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 - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 10 – DNA: The Chemical Nature of the Gene
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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?
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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
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Fred Griffith 1928• Worked with different
strains of the bacteria Streptococcus pneumoniae
• Transformation – bacteria acquired genetic information from dead strain which permanently changed bacteria
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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
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Alfred Hershey and Martha Chase 1952
• DNA or protein genetic material?
• Conclusion: phage injects DNA, not protein, into bacteria; DNA genetic material
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Maurice Wilkins and Rosalind Franklin early 1950s
• Worked independently on X ray crystallography
• Diffraction pattern gives information on molecular structure
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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
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Heinz Fraenkel Conrat and Bea Singer 1956
• RNA can serve as genetic material in viruses
• Created hybrid virsuses; progeny particles were of RNA type
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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
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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)
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Nucleotide structure
• Nucleoside– Base + sugar
• Nucleotide – Nucleoside +
phosphate
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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)
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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
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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?
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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
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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
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Special structures• Cruciform
– Double-stranded– Hairpins form on
both strands due to palindrome sequences
• Complex structures can form within a single strand
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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
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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