DNA Structure

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DNA Structure

• Thursday, July 31

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Nucleic Acids polymers of nucleotides

• DNA deoxyribonucleic acid
• Directs DNA replication
• Directs RNA synthesis (transcription)
• RNA ribonucleic acid
• Directs protein synthesis (translation)
• Flow of genetic information
• DNA ? RNA ? protein
• Nucleus ? Cytoplasm

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Figure 5.28 DNA ? RNA ? protein overview of
information flow in a cell
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Nucleotides

• Nitrogenous base (organic molecule of C N)
• Pyrimidines 6-membered ring
• Cytosine (C), Thymine (T), and Uracil (U)
• Purines 6-membered ring fused to a 5-membered
  ring
• Adenine (A) and Guanine (G)
• Pentose (five carbon sugar)
• Ribose in RNA
• Deoxyribose in DNA (lacks an oxygen atom on C2)
• Phosphate group (on the C5 of the sugar)

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Figure 5.29 The components of nucleic acids
Nucleotides are joined by phosphodiester bonds
between phosphates and sugars
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Nucleic Acid Structure

• RNA is a single stranded polynucleotide chain
• DNA is double stranded where the two chains
  spiral around an imaginary axis, forming a double
  helix
• The sugar-phosphate backbones are on the outside
  of the helix, while the bases are stacked and
  paired on the inside
• The two strands are held together by hydrogen
  bonds between the paired bases and van der Waals
  interactions between the stacked bases

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Figure 16.3 The structure of a DNA strand
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Figure 16.5 The double helix
The helix is right-handed, meaning the direction
of the spiral curves to the right. The helix
makes one full turn every 3.4 nm along its
length Bases are stacked 0.34 nm apart, allowing
ten layers of base pairs per turn
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Composition of DNA Chargaffs Rule

• Base composition varies between species
• Number of adenines always equaled the number of
  thymines (AT)
• Number of guanines always equaled the number of
  cytosines (GC)
• Humans DNA
• A 30.9, T 29.4
• G 19.9, C 19.8

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Figure 16.6 Base pairing in DNA
Purines pair with pyrimidines The helix has a
2nm diameter which corresponds in width to one
purine and one pyrimidine. Further specificity
in pairing comes from the hydrogen bonds able to
form between A-T and G-C
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Figure 5.30 The DNA double helix
The sequence of the bases along the length of the
strand encode information for the synthesis of
proteins. The strands are complementary in
sequence ACCAGTTGAA on one strand pairs with
TGGTCAACTT on the other strand
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Figure 5.30 The DNA double helix and its
replication
The complementarity of the two strands allows for
precise copying of the DNA into new strands of
DNA (replication) or RNA (transcription) The
strands serve as templates for synthesizing new
strands of identical sequence.
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Genetic Material

• Hereditary material reqirements
• Great heterogeneity
• Specificity of function
• Protein or DNA??
• Worked out in bacteria and viruses

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Figure 16.1 Transformation of bacteria (Griffith)

• Streptococcus pneumoniae 2 strains
• Pathogenic (S strain)
• Nonpathogenic (R strain)

Some pathogenicity factor produced by the S
strain transformed the R strain to the pathogenic
form
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What is the transforming agent?

• Purified substances from the heat-killed,
  pathogenic bacteria
• Tried to transform live, nonpathogenic bacteria
  with each one
• Only DNA worked

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More evidence viruses

• Viruses are DNA or RNA enclosed in a protein coat
  that infect a cell and use its machinery to
  reproduce itself
• Viruses that infect bacteria bacteriophages
• Hershey and Chase phage T2 infects E.coli and
  makes more T2
• Which component, protein or DNA, was responsible?
• Individually labeled protein and DNA to show that
  only DNA actually entered the cell

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Figure 16.2b The Hershey-Chase experiment
Labels protein (contains sulfur)
Labels DNA (contains phosphate)
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Figure 16.2a The Hershey-Chase experiment phages