BY John Roderick

STORES AND PASSES ON GENETIC INFORMATION FROM ONE GENERATION TO ANOTHER .

DNA DEOXYRIBONUCLEIC ACID

SOME GENERAL REVIEWSOME 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 RIBOSE IS A SUGAR LIKE GLUCOSE, BUT WITH ONLY 5 CARBON ATOMS IN ITS WITH ONLY 5 CARBON ATOMS IN ITS MOLECULE.MOLECULE.

• DEOXYRIBOSE IS ALMOST THE SAME DEOXYRIBOSE IS ALMOST THE SAME BUT LACKS 1 OXYGEN ATOM.BUT LACKS 1 OXYGEN ATOM.

• BOTH MOLECULES MAY BE BOTH MOLECULES MAY BE REPRESENTED BY THE SYMBOL: REPRESENTED BY THE SYMBOL:

NNitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T)

DNA STRUCTURE

CH2

O

C1C4

C3 C2

5

SugarSugar(deoxyribose)(deoxyribose)

OO=P-O O

PhosphatePhosphate GroupGroup

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 = A2. GUANINE = G3. CYTOSINE = C4. THYMINE = T

THEY ARE REPRESENTED BYTHEIR CAPITAL LETTER.

DNA STRUCTURE

THERE IS ONLY ONE WAY THE NITROGEN BASES CAN FORM

ADENINE PAIRS WITH THYMINEGUANINE PAIRS WITH CYTOSINE

A==TG==C

DNA STRUCTUREJOINED NUCLEOTIDESJOINED NUCLEOTIDES

A MOLECULE OF A MOLECULE OF

DNA IS FORMED BY DNA IS FORMED BY

MILLIONS OF MILLIONS OF

NUCLEOTIDES NUCLEOTIDES

JOINED TOGETHER JOINED TOGETHER

IN A LONG CHAININ A LONG CHAIN

HOW MANY DO WE HOW MANY DO WE

SEE IN THIS SLIDE?SEE IN THIS SLIDE?

PO4

PO4

PO4

PO4

sugar-phosphate backbone

+ bases

2 STRAN DNA

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

PO4

DNA STRUCTUREQUESTION ?

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 STRAND3. TWO NEW DNA CHAINS ARE FORMED

JAMES WATSON AND JAMES WATSON AND FRANCIS CRICKFRANCIS CRICK

• THE YEAR 1953 COULD BE THE YEAR 1953 COULD BE SAID TO MARK, IN SAID TO MARK, IN BIOLOGY AT LEAST, THE BIOLOGY AT LEAST, THE END OF HISTORY. END OF HISTORY.

• JAMES WATSON AND JAMES WATSON AND FRANCIS CRICK FRANCIS CRICK PUBLISHED A PAPER ON PUBLISHED A PAPER ON THE STRUCTURE OF DNA THE STRUCTURE OF DNA WHICH BEGAN A NEW ERA WHICH BEGAN A NEW ERA IN ITS UNDERSTANDING.IN ITS UNDERSTANDING.

RACE TO DETERMINE RACE TO DETERMINE 3D STRUCTURE3D STRUCTURE

IN THE EARLY 1950’S IN THE EARLY 1950’S

THE RACE FOCUSED ON THE RACE FOCUSED ON

DISCOVERING THE 3DISCOVERING THE 3

D STRUCTURE.D STRUCTURE.

