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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Chapter 10 – DNA: The Chemical Nature of the Gene

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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

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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