Mod 3 Slide Set 1Mod 3 Slide Set 1

Chapter 10Chapter 10

DNA Structure and FunctionDNA Structure and Function

There are a lot of activities at Mastering Biology for Ch 10; Please do them.There are a lot of activities at Mastering Biology for Ch 10; Please do them.

DNADNA(Deoxyribonucleic Acid)(Deoxyribonucleic Acid)

What is it?What is it? Read Chap 10Read Chap 10

How does it work?How does it work? Read Chap 10Read Chap 10

How do we know?How do we know? Read 10 & take good class class notesRead 10 & take good class class notes

Darwin and MendelDarwin and Mendel

1809-18821809-1882

1822-18841822-1884

Mendel was encouraged to enter the monastery partly Mendel was encouraged to enter the monastery partly because of his demonstrated abilities in mathematics. He was because of his demonstrated abilities in mathematics. He was

an Augustinian Friar in Brno, Czech Republic (Moravia).an Augustinian Friar in Brno, Czech Republic (Moravia).

Studied Physics at U. Vienna and was a contemporary of Semmelweis !Studied Physics at U. Vienna and was a contemporary of Semmelweis !

““Factors” of inheritance.Factors” of inheritance.“Particles” of inheritance.“Particles” of inheritance.

Discrete units.Discrete units.

Application of Application of mathematics to breedingmathematics to breeding

So, what are these “factors” of Mendel??So, what are these “factors” of Mendel??Well, we know today that they are Well, we know today that they are genesgenes..

What is Life?What is Life?Erwin Shrödinger, 1944Erwin Shrödinger, 1944

Shrödinger is a world-renowned Shrödinger is a world-renowned Nobel-laureate physicist who Nobel-laureate physicist who crosses over to the “dark side” crosses over to the “dark side” to take on the world of biology.to take on the world of biology.

““Physics is the only real science. The rest are just stamp collecting.” --Physics is the only real science. The rest are just stamp collecting.” --Ernest RutherfordErnest Rutherford

In a small book entitled “What is In a small book entitled “What is Life?” He suggests that genes Life?” He suggests that genes are a special type of are a special type of PROTEINPROTEIN..

Was he right about that?Was he right about that?

What is a protein?What is a protein?

cytoplasm

nucleus

plasma membrane

nuclear envelope

nucleoplasm

nucleolus

chromatin

The The NucleusNucleus

Nuclear DNANuclear DNA The changing appearance of a chromosome. Sometime The changing appearance of a chromosome. Sometime

DNA looks like spaghetti and sometimes it looks like rotini.DNA looks like spaghetti and sometimes it looks like rotini.

p.61

““The Tempting Template Hypothesis”The Tempting Template Hypothesis”slide 1slide 1

Cells (and therefore organisms) function Cells (and therefore organisms) function primarily because of their proteins. (enzymes, primarily because of their proteins. (enzymes, protein receptors, some hormones like insulin, protein receptors, some hormones like insulin, transport, contractile, structural, etc.)transport, contractile, structural, etc.)

How do cells “know how” to make all these How do cells “know how” to make all these proteins?proteins?

““The Tempting Template Hypothesis”The Tempting Template Hypothesis”slide 2slide 2

Proteins made from amino acids Proteins made from amino acids (there are 20 aa’s in biology).(there are 20 aa’s in biology).

The nucleus seems to control the cell.The nucleus seems to control the cell. The nucleus contains chromosomes The nucleus contains chromosomes (colored things)(colored things) and and

chromatin chromatin (colored stuff) (colored stuff) and these are made of DNA and these are made of DNA andand Protein.Protein.

““The Tempting Template Hypothesis”The Tempting Template Hypothesis”slide 3slide 3

IF…IF… The nucleus controls the cell… The nucleus controls the cell…

and and IF…IF… The main thing in the nucleus is chromosomes… The main thing in the nucleus is chromosomes…

andand IF…IF… Cell functioning depends mostly on proteins… Cell functioning depends mostly on proteins…

and and IF…IF… Chromosomes are made of protein and DNA… Chromosomes are made of protein and DNA…

THEN…THEN…

““The Tempting Template Hypothesis”The Tempting Template Hypothesis”slide 4slide 4

THEN…THEN… It seems likely that the controllers of the cell are It seems likely that the controllers of the cell are

the chromosomal proteins the chromosomal proteins rather than rather than the the chromosomal DNA, i.e. that genes are proteins.chromosomal DNA, i.e. that genes are proteins.(This is a logical idea, but…eventually it’s shown to be wrong)(This is a logical idea, but…eventually it’s shown to be wrong)

The moral of the storyThe moral of the story: : Some ideas that Some ideas that seem great at the time end up sucking! seem great at the time end up sucking!

