BY John Roderick
DNA
DEOXYRIBONUCLEIC
ACID
STORES AND PASSES ON
GENETIC INFORMATION
FROM ONE GENERATION TO
ANOTHER .
SOME GENERAL REVIEW
DNA STRUCTURE
THE DNA STRUCTURE IS SHAPED
LIKED A SPIRAL STAIRCASE, OR
TWISTED LADDER.
IMAGES OF DNA STRUCTURE
DNA STRUCTURE
THE OUTER PARTS OF THE OF THE
LADDER ARE MADE OF SUGARS.
DEOXYRIBOSE + PHOSPHATE
THESE ARE JOINED TOGETHER BY
ESTER BONDS
DNA STRUCTURE
• RIBOSE IS A SUGAR LIKE GLUCOSE,
BUT WITH ONLY 5 CARBON ATOMS
IN ITS MOLECULE.
• DEOXYRIBOSE IS ALMOST THE
SAME BUT LACKS 1 OXYGEN ATOM.
• BOTH MOLECULES MAY BE
REPRESENTED BY THE SYMBOL:
DNA STRUCTURE
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
C2
Nitrogenous base
(A, G, C, or T)
DNA STRUCTURE
THE STEPS OF THE LADDER ARE
MADE OF PAIRS OF NITROGEN
BASES.
ERWIN CHARGAFF (1947) WROTE
THAT IN THE DNA THE AMOUNTS OF
THE NITROGEN BASES ARE NOT ALL
EQUAL BUT ARE PRESENT IN
CHARACTERISTIC RATIO.
A=30.9% -T= 29.4%, G=19.9% - C=19.8%
DNA STRUCTURE
1. ADENINE = A
2. GUANINE = G
3. CYTOSINE = C
4. THYMINE = T
THEY ARE REPRESENTED BY
THEIR CAPITAL LETTER.
DNA STRUCTURE
THERE IS ONLY ONE WAY THE
NITROGEN BASES CAN FORM
ADENINE PAIRS WITH THYMINE
GUANINE PAIRS WITH CYTOSINE
A==T
G==C
DNA STRUCTURE
JOINED NUCLEOTIDES
PO4
PO4
PO4
HOW MANY DO WE
SEE IN THIS SLIDE?
PO4
sugar-phosphate
backbone
A MOLECULE OF
DNA IS FORMED BY
MILLIONS OF
NUCLEOTIDES
JOINED TOGETHER
IN A LONG CHAIN
+ bases
2 STRAN DNA
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO
PO4
PO4
PO4
PO4
PO4
PO4
DNA STRUCTURE
QUESTION ?
CAN C JOIN WITH A
AND G JOIN WITH T
YES
NO
CORRECT
MOVE ON
SORRY
REVIEW MORE
DNA STRUCTURE
ACTCATGGTCATG
TGAGTACCAGTAC
THIS IS AN EXAMPLE OF A
GENETIC CODE THAT MAY BE
FOUND IN YOUR BODY.
DNA STRUCTURE
THE COMBINATIONS OF THE
NITROGEN BASES IS THE CODE
THAT PRODUCES THE GENES FOR
A PARTICULAR TRAIT.
DNA STRUCTURE
BECAUSE THERE CAN BE SO
MANY DIFFERENT
COMBINATIONIS OF THE BASES
THE CODE CAN BE LIMITLESS
DNA STRUCTURE
EXAMPLE:
ATCGTCAGG MAY BE FOR HAIR
COLOR.
BUT
ATCGTCAGC MAY BE FOR EYE
COLOR
DNA REPLICATION
REPLICATION: IS
THE PROCESS
WHERE DNA
MOLECULES
FORM EXACT
DUPLICATS.
DNA REPLICATION
1. THE LADDER SPLITS
2. FREE FLOATING NITROGEN
BASES BEGIN TO PAIR WITH
EACH STRAND
3. TWO NEW DNA CHAINS ARE
FORMED
JAMES WATSON AND
FRANCIS CRICK
• THE YEAR 1953 COULD BE
SAID TO MARK, IN
BIOLOGY AT LEAST, THE
END OF HISTORY.
• JAMES WATSON AND
FRANCIS CRICK
PUBLISHED A PAPER ON
THE STRUCTURE OF DNA
WHICH BEGAN A NEW ERA
IN ITS UNDERSTANDING.
RACE TO DETERMINE
3D STRUCTURE
IN THE EARLY 1950’S
THE RACE FOCUSED ON
DISCOVERING THE 3
D STRUCTURE.
