The Replication Process

30 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes DNA Replication Video

32 Lesson Overview Lesson Overview Identifying the Substance of Genes 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 Lesson Overview Lesson Overview Identifying the Substance of Genes 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. kYnp0 Prokaryotic DNA Replication Animation

34 Lesson Overview Lesson Overview Identifying the Substance of Genes 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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