““The Tempting Template Hypothesis”The Tempting Template Hypothesis”slide 5slide 5

MORE LOGICAL IDEA:MORE LOGICAL IDEA:

Chromosomal PROTEINS are responsible for Chromosomal PROTEINS are responsible for making CELLULAR proteins.making CELLULAR proteins.

LESS LOGICAL IDEA:LESS LOGICAL IDEA:

Chromosomal DNA is responsible for making Chromosomal DNA is responsible for making CELLULAR proteins.CELLULAR proteins.

Proteins Proteins Proteins Proteins DNA DNA Proteins Proteins

Johann Friedrich MiesscherJohann Friedrich MiesscherDiscovers DNA, 1869Discovers DNA, 1869

Discovers an acidic chemical in cell nucleiDiscovers an acidic chemical in cell nuclei He names this chemical “Nuclein”He names this chemical “Nuclein” This is later re-named DNAThis is later re-named DNA

Joachim Hammerling Joachim Hammerling 1930’s1930’s

Shows that the nucleus controls genetic expressionShows that the nucleus controls genetic expression of the cell. How does he do that? of the cell. How does he do that?Acetabularia,Acetabularia, a green alga.It’s one large cell ! a green alga.It’s one large cell !

Hammerling’s expt’s Hammerling’s expt’s with with AcetabulariaAcetabularia

Phoebus Aaron Theodor Levene Phoebus Aaron Theodor Levene Fishel Aaronovich LevinFishel Aaronovich Levin P.A.T. LeveneP.A.T. Levene

P.A.T. Levene #1P.A.T. Levene #1First to do a detailed study of DNA’s chemistryFirst to do a detailed study of DNA’s chemistry

Levene determines that:Levene determines that: DNA is a very, very, long molecule.DNA is a very, very, long molecule. But made of just 4 building blocks, NUCLEOTIDES.But made of just 4 building blocks, NUCLEOTIDES.

• 1. Adenine nucleotides,1. Adenine nucleotides,• 2. Guanine nucleotides,2. Guanine nucleotides,• 3. Cytosine nucleotides and3. Cytosine nucleotides and• 4. Thymine nucleotides.4. Thymine nucleotides.

and this is CORRECT !!!and this is CORRECT !!!

NUCLEOTIDES are the NUCLEOTIDES are the building blocks of DNAbuilding blocks of DNA !! !!

P.A.T. Levene #2P.A.T. Levene #2First to do a detailed study of DNA’s chemistryFirst to do a detailed study of DNA’s chemistry

Levene Levene alsoalso determines: determines: The relative amounts of A, G, C, T, nucleotides are The relative amounts of A, G, C, T, nucleotides are

equal, 25% each. 25%A 25%G 25%C & 25%T equal, 25% each. 25%A 25%G 25%C & 25%T • In other words, A=G=C=TIn other words, A=G=C=T

LATER ON, THIS TURNS OUT TO BE WRONGLATER ON, THIS TURNS OUT TO BE WRONG !! !!

P.A.T. Levene #3P.A.T. Levene #3

Levene’s Tetranucleotide HypothesisLevene’s Tetranucleotide Hypothesis What does Levene conclude from the What does Levene conclude from the

apparent fact that A=G=C=T ?apparent fact that A=G=C=T ? B=G 50% each N=B=W 33.33% eachB=G 50% each N=B=W 33.33% each

P.A.T. Levene #4P.A.T. Levene #4 The nucleotides The nucleotides go togethergo together as “tetra- as “tetra-

nucleotides”nucleotides” So he concludes that the DNA So he concludes that the DNA

molecule is BO-O-O-O-ring !! Too molecule is BO-O-O-O-ring !! Too boring to carry any genetic code. boring to carry any genetic code.

You can’t have a rich language with You can’t have a rich language with only 4 letters that endlessly repeat !only 4 letters that endlessly repeat !AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT- AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT…AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT-AGCT…

This further diminishes DNA as the This further diminishes DNA as the possible genetic molecule.possible genetic molecule.

