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DNA Information and Heredity, Cellular Basis of Life Chapter 12(M)

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Presentation on theme: "DNA Information and Heredity, Cellular Basis of Life Chapter 12(M)"— Presentation transcript:

1 DNA Information and Heredity, Cellular Basis of Life Chapter 12(M)

2 Genes are made of DNA The following scientists proved that DNA is the genetic material Fredrick Griffith (1928) Oswald Avery ( 1944) Hershey and Chase (1952)

3 Fredrick Griffith (1928) The discovery of the genetic role of DNA began with research by Frederick Griffith. He studied Streptococcus pneumoniae, a bacterium that causes pneumonia in mammals.  One strain, the R strain, was harmless.  The other strain, the S strain, was pathogenic.

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8 When Griffith mixed heat-killed S strain with live R strain bacteria and injected this into a mouse it died. He recovered the pathogenic strain from the dead mouse’s blood. Some harmless bacteria had been “transformed” into the deadly strain For the next 14 years scientists tried to identify the transforming substance.

9 Transforming Substance? Could be:  DNA  Proteins Because scientists already knew chromosomes consist of these substances. So the debate started.

10 Oswald Avery ( 1944) Treated Griffiths mixture with  Protein digesting enzymes  remove all proteins  DNA digesting enzymes  remove all DNA

11 Avery contd. Is Protein the transforming factor?  treated Griffith’s mixture of heat treated deadly strain and live harmless strains with protein-destroying enzymes  grew the strains  The bacterial colonies were still transformed Concluded that protein could not be the transforming factor

12 Avery contd. Is DNA the transforming factor?  treated the mixture with DNA- destroying enzymes  grew the strains  The bacterial colonies failed to transform Concluded that DNA is the genetic material of the cell Scientists were still skeptical  proteins made of 20 AAs, DNA only 4 bases

13 Hershey and Chase (1952) Used viruses to prove that DNA is the genetic material.  Viruses consist of a DNA (sometimes RNA) enclosed by a protective coat of protein.  To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery.  Viruses that specifically attack bacteria are called bacteriophages or just phages.

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16 Conclusion Phage DNA entered the bacterial cell, proteins did not DNA carries the genetic information.

17 The Role of DNA Storing Information  The genetic material stores information needed by every living cell Copying Information  before a cell divides this info must be copied Transmitting Information  Each daughter cell must receive a complete copy of all the information

18 Structure of DNA Ch 12.2

19 Building Blocks Of DNA Nucleotides A ring-shaped sugar called deoxyribose A phosphate group (a phosphorus atom surrounded by four oxygen atoms) A nitrogenous base ("nitrogen-containing") : a single or double ring of carbon and nitrogen atoms with functional groupsnitrogenous base

20 Nitrogenous Bases The four nucleotides in DNA differ only in their nitrogenous bases Bases:  Thymine (T)  single ring  Cytosine (C)  single ring  Adenine (A)  double ring  Guanine (G)  double ring

21 Bases

22 Structure of DNA Early 1950s R. Franklin DNA  helix with2 strands with the phosphate linked to the sugar, diameter is 2nm, each turn has 10 bases Watson & Crick (1953) Using Franklins work built a model of DNA Each strand is complementary to the other A pairs with T, G with C

23 Base Pairing Chargaff’s Rule  Adenine forms a base pair with Thymine  Guanine forms a base pair with Cytosine  Amounts are about the same

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25 The Double Helix

26 Figure 16.5 The double helix

27 DNA Strands Two strands  double helix Compared to a ladder Sides  Sugar phosphate backbone Rungs  pairs of nitrogenous bases Order of bases  genetic code Difference in order gives individuality to each living organism Base pairing is the key that allows DNA to be copied A pairs with T, G with C

28 Replication of DNA Ch 12.3

29 Replication Complete set of genetic instructions passes from one generation to the next The DNA molecule must be copied  Base pairing allows DNA to be copied  DNA  2 strands, one is used as a template Replication  Process by which DNA is copied

30 Mechanism of Replication Takes place in the nucleus DNA untwists & both strands are replicated almost simultaneously 50 bases /sec are added The parent DNA strands serve as a template for making a new strand

31 DNA polymerase unzips the 2 strands Each strand is used as a template Free nucleotides present in the nucleus, pair with the exposed bases A with T, G with C As bases pair DNA Ligase an enzyme links the phosphate of each nucleotide to the sugar of the previous one Pairing & bonding continue till 2 new strands are formed.

32 Enzyme works from 5’  3’ Synthesis is anti parallel 5’  3’ and 3’  5’ As bases pair DNA Ligase an enzyme links the phosphate of each nucleotide to the sugar of the previous one Pairing & bonding continue till 2 new strands are formed.

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35 DNA Replication is Semi conservative

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