DNA Information and Heredity, Cellular Basis of Life

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DNAInformation and Heredity, Cellular Basis of
Life

• Chapter 12(M)

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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)

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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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• 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
  mouses blood.
• Some harmless bacteria had been transformed
  into the deadly strain
• For the next 14 years scientists tried to
  identify the transforming substance.

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Transforming Substance?

• Could be
• DNA
• Proteins
• Because scientists already knew chromosomes
  consist of these substances. So the debate
  started.

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Oswald Avery ( 1944)

• Treated Griffiths mixture with
• Protein digesting enzymes ? remove all proteins
• DNA digesting enzymes? remove all DNA

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Avery contd.

• Is Protein the transforming factor?
• treated Griffiths 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

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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

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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 cells metabolic machinery.
• Viruses that specifically attack bacteria are
  called bacteriophages or just phages.

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Conclusion

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

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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

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Structure of DNA

• Ch 12.2

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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 groups

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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

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Bases
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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

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Base Pairing

• Chargaffs Rule
• Adenine forms a base pair with Thymine
• Guanine forms a base pair with Cytosine
• Amounts are about the same

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The Double Helix
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Figure 16.5 The double helix
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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

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Replication of DNA

• Ch 12.3

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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

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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

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• 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.

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• 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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DNA Replication is Semi conservative
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