chapter 10 dna the chemical nature of the gene
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Chapter 10 – DNA: The Chemical Nature of the Gene

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Chapter 10 – DNA: The Chemical Nature of the Gene. Early DNA studies. Johann Friedrich Meischer – late 1800s Studied pus (contains white blood cells) Isolated nuclear material Slightly acidic, high phosphorous content Consisted of DNA and protein

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early dna studies
Early DNA studies
  • Johann Friedrich Meischer – late 1800s
    • Studied pus (contains white blood cells)
    • Isolated nuclear material
      • Slightly acidic, high phosphorous content
      • Consisted of DNA and protein
        • Called in “nuclein” – later renamed nucleic acid
  • By late 1800s
    • Chromatin thought to be genetic material, but protein or DNA?
early dna studies1
Early DNA studies
  • Tetranucleotide theory
    • DNA made up of 4 different nucleotides in equal amounts
      • Nucleotide – pentose sugar, phosphate group, nitrogenous base
    • Under this assumption, DNA doesn’t have the variety needed for genetic material
      • Protein composed of 20 different amino acids; complex structures
  • Erwin Chargaff 1940s
    • Base composition of DNA among different species had great variety, but consistent within a single species
    • Adenine amount roughly equals thymine amount; guanine amount roughly equals cytosine amount
fred griffith 1928
Fred Griffith 1928
  • Worked with different strains of the bacteria Streptococcus pneumoniae
  • Transformation – bacteria acquired genetic information from dead strain which permanently changed bacteria
oswald avery published 1944
Oswald Avery published 1944
  • Based on Griffith’s findings
  • What was transforming principle – protein, RNA, or DNA?
  • Conclusion: when DNA is degraded, no transformation occurs; DNA genetic material
alfred hershey and martha chase 1952
Alfred Hershey and Martha Chase 1952
  • DNA or protein genetic material?
  • Conclusion: phage injects DNA, not protein, into bacteria; DNA genetic material
maurice wilkins and rosalind franklin early 1950s
Maurice Wilkins and Rosalind Franklin early 1950s
  • Worked independently on X ray crystallography
  • Diffraction pattern gives information on molecular structure
james watson and francis crick
James Watson and Francis Crick
  • Published paper detailing DNA structure in 1953
    • Based on published data and unreleased information
  • 1962 won Nobel prize along with Maurice Wilkins
heinz fraenkel conrat and bea singer 1956
Heinz Fraenkel Conrat and Bea Singer 1956
  • RNA can serve as genetic material in viruses
  • Created hybrid virsuses; progeny particles were of RNA type
nucleotide structure
Nucleotide structure
  • Pentose (5 carbon) sugar
    • 1′ to 5′ “′” refers to carbon in sugar (not base)
    • RNA – ribose
      • -OH at 2′ carbon
      • Less stable
    • DNA – deoxyribose
      • -H at 2′ carbon
  • Phosphate group
    • Phosphorous and 4 oxygen
    • Negatively charged
    • Attached to 5′ carbon
nucleotide structure1
Nucleotide structure
  • Nitrogenous base
    • Covalently bonded to 1′ carbon
    • Purine
      • Double-ringed; six- and five-sided rings
      • Adenine
      • Guanine
    • Pyrimidine
      • Single-ringed; six-sided ring
      • Cytosine
      • Thymine (DNA only)
      • Uracil (RNA only)
nucleotide structure2
Nucleotide structure
  • Nucleoside
    • Base + sugar
  • Nucleotide
    • Nucleoside + phosphate
polynucleotide strands
Polynucleotide strands
  • Nucleotides covalently bonded – phosphodiester bonds
    • Phosphate group of one nucleotide bound to 3′C of previous sugar
  • Backbone consists of alternating phosphates and sugars
    • Always has one 5′ end (phosphate) and one 3′ end (sugar –OH)
dna double helix
DNA double helix
  • 2 antiparallel strands with bases in interior
  • Bases held together by hydrogen bonds
    • 2 between A and T; 3 between G and C
  • Complementary base pairing; complementary strands
helices
Helices
  • B-DNA
    • Watson and Crick model
    • Shape when plenty of water is present
    • Right hand/clockwise turn; approx 10 bases per turn
  • A-DNA
    • Form when less water is present; no proof of existence under physiological conditions
    • Shorter and wider than B form
    • Right hand/clockwise turn; approx 11 bases per turn
  • Z-DNA
    • Left hand/counterclockwise turn
    • Approx 12 bases per turn
    • Found in portions with specific base pair sequences (alternating G and C)
    • Possible role in transcription regulation?
genetic implications
Genetic implications
  • Watson and Crick indicated structure revealed mode of replication
    • H bonds break and each strand serves as a template for new strand due to complementary base pairing
  • Central dogma
    • Replication
      • DNA from DNA
    • Transcription
      • RNA from DNA
    • Translation
      • Polypeptide/protein from mRNA
special structures
Special structures
  • Sequences with a single strand of nucleotides may be complementary and pair – forming double-stranded regions
  • Hairpin
    • Region of complementary bases form base; loop formed by unpaired bases in the middle
  • Stem
    • No loop of hairpin
special structures1
Special structures
  • Cruciform
    • Double-stranded
    • Hairpins form on both strands due to palindrome sequences
  • Complex structures can form within a single strand
dna methylation
DNA methylation
  • Addition of methyl groups to certain bases
  • Bacteria is frequently methylated
    • Restriction endonucleases cleave unmethylated sequences
  • Amount of methylation varies among organisms
    • Yeast – 0%
    • Animals – 5%
    • Plants – approx 50%
  • Methylation in eukaryotic cells is associated with gene expression
    • Methylated sequences are low/no transcription
bends in dna
Bends in DNA
  • Series of 4 or more A-T base pairs cause DNA to bend
    • Affects ability of proteins to bind to DNA’ affects transcription
  • SRY gene
    • Produces SRY protein
      • Binds to certain DNA sequences; bends DNA
        • Facilitates binding of transcription proteins; activates genes for male traits
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