P.A.T. Levene P.A.T. Levene #5#5

Summary of Levine:Summary of Levine:

First guy to do a detailed study of DNA’s chemistryFirst guy to do a detailed study of DNA’s chemistry He finds that DNA is a great long molecule built of He finds that DNA is a great long molecule built of

A, G, C & T nucleotides. A, G, C & T nucleotides. (correct)(correct)

The A’s, G’s, C’s, & T’s are equal (A=G=C=T) and The A’s, G’s, C’s, & T’s are equal (A=G=C=T) and therefore they “go together.” therefore they “go together.” (incorrect)(incorrect)

MORAL of the STORYMORAL of the STORY: : Like everybody else, sometimes scientists make Like everybody else, sometimes scientists make

mistakes. And sometimes these mistakes affect mistakes. And sometimes these mistakes affect other scientists. other scientists.

So what’s the score?So what’s the score?Are genes made of protein or are they made of DNA?Are genes made of protein or are they made of DNA?

How much good, hard, empirical evidence supports either hypothesis?How much good, hard, empirical evidence supports either hypothesis?

11

MiesscherMiesscher

18691869

22

HammerlingHammerling

1930’s1930’s

33

LeveneLevene

1910, 19291910, 1929

PROTEINPROTEINThe smart $$ bets The smart $$ bets

on the protein ideaon the protein idea00 00 00

DNADNANo one bets on No one bets on

these losers!these losers!00 00 00

Curt Schilling ’04 W.S.Curt Schilling ’04 W.S.No one, in 1928, thought the DNA was the genetic code and, additionally, No one, in 1928, thought the DNA was the genetic code and, additionally, because of Levine’s discoveries, DNA seemed too boring and pointless to because of Levine’s discoveries, DNA seemed too boring and pointless to

carry the genetic code needed to make a human or even an amoeba. carry the genetic code needed to make a human or even an amoeba.

On the other hand, there’s an awful lot of the stuff (DNA) in any cell, so On the other hand, there’s an awful lot of the stuff (DNA) in any cell, so “pointless” was very likely not a good evaluation of DNA.“pointless” was very likely not a good evaluation of DNA.

Frederick Griffith Frederick Griffith 19281928Fred discovers something weird… “TRANSFORMING FACTOR”Fred discovers something weird… “TRANSFORMING FACTOR”

This is a story of “dumb luck.” This is a story of “dumb luck.”

(the nice word for that is SERENDIPITY)(the nice word for that is SERENDIPITY)

Griffith is trying to make a vaccine against pneumonia.Griffith is trying to make a vaccine against pneumonia. So, how do you make a vaccine??So, how do you make a vaccine??

Griffith’s Experiments Griffith’s Experiments 19281928

Griffith’s Experiments Griffith’s Experiments 19281928How do you interpret Fred’s results?How do you interpret Fred’s results?

Either Either the dead cells that HAVE a the dead cells that HAVE a

capsule are coming back to lifecapsule are coming back to life

OrOr something is getting out of the something is getting out of the

dead ones that DO HAVE a dead ones that DO HAVE a capsule and into the live ones that capsule and into the live ones that DON’T and giving them the ability DON’T and giving them the ability to make a capsuleto make a capsule

Griffith’s Experiments Griffith’s Experiments 19281928

Either:Either: 1. Dead cells come back to life. 1. Dead cells come back to life.

Or:Or: 2. Some substance from the live ones gets into the dead 2. Some substance from the live ones gets into the dead ones and transforms them.ones and transforms them.

Griffith chooses #2. He writes up this curious discovery and Griffith chooses #2. He writes up this curious discovery and

calls the substance calls the substance TRANSFORMING FACTORTRANSFORMING FACTOR

Griffith’s Experiments Griffith’s Experiments 19281928

So:So: Some substance from the live ones gets into Some substance from the live ones gets into the dead ones and transforms them !!!the dead ones and transforms them !!!

He writes up this curious discovery and calls the He writes up this curious discovery and calls the substance substance TRANSFORMING FACTORTRANSFORMING FACTOR

He then returns his attention to finding a He then returns his attention to finding a pneumonia vaccine. pneumonia vaccine. (He never succeeded at that)(He never succeeded at that)

The MORAL of this STORYThe MORAL of this STORY

Sometimes we stumble onto important discoveries Sometimes we stumble onto important discoveries serendipitouslyserendipitously (by good ol’ dumb luck). (by good ol’ dumb luck).

So what’s so important about So what’s so important about Transforming Factor?Transforming Factor?

