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

– Called in “nuclein” – later renamed nucleic acid

• By late 1800s– Chromatin thought to be genetic material, but

protein or DNA?

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• 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• 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

• DNA or protein genetic material?

• Conclusion: phage injects DNA, not protein, into bacteria; DNA genetic material

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

• 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

• RNA can serve as genetic material in viruses

• Created hybrid virsuses; progeny particles were of RNA type

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 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 structure

• Nucleoside– Base + sugar

• Nucleotide – Nucleoside +

phosphate

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• 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• 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

• 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 • 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 structures• Cruciform

– Double-stranded– Hairpins form on

both strands due to palindrome sequences

• Complex structures can form within a single strand

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

• 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