Download DNA Structure

Ch 5 and16
A Close Look at the Hereditary Molecules
• Protein sequence-->programmed by genes
• Genes are made of DNA, a nucleic acid
LE 5-25
Flow of genetic information
DNA
DNA
Synthesis of
mRNA in the nucleus
mRNA
RNA
NUCLEUS
CYTOPLASM
Protein
mRNA
Movement of
mRNA into cytoplasm
via nuclear pore
Ribosome
Synthesis
of protein
Polypeptide
Amino
acids
The Roles of Nucleic Acids
• Two types:
– Deoxyribonucleic acid (DNA)
– Ribonucleic acid (RNA)
• DNA provides directions for its own replication.
• DNA directs synthesis of messenger RNA
(mRNA)
• mRNA controls protein synthesis.
• Protein synthesis occurs on ribosomes.
LE 5-26a
5 end
Nucleic acid building block
Nucleoside
Nitrogenous
base
Phosphate
group
Nucleotide
3 end
Polynucleotide, or
nucleic acid
Pentose
sugar
Nucleic Acid Structure
Monomers
nucleotide (3 parts)
1. nitrogenous base
nucleoside
2. 5 C sugar
3. Phosphate
Polymer
polynucleotide or nucleic acid
LE 5-26b
Nitrogenous bases
Pyrimidines
Cytosine
C
Thymine (in DNA) Uracil (in RNA)
U
T
Purines
Adenine
A
Guanine
G
Pentose sugars
Deoxyribose (in DNA)
Nucleoside components
Ribose (in RNA)
Important Nucleic Acid Distinctions
Two kinds of bases
Pyrimidines-one ring (T,U,C)
Purines- two rings (G,A)
DNA
the sugar = deoxyribose
NO 2’ OH (hydroxyl)
RNA
the sugar= ribose
YES 2’ OH
Nucleotide Polymers
• Nucleotides (nt) connect through
phosphodiester bond
5’ Phosphate--> 3’OH
• Creation of a sugar-phosphate backbone with
bases as appendages.
• Sequence of bases along DNA or mRNA
polymer unique for each gene.
LE 16-7
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
Key features of DNA structure
5 end
Partial chemical structure
Space-filling model
Two DNA strands bind together
through complementary base-pairing.
Structure of DNA double helix: published in 1953
Francis
Crick
James
Watson
Watson JD, Crick FHC. 1953. Molecular structure of nucleic acids: a
structure for deoxyribonucleic acids. Nature 171:738.
LE 16-6
Partly based on Franklin’s x-ray diffraction data
Rosalind Franklin
Franklin’s X-ray diffraction
photograph of DNA
LE 16-8
Chargaff’s rules (1940s):
Amount of
A=T
G=C
LE 16-UN298
Watson & Crick: built model of DNA and
tested possible combinations of bases
Did model support Chargaff’s observations and
Franklin’s x-ray diffraction data?
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
LE 16-7
Antiparallel DNA strands
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
Key features of DNA structure
5 end
Partial chemical structure
Space-filling model
Two DNA strands bind together
through complementary base-pairing.
The DNA Double Helix
• Two polynucleotides (strands) base-paired
together GC, AT (complementary base-pairing)
• Double helix
• Two sugar-phosphate backbones run in
opposite 5´ to 3´ directions - antiparallel
• One DNA molecule includes many genes
Complementary base pairs
A=T
2 H-bonds
Sugar
Adenine (A)
Sugar
Thymine (T)
G=C
3 H-bonds
Sugar
Sugar
Guanine (G)
Cytosine (C)
Behavior of DNA
Draw a 10 base pair double-stranded
DNA (dsDNA) that is rich in AT.
Draw a 10 base pair double-stranded
DNA (dsDNA) that is rich in GC.
If these were placed in a tube of boiling water
what would happen?
DNA would become single stranded (ssDNA)
(denatured or melted).
Which DNA would denature first. Why?
AT rich fragment less stable
2 H-bonds/bp versus 3 H-bonds/bp
DNA Used as Evolutionary Ruler
• Linear sequences of DNA in chromosomes
– passed from parents to offspring
• Two closely related species are more similar in DNA
sequence than distantly related species
• Similarity of DNA sequence
– Determines evolutionary relatedness
1. Compare the human sequence to the frog and mouse.
Which sequence is most similar to human?
human
5’ GAACCTTCCAATTGATCT3’
5’ GAACCAACCAATTAAACT3’
frog
5’ GAACCTTCGAATTGATCT3’
mouse
2. Write in the complementary strand for each.
Earlier data suggested that DNA was
hereditary material
Model system: Drosophila melanogaster
Investigator: Thomas Hunt Morgan (early 1900’s)
Evidence: white eye phenotype associated with X-chromosome
Model system: bacteria and viruses
Investigators: Many
Evidence: various
Evidence That DNA Can Transform Bacteria
Evidence for genetic role of DNA (Frederick Griffith,1928)
Heat-killed pathogenic “S” Streptococcus pneumoniae
+
“R”non-pathogenic bacterial strain
Some living bacteria became pathogenic
Transformation of “R’ to ‘S”,
How could one determine pathogenicity experimentally?
LE 16-2
Living S cells
(control)
Living R cells
(control)
Heat-killed
S cells (control)
Mixture of heat-killed
S cells and living
R cells
RESULTS
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells
are found in
blood sample
What molecule was responsible for conferring a new phenotype
into an organism?
• Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944)
• Published results
– Showed DNA from bacteria NOT protein--> caused
transformation of “R” to “S”
Independent confirmation
• Alfred Hershey and Martha Chase (1952)
– Used bacterial virus (bacteriophage) (T2) to ask
whether DNA or protein was hereditary material
LE 16-3
Phage
head
Tail
Tail fiber
Bacterial
cell
100 nm
DNA
LE 16-4
Hershey & Chase labeling experiment
Phage
Radioactive
protein
Empty
protein shell
Radioactivity
(phage protein)
in liquid
Bacterial cell
Batch 1:
Sulfur (35S)
DNA
Phage
DNA
Protein
radiolabelled
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
Phosphorus (32P)
DNA
radiolabelled
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Phage produced in and released from bacteria
with radioactive DNA.
Hershey & Chase results
-Suggest that DNA, not protein, is transferred to
bacteria by phage.
-DNA programs the reproduction of more phage.
Contains important genetic instructions.
I’m a pretty
cool molecule but
I’ll still answer your
questions.