AMONG THE SCIENTISTS AMONG THE SCIENTISTS

WORKING ON THE WORKING ON THE

PROBLEM WERE LINUS PROBLEM WERE LINUS

PAULING, MAURICE PAULING, MAURICE

WILKINS AND ROSALIND WILKINS AND ROSALIND

FRANKLINFRANKLIN

THE WINNERS OF THE RACETHE WINNERS OF THE RACE

THE FIRST TO THE FINISH LINE, THE FIRST TO THE FINISH LINE,

HOWEVER WERE TWO SCIENTISTS HOWEVER WERE TWO SCIENTISTS

WHO WERE RELATIVELY UNKOWN AT WHO WERE RELATIVELY UNKOWN AT

THE TIME: JAMES WATSON AND THE TIME: JAMES WATSON AND

FRANCIS CRICKFRANCIS CRICK

FRIENDSHIP BETWEEN THEMFRIENDSHIP BETWEEN THEM

THEIR FRIENDSHIP BEGAN THEIR FRIENDSHIP BEGAN

WHEN WATSON WENT TOWHEN WATSON WENT TO

CAMBRIDGE UNIVERSITY,CAMBRIDGE UNIVERSITY,

WHERE CRICK WAS WHERE CRICK WAS

STUDYING PROTEIN STUDYING PROTEIN

STRUCTURE WITH A STRUCTURE WITH A

TECHNIQUE CALLED X-TECHNIQUE CALLED X-

RAY CRYSTALLOGRAPHY.RAY CRYSTALLOGRAPHY.

INTERPRETING THE DATAINTERPRETING THE DATA

WATSON AND CRICK WATSON AND CRICK

BASED THEIR MODEL OF BASED THEIR MODEL OF

DNA ON THE DATA THEYDNA ON THE DATA THEY

WERE ABLE TO EXTRACT.WERE ABLE TO EXTRACT.

FROM FRANKLIN’S X-RAYFROM FRANKLIN’S X-RAY

DIFFRACTION PHOTO.DIFFRACTION PHOTO.

THEY INTERPRETED THETHEY INTERPRETED THE

X-RAY TO MEAN THAT X-RAY TO MEAN THAT

DNA WAS HELICAL IN DNA WAS HELICAL IN

SHAPE.SHAPE.

A CRITIQUE OF PAULING A CRITIQUE OF PAULING AND COREYAND COREY

THE WIDTH DEDUCED THE WIDTH DEDUCED FROM THE IMAGE LED FROM THE IMAGE LED THEM TO SUGGEST THAT THEM TO SUGGEST THAT IT WAS MADE UP OF TWOIT WAS MADE UP OF TWOSTRANDS, CONTRARY TO STRANDS, CONTRARY TO THE THREE STRAND THE THREE STRAND MODEL THAT LINUS MODEL THAT LINUS PAULING HAD PROPOSED.PAULING HAD PROPOSED.

CREATION OF A MODELCREATION OF A MODEL

USING MODELS MADE USING MODELS MADE

OF WIRE, THEY BEGAN OF WIRE, THEY BEGAN

BUILDING MODELS OF BUILDING MODELS OF

DOUBLE HELIX THAT DOUBLE HELIX THAT

WOULD CONFORM TO WOULD CONFORM TO

THE DATA FROM X-RAY, THE DATA FROM X-RAY,

AND WHAT WAS KNOWN AND WHAT WAS KNOWN

ABOUT THE CHEMISTRY ABOUT THE CHEMISTRY

OF DNA.OF DNA.

CREATION OF A MODELCREATION OF A MODEL

AFTER FAILING TO MAKE AFTER FAILING TO MAKE A SATISFACTORY MODELA SATISFACTORY MODELTHAT PLACED THE SUGAR THAT PLACED THE SUGAR PHOSPHATE ON THE PHOSPHATE ON THE INSIDE OF THE CHAINS, INSIDE OF THE CHAINS, WATSON TRIED PUTTING WATSON TRIED PUTTING THEM ON THE OUTSIDE THEM ON THE OUTSIDE AND FORCING THE AND FORCING THE NITROGENOUS BASES TO NITROGENOUS BASES TO MOVE TO THE INTERIOR.MOVE TO THE INTERIOR.

CREATION OF A MODELCREATION OF A MODEL

FRANKLIN’S X-RAY DATA FRANKLIN’S X-RAY DATA

INDICATED THAT THE INDICATED THAT THE

HELIX MAKES ONE FULL HELIX MAKES ONE FULL

TURN EVERY 3.4nm.TURN EVERY 3.4nm.

BECAUSE BASES AREBECAUSE BASES ARE

STACKED 0.34nm APART STACKED 0.34nm APART

THERE ARE TEN LAYERS THERE ARE TEN LAYERS

OF BASE PAIRS IN EACH OF BASE PAIRS IN EACH

TURN OF THE HELIX.TURN OF THE HELIX.