AMONG THE SCIENTISTS
WORKING ON THE
PROBLEM WERE LINUS
PAULING, MAURICE
WILKINS AND ROSALIND
FRANKLIN
THE WINNERS OF THE RACE
THE FIRST TO THE FINISH LINE,
HOWEVER WERE TWO SCIENTISTS
WHO WERE RELATIVELY UNKOWN AT
THE TIME: JAMES WATSON AND
FRANCIS CRICK
FRIENDSHIP BETWEEN THEM
THEIR FRIENDSHIP BEGAN
WHEN WATSON WENT TO
CAMBRIDGE UNIVERSITY,
WHERE CRICK WAS
STUDYING PROTEIN
STRUCTURE WITH A
TECHNIQUE CALLED XRAY CRYSTALLOGRAPHY.
INTERPRETING THE DATA
WATSON AND CRICK
BASED THEIR MODEL OF
DNA ON THE DATA THEY
WERE ABLE TO EXTRACT.
FROM FRANKLIN’S X-RAY
DIFFRACTION PHOTO.
THEY INTERPRETED THE
X-RAY TO MEAN THAT
DNA WAS HELICAL IN
SHAPE.
A CRITIQUE OF PAULING
AND COREY
THE WIDTH DEDUCED
FROM THE IMAGE LED
THEM TO SUGGEST THAT
IT WAS MADE UP OF TWO
STRANDS, CONTRARY TO
THE THREE STRAND
MODEL THAT LINUS
PAULING HAD PROPOSED.
CREATION OF A MODEL
USING MODELS MADE
OF WIRE, THEY BEGAN
BUILDING MODELS OF
DOUBLE HELIX THAT
WOULD CONFORM TO
THE DATA FROM X-RAY,
AND WHAT WAS KNOWN
ABOUT THE CHEMISTRY
OF DNA.
CREATION OF A MODEL
AFTER FAILING TO MAKE
A SATISFACTORY MODEL
THAT PLACED THE SUGAR
PHOSPHATE ON THE
INSIDE OF THE CHAINS,
WATSON TRIED PUTTING
THEM ON THE OUTSIDE
AND FORCING THE
NITROGENOUS BASES TO
MOVE TO THE INTERIOR.
CREATION OF A MODEL
FRANKLIN’S X-RAY DATA
INDICATED THAT THE
HELIX MAKES ONE FULL
TURN EVERY 3.4nm.
BECAUSE BASES ARE
STACKED 0.34nm APART
THERE ARE TEN LAYERS
OF BASE PAIRS IN EACH
TURN OF THE HELIX.
CREATION OF A MODEL
THIS ARRANGEMENT WAS PPEALING BECAUSE
IT PUT THE HYDROPHOBIC NITROGEN BASES IN
THE MOLECULE’S INTERIOR AND THUS AWAY
FROM THE SURROUNDING AQUEOUS MEDIUM .
CREATION OF A MODEL
• THE NITROGENOUS BASES OF THE DOUBLE HELIX
ARE PAIRED IN SPECIFIC COMBINATIONS. IT WAS
BY TRIAL AND ERROR THAT WATSON AND CRICK
ARRIVED AT THIS KEY FEATURE OF DNA. AT FIRST
WATSON IMAGINED THE BASES WERE PAIRED
LIKE WITH LIKE: A-A, C-C.
• THIS WAS INCONSISTENT BECAUSE ADENINE AND
GUANINE ARE PURINES, NITROGEN BASES WITH
TWO RINGS. WHEREAS CYTOSINE AND THYMINE
ARE PYRIMIDINES WHICH HAVE A SINGLE RING.
THUS PURINES ARE TWICE AS BIG.
CREATION OF A MODEL
• WATSON AND CRICK REASONED THAT THERE
MUST BE ADDITIONAL SPECIFICITY OF PAIRING
DICTATED BY THE STRUCTURE OF BASES.
• EACH BASE HAS CHEMICAL SIDE GROUPS THAT
CAN FORM HYDROGEN BONDS WITH ITS
APPROPRIATE PARTNER: ADENINE CAN FORM
TWO HYDROGEN BONDS WITH THYMINE AND
ONLY THYMINE; GUANINE FORMS THREE
HYDROGEN BONDS WITH CYTOSINE AND ONLY
CYTOSINE.
CHEMICAL BONDING
BETWEEN BASES
H-bonds
G
T
C
A
FURTHER DEVELOPMENT
• THE WATSON-CRICK MODEL EXPLAINED
CHARGAFF’S RULES. WHEREVER ONE STRAND OF
A DNA MOLECULE HAS AN A, THE PARTNER
STRAND HAS A T. AND A G IN ONE STRAND IS
ALWAYS PARTNERED WITH A C IN THE
COMPLEMENTARY STRAND.