Lesson Overview Identifying the Substance of Genes

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1 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(no capsule); harmless or avirulent S-strain smooth strain(capsule); harmful or virulent

3 Griffith s Experiments 1. Live R-strain + mouse = mouse lives 2. Live S-strain + mouse = mouse dies 3. Heat-killed S-strain + mouse = mouse lives 4. Live 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 heatkilled bacteria was most important for transformation. AVERY S EXPERIMENT 1. Extracted a mixture of various molecules from the heatkilled 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 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 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 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, a. it injects its genetic information into it, b. uses its viral genes to produce many new bacteriophages, which gradually destroy the bacterium, c. splits the cell open, hundreds of new viruses burst out.

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: 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. 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.

13 Hershey and Chase Experiment cont. 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.


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 long, slightly acidic molecules (1 st identified in the nucleus). Nucleic acids are made up of nucleotides, linked together by covalent bonds. 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 Original DNA 1- ATT CCG TTA GAT Original DNA 2- TAA GGC AAT CTA

30 Original DNA 1- ATT CCG TTA GAT Original DNA 2- TAA GGC AAT CTA Original DNA 1- ATT CCG TTA GAT New DNA strand- New DNA strand- Original DNA 2- TAA GGC AAT CTA

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

32 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.

33 DNA helicase Unwinds the double helix of the DNA in order to provide a single-stranded DNA for replication,

34 Semi Conservative Replication of DNA This means that each daughter DNA strand contains one strand from the parent and one that is newly synthesized.

35 DNA Replication Video

36 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. Telomeres

37 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 kynp0

38 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.