Download Pierce chapter 10

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
• B-DNA
Helices
– 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 doublestranded 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