Oswald T. Avery 1944 Oswald T. Avery 1944 (and McCarty and Mcleod)(and McCarty and Mcleod)

In 1944 Oswald T. Avery finally figures out that In 1944 Oswald T. Avery finally figures out that Griffith’s Transforming Factor is DNA !!Griffith’s Transforming Factor is DNA !!

BAM!!! Home Run!BAM!!! Home Run!

So what’s the score?So what’s the score?Are genes made of protein or are they made of DNA?Are genes made of protein or are they made of DNA?

11

MiesscherMiesscher

18691869

22HammerlingHammerling

1930’s1930’s

33

LeveneLevene

1910,19291910,1929

44

GriffithGriffith

19281928

55

AveryAvery

19441944

PROTEINPROTEIN 00 00 00 00 00

DNADNA 00 00 00 00 11

Avery proves that transforming factor is DNA!Avery proves that transforming factor is DNA! After ~15 years of meticulous After ~15 years of meticulous

research, Avery shows that research, Avery shows that Streptococcus pneumoniae Streptococcus pneumoniae Transforming Factor… Transforming Factor… Is notIs not destroyed by lipase or destroyed by lipase or

protease enzymes, BUTprotease enzymes, BUT Is destroyed by DN’aseIs destroyed by DN’ase T.F. = DNA !!!!!!!!!!! But… T.F. = DNA !!!!!!!!!!! But…

Jeffrey Maier, Jeter, Tarasco, 1996Jeffrey Maier, Jeter, Tarasco, 1996

Anyway, it seems that the Anyway, it seems that the S. pneumoniae capsule S. pneumoniae capsule gene is gene is DNADNA..

So what does this say about So what does this say about YOUR brown eyes?YOUR brown eyes?

Avery proves that Transforming Factor is DNA !!Avery proves that Transforming Factor is DNA !!

After ~15 years of meticulous research, Avery shows that After ~15 years of meticulous research, Avery shows that S. pneumoniae S. pneumoniae Transforming Factor is DNA. Transforming Factor is DNA.

THE MORAL OF THE STORYTHE MORAL OF THE STORYHard work pays off !!!Hard work pays off !!!

alsoalso

Be Patient !!!Be Patient !!!Because of politics in the scientific world, Avery never received a Nobel Prize. Because of politics in the scientific world, Avery never received a Nobel Prize.

In a fitting tribute, one of the discovers of the structure of DNA, Francis In a fitting tribute, one of the discovers of the structure of DNA, Francis Crick, recognized Avery’s work by declaring: Avery should have received Crick, recognized Avery’s work by declaring: Avery should have received

TWO Nobel Prizes !!!TWO Nobel Prizes !!!

So what’s the big deal about TF?So what’s the big deal about TF?

This is the This is the firstfirst time ever in the history of history that time ever in the history of history that anyone showed that anyone showed that ANYANY genetic characteristic (i.e. the genetic characteristic (i.e. the presence or absence of a sugary coat on pneumonia presence or absence of a sugary coat on pneumonia bacteria) is carried by DNA.bacteria) is carried by DNA.

It’s the first time a gene was recognized as being made of It’s the first time a gene was recognized as being made of DNA. (Everyone thought genes are made of protein)DNA. (Everyone thought genes are made of protein)

But, again, what does this say about But, again, what does this say about youryour green eyes or green eyes or brown hair or big ears? Well… nothing… yet.brown hair or big ears? Well… nothing… yet.

But DNA is suddenly not boring anymore… it’s interesting.But DNA is suddenly not boring anymore… it’s interesting.

Viruses Help Us Understand DNAViruses Help Us Understand DNASalvador Luria, Max Delbrück, and Alfred HersheySalvador Luria, Max Delbrück, and Alfred Hershey

Salvador Luria, Max Delbrück Salvador Luria, Max Delbrück 1940’s1940’s

Sal and Max are virologists; they study viruses.Sal and Max are virologists; they study viruses. Known as “The Fathers of Virology”Known as “The Fathers of Virology” They study “phages”They study “phages”

A phage A phage (bacteriophage)(bacteriophage) is a virus that infects bacteria. is a virus that infects bacteria. They studied the T-even (T-2 and T-4) phages that They studied the T-even (T-2 and T-4) phages that

are viruses that infect are viruses that infect E.coliE.coli bacteria. bacteria.

