DNA Structure and Function

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DNA Structure and Function
The Structure, Replication and Repair of DNA

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life

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Key Concepts

• DNA contains the information of heritable traits
  in all cells
• Each DNA strand consists of two strands of
  nucleotides twisted together
• Hereditary information is encoded in the sequence
  of nucleotides
• Nitrogen bases have specific pairing arrangements
• DNA is replicated before a cell divides

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Key Questions

• How did biologists discover what genes were made
  of and what they did?
• What is the structure of DNA?
• How does DNAs structure allow it to act as a
  template for its own replications?
• What is a mutation and why are mutations
  important?

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Discovery of DNA Function

• Fred Griffith - Discovered hereditary
  transformation
• Oswald Avery - DNAase blocks transformation
• Hershey and Chase - Bacteriophage injects
  DNA--not protein--into bacterium

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Discovery of DNA Function

• Griffiths experiments
• Transfer of hereditary material from dead S cells
  to living R cells (transformation)

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Discovery of DNA FunctionHershey-Chase

• Radioisotope label protein and DNA of
  bacteriophage
• Radioactive phophorus is detected inside of
  bacteria

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DNA Structure

• Nucleotides
• Deoxyribose
• Phosphate Group
• Nitrogen base
• Adenine A
• Guanine G
• Thymine T
• Cytosine C

• Pairing Arrangement
• A - T
• C - G

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DNA long chain of NUCLEOTIDES
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Components of DNA

• Four types of nucleotides
• Adenine (A) (base with double ring)
• Guanine (G) (base with double ring)
• Thymine (T) (base with single ring)
• Cytosine (C) (base with single ring
• Amount of AT and CG

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Chargaffs Rules

• Found the proportions of the bases in many
  different species
• The amount of A is equal to T
• The amount of G is equal to C

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Patterns of Base Pairing

• DNA consists of two strands of nucleotides held
  together at bases by hydrogen bonds
• Two kinds of base pairs form
• A-T and C-G
• Base with double-ring structure always binds with
  base with single-ring structure
• The sugar and phosphate units are linked together
  alternately (sugar-phosphate-sugar-phosphate)
  into a backbone
• The 2 parallel backbones run in opposite
  directions

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What Is a Gene?

• To Mendel, in 1865, it was just an abstraction
• Places on chromosomes
• Pure information
• Important to understand genes in chemical terms

Using Chargaffs Rules

• Here is 1 side of the DNA strand
• AATCCGTATCGGCCTA
• What would the sequence of bases be for the
  complementary strand?

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• George Beadle and Edward Tatum,
• using Neurospora, showed that 1 gene
• could specify 1 enzyme

• Genes influence phenotype by specifying
  polypeptides

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DNA Replication and Repair

• Enzyme regulated
• Hydrogen bonds break
• Attachment of nucleotides to new strands
• DNA polymerases DNA ligases
• New strand is half old, half new

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Deeper Look at Replication

• Strand assembly is continuous on one strand,
  discontinuous on other
• Orientation of 5-C sugar determines direction of
  assembly - Nucleotides can only be added in the
  5 to 3 direction
• Semiconservative replication
• Each half of the double helix acquires a new mate
• Each new DNA molecule, then, is really half old
  and half new

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DNA Polymerization
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Creating Clones

• Dolly the sheep was first mammal to be cloned
  from a differentiated cell
• Nucleus from sheep udder cell was transferred
  into enucleated unfertilized egg
• Egg grew into sheep by mitotic divisions
• Mice and cows have now also been cloned from
  adult cells

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In Conclusion

• Hereditary information is located in DNA
• DNA consists of nucleotides
• DNA molecule consists of two nucleotide strands
  twisted into a double helix
• The bases of DNA strands pair in a constant
  fashion
• DNA of one species has specific nucleotide
  sequences

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In Conclusion

• Enzymes unwind the two strands of DNA and
  assemble a new strand during DNA replication
• Resulting new DNA molecule has an old strand and
  a new strand
• Some of the enzymes in DNA replication also
  repair DNA