CREATION OF A MODELCREATION OF A MODEL

THIS ARRANGEMENT WAS PPEALING BECAUSE THIS ARRANGEMENT WAS PPEALING BECAUSE

IT PUT THE HYDROPHOBIC NITROGEN BASES IN IT PUT THE HYDROPHOBIC NITROGEN BASES IN

THE MOLECULE’S INTERIOR AND THUS AWAY THE MOLECULE’S INTERIOR AND THUS AWAY

FROM THE SURROUNDING AQUEOUS MEDIUMFROM THE SURROUNDING AQUEOUS MEDIUM . .

CREATION OF A MODELCREATION OF A MODEL

• THE NITROGENOUS BASES OF THE DOUBLE HELIX THE NITROGENOUS BASES OF THE DOUBLE HELIX ARE PAIRED IN SPECIFIC COMBINATIONS. IT WAS ARE PAIRED IN SPECIFIC COMBINATIONS. IT WAS BY TRIAL AND ERROR THAT WATSON AND CRICK BY TRIAL AND ERROR THAT WATSON AND CRICK ARRIVED AT THIS KEY FEATURE OF DNA. AT ARRIVED AT THIS KEY FEATURE OF DNA. AT FIRST WATSON IMAGINED THE BASES WERE FIRST WATSON IMAGINED THE BASES WERE PAIRED LIKE WITH LIKE: A-A, C-C.PAIRED LIKE WITH LIKE: A-A, C-C.

• THIS WAS INCONSISTENT BECAUSE ADENINE AND THIS WAS INCONSISTENT BECAUSE ADENINE AND GUANINE ARE PURINES, NITROGEN BASES WITH GUANINE ARE PURINES, NITROGEN BASES WITH TWO RINGS. WHEREAS CYTOSINE AND THYMINE TWO RINGS. WHEREAS CYTOSINE AND THYMINE ARE PYRIMIDINES WHICH HAVE A SINGLE RING. ARE PYRIMIDINES WHICH HAVE A SINGLE RING. THUS PURINES ARE TWICE AS BIG.THUS PURINES ARE TWICE AS BIG.

CREATION OF A MODELCREATION OF A MODEL

• WATSON AND CRICK REASONED THAT THERE WATSON AND CRICK REASONED THAT THERE MUST BE ADDITIONAL SPECIFICITY OF PAIRING MUST BE ADDITIONAL SPECIFICITY OF PAIRING DICTATED BY THE STRUCTURE OF BASES.DICTATED BY THE STRUCTURE OF BASES.

• EACH BASE HAS CHEMICAL SIDE GROUPS THAT EACH BASE HAS CHEMICAL SIDE GROUPS THAT CAN FORM HYDROGEN BONDS WITH ITS CAN FORM HYDROGEN BONDS WITH ITS APPROPRIATE PARTNER: ADENINE CAN FORM APPROPRIATE PARTNER: ADENINE CAN FORM TWO HYDROGEN BONDS WITH THYMINE AND TWO HYDROGEN BONDS WITH THYMINE AND ONLY THYMINE; GUANINE FORMS THREE ONLY THYMINE; GUANINE FORMS THREE HYDROGEN BONDS WITH CYTOSINE AND ONLY HYDROGEN BONDS WITH CYTOSINE AND ONLY CYTOSINE. CYTOSINE.

CHEMICAL BONDING CHEMICAL BONDING BETWEEN BASESBETWEEN BASES

CG

H-bonds

T A

FURTHER DEVELOPMENTFURTHER DEVELOPMENT

• THE WATSON-CRICK MODEL EXPLAINED THE WATSON-CRICK MODEL EXPLAINED CHARGAFF’S RULES. WHEREVER ONE STRAND OF CHARGAFF’S RULES. WHEREVER ONE STRAND OF A DNA MOLECULE HAS AN A, THE PARTNER A DNA MOLECULE HAS AN A, THE PARTNER STRAND HAS A T. AND A G IN ONE STRAND IS STRAND HAS A T. AND A G IN ONE STRAND IS ALWAYS PARTNERED WITH A C IN THE ALWAYS PARTNERED WITH A C IN THE COMPLEMENTARY STRAND. COMPLEMENTARY STRAND.