Viruses are small, 10-100x Viruses are small, 10-100x smaller than bacteriasmaller than bacteria

Salvador Luria, Max Delbrück Salvador Luria, Max Delbrück 1940’s1940’s

The point is: They foster in the era of The point is: They foster in the era of studying viruses including…studying viruses including…

The experiment of Hershey and Chase…The experiment of Hershey and Chase…

Alfred Hershey and Martha ChaseAlfred Hershey and Martha Chase19521952

They do radioisotope tracer They do radioisotope tracer studies to see if viruses studies to see if viruses carry their genetic carry their genetic information on DNA or information on DNA or protein.protein.

Hershey and ChaseHershey and ChaseBased on these two facts:Based on these two facts:

1. DNA contains 1. DNA contains PPhosphorus but never hosphorus but never SSulfurulfurandand

2. Protein contains 2. Protein contains SSulfur but never ulfur but never PPhosphorushosphorus So, they create one batch of viruses with “hot” DNA So, they create one batch of viruses with “hot” DNA

and one batch of viruses with “hot” protein.and one batch of viruses with “hot” protein. ““Hot” = radioactive. Hot” = radioactive.

Hershey and ChaseHershey and Chase

Hershey and ChaseHershey and Chase

Every bit of genetic information needed to Every bit of genetic information needed to make a virus is carried on make a virus is carried on DNADNA..

ANDAND

Not one smidgen of it is carried by Protein.Not one smidgen of it is carried by Protein.

Bada Bing…Bada Bing… Game, Set, Match !!! Game, Set, Match !!! DNA’s the stuff !!!DNA’s the stuff !!! Fahgeddaboudit.Fahgeddaboudit.

So what’s the score now ?So what’s the score now ?Are genes made of protein or are they made of DNA?Are genes made of protein or are they made of DNA?

Ka-Pow !Ka-Pow !

Grand Slam !!Grand Slam !!

Genes are Genes are DNADNA !!! !!!

So what’s the score now ?So what’s the score now ?Are genes made of protein or are they made of DNA?Are genes made of protein or are they made of DNA?

11

MiesscherMiesscher

18691869

22HammerlingHammerling

1930’s1930’s

33

LeveneLevene

19291929

44

GriffithGriffith

19281928

55

AveryAvery

19441944

66

Hershey Hershey & Chase& Chase

19521952

PROTEINPROTEIN00 00 00 00 00 00 00

DNADNA00 00 00 00 11 44 55

Two more critical investigations…Two more critical investigations…

Before turning to their royal highnesses Before turning to their royal highnesses James Watson and Francis Crick…James Watson and Francis Crick…

1) Maurice Wilkins and Rosalind Franklin1) Maurice Wilkins and Rosalind Franklin

2) Erwin Chargaff2) Erwin Chargaff

Wilkins and FranklinWilkins and FranklinX-ray crystallographyX-ray crystallography

How X-ray Crystallography WorksHow X-ray Crystallography Works

Wilkins and FranklinWilkins and FranklinX-ray crystallographyX-ray crystallography

Wilkins and FranklinWilkins and Franklin19521952

The x-ray photographs of DNA, especially The x-ray photographs of DNA, especially those of Rosalind Franklin, were critical for those of Rosalind Franklin, were critical for Watson an Crick to help them figure out Watson an Crick to help them figure out DNA’s structure.DNA’s structure.

Many people think that Watson and Crick Many people think that Watson and Crick and Wilkins “dissed her.” More on that and Wilkins “dissed her.” More on that later.later.

Erwin ChargaffErwin Chargaffandand

The Base Pairing RulesThe Base Pairing RulesUsing more sophisticated techniques than Levene Using more sophisticated techniques than Levene

had in 1928, Chargaff analyzes the AGCT ratios.had in 1928, Chargaff analyzes the AGCT ratios.

