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Chapter 10 – DNA: The Chemical Nature of the Gene

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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Chapter 10 – DNA: The Chemical Nature of the Gene

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  1. Chapter 10 – DNA: The Chemical Nature of the Gene

  2. 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?

  3. 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

  4. Fred Griffith 1928 • Worked with different strains of the bacteria Streptococcus pneumoniae • Transformation – bacteria acquired genetic information from dead strain which permanently changed bacteria

  5. 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

  6. Alfred Hershey and Martha Chase 1952 • DNA or protein genetic material? • Conclusion: phage injects DNA, not protein, into bacteria; DNA genetic material

  7. Maurice Wilkins and Rosalind Franklin early 1950s • Worked independently on X ray crystallography • Diffraction pattern gives information on molecular structure

  8. 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

  9. Heinz Fraenkel Conrat and Bea Singer 1956 • RNA can serve as genetic material in viruses • Created hybrid virsuses; progeny particles were of RNA type

  10. 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

  11. 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)

  12. Nucleotide structure • Nucleoside • Base + sugar • Nucleotide • Nucleoside + phosphate

  13. 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)

  14. 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

  15. 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?

  16. 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

  17. 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

  18. Special structures • Cruciform • Double-stranded • Hairpins form on both strands due to palindrome sequences • Complex structures can form within a single strand

  19. 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

  20. 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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