Erwin Chargaff Erwin Chargaff 19521952

The Base Pairing RulesThe Base Pairing RulesAnalyze this…Analyze this…

Chargaff's Results, 1952Chargaff's Results, 1952

COMPARISON OF BASE RATIOS OF THE DNA OF SEVERAL SPECIESCOMPARISON OF BASE RATIOS OF THE DNA OF SEVERAL SPECIES

SourceSource

of DNAof DNA

AdenineAdenine

(A)(A)Guanine Guanine

(G)(G)Cytosine Cytosine

(C)(C)ThymineThymine

(T)(T)

HumanHuman 30.4%30.4% 19.6%19.6% 19.9%19.9% 30.1%30.1%

OxOx 29.0%29.0% 21.2%21.2% 21.2%21.2% 28.7%28.7%

Salmon spermSalmon sperm 29.7%29.7% 20.8%20.8% 20.4%20.4% 29.1%29.1%

Wheat germWheat germ 28.1%28.1% 21.8%21.8% 22.7 % 22.7 % 27.4%27.4%

E. coliE. coli 24.7%24.7% 26.0%26.0% 25.7%25.7% 23.6%23.6%

Sea UrchinSea Urchin 32.8%32.8% 17.7%17.7% 17.3%17.3% 32.1%32.1%

Chargaff Chargaff andand The Base Pairing The Base Pairing RulesRules Levine’s tetranucleotide idea is wrong !Levine’s tetranucleotide idea is wrong !

A=G=C=T is not right,A=G=C=T is not right, but what is right is that but what is right is that A=T A=T and and G=CG=C Levine applied good logic to bad data; so apply Levine applied good logic to bad data; so apply

Levene’s good logic to Chargaff’s good data.Levene’s good logic to Chargaff’s good data.

James Watson and Francis Crick 1953James Watson and Francis Crick 1953

Watson and Crick were theoretical biologists.Watson and Crick were theoretical biologists.

They did not so much work with white mice and They did not so much work with white mice and bubbling test tubes in the lab but rather took the bits bubbling test tubes in the lab but rather took the bits and pieces of information known about DNA and and pieces of information known about DNA and tried to make sense of it (in the office.)tried to make sense of it (in the office.)

James Watson and Francis Crick 1953James Watson and Francis Crick 1953 Watson and Crick were theoretical biologists… Watson and Crick were theoretical biologists…

Theoretically, the genetic molecule Theoretically, the genetic molecule mustmust…… Carry genetic information from cell to cell and from Carry genetic information from cell to cell and from

generation to generation and it generation to generation and it must carry a great amount of must carry a great amount of information.information.

It must carry information to copy itself and be able to do so It must carry information to copy itself and be able to do so with great precision.with great precision.

BUT... it must also make mistakes sometimes (mutate). BUT... it must also make mistakes sometimes (mutate). Mistakes (mutations) must then be copied as faithfully as the Mistakes (mutations) must then be copied as faithfully as the original. If it could not copy its mutations, there could be no original. If it could not copy its mutations, there could be no evolution by natural selection.evolution by natural selection.

There must be some mechanism for decoding the stored There must be some mechanism for decoding the stored genetic information and translating it into action in the living genetic information and translating it into action in the living cell.cell.

James Watson and Francis CrickJames Watson and Francis Crick 19531953 ……took the bits and pieces of information known took the bits and pieces of information known

about DNA and tried to make sense of it…about DNA and tried to make sense of it… The DNA molecule known to be a very large, long, and The DNA molecule known to be a very large, long, and

very thin molecule composed of four sub-units: adenine very thin molecule composed of four sub-units: adenine nucleotides, guanine nucleotides, cytosine nucleotides and nucleotides, guanine nucleotides, cytosine nucleotides and thymine nucleotides.thymine nucleotides.

Levine thought these nucleotides were arranged in Levine thought these nucleotides were arranged in repeating units of four--"tetranucleotides" and thought the repeating units of four--"tetranucleotides" and thought the molecule was therefore pretty uninteresting=boring!molecule was therefore pretty uninteresting=boring!

Linus Pauling had shown that protein molecules were often Linus Pauling had shown that protein molecules were often great large helices, and suggested that DNA may also be a great large helices, and suggested that DNA may also be a helix.helix.

Wilkin's and Franklin's X-ray diffraction photos showed Wilkin's and Franklin's X-ray diffraction photos showed patterns that very strongly suggested that DNA was helical.patterns that very strongly suggested that DNA was helical.

Chargaff's data, which contradicted Levine's, suggested Chargaff's data, which contradicted Levine's, suggested that A=T and C=G.that A=T and C=G.

By Jove, we’ve got it !!!By Jove, we’ve got it !!!

Watson and Crick’s PropasalWatson and Crick’s Propasal

Watson and Crick’s ProposalWatson and Crick’s Proposal

DNA ReplicationDNA Replication It must carry information to copy itself and be It must carry information to copy itself and be

able to do so with great precision.able to do so with great precision.

The 1962 Nobel PrizeThe 1962 Nobel Prize