Discovering DNA: Structure and Replication

Discovering DNA: Discovering DNA: Structure and ReplicationStructure and Replication

A collaborative effort!A collaborative effort!

The discovery of DNA resulted from The discovery of DNA resulted from the combination of contributions the combination of contributions from several scientists.from several scientists.

Each conducted experiments that Each conducted experiments that provided different pieces of provided different pieces of information needed to solve the information needed to solve the puzzle of the role and structure of puzzle of the role and structure of DNA.DNA.

Good things happen when Good things happen when you’re not looking…….you’re not looking…….

Griffith’s ExperimentGriffith’s Experiment 1928 – Frederick Griffith conducted 1928 – Frederick Griffith conducted

experiments with mice to study the experiments with mice to study the effects of pneumonia bacteria.effects of pneumonia bacteria.

Griffith isolated two strains of Griffith isolated two strains of pneumonia bacteria – one caused the pneumonia bacteria – one caused the lung disease (pneumonia) and the other lung disease (pneumonia) and the other did not.did not.

Griffith’s ExperimentGriffith’s Experiment Mice injected with the Mice injected with the

disease causing disease causing bacteria died.bacteria died.

Mice injected with the Mice injected with the harmless bacteria lived.harmless bacteria lived.

Mice injected with the Mice injected with the heat-killed disease heat-killed disease causing bacteria also causing bacteria also lived.lived.

Griffith then mixed the Griffith then mixed the live harmless bacteria live harmless bacteria with heat-killed disease with heat-killed disease causing bacteria.causing bacteria.

Mice injected with this Mice injected with this combination died.combination died.

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Griffith’s ExperimentGriffith’s Experiment

Based on his results, Griffith Based on his results, Griffith hypothesized that when the harmless hypothesized that when the harmless and heat-killed bacteria where and heat-killed bacteria where mixed, some factor was exchanged mixed, some factor was exchanged between them, making the live between them, making the live harmless bacteria deadly.harmless bacteria deadly.

TransformationTransformation – process in which – process in which one strain of bacteria is changed by one strain of bacteria is changed by the gene(s) of another bacteriathe gene(s) of another bacteria

Avery modifies Griffith’s Avery modifies Griffith’s experiment.experiment. In 1943, Avery, with fellow In 1943, Avery, with fellow

scientists, conducted an scientists, conducted an experiment similar to experiment similar to Griffith’s, except they used Griffith’s, except they used enzymes to selectively enzymes to selectively destroy molecules one at a destroy molecules one at a time.time.

When they injected harmless When they injected harmless bacteria with only lipids, bacteria with only lipids, carbs, or proteins: carbs, or proteins: transformation did not occur. transformation did not occur.

When they used the nucleic When they used the nucleic acids (DNA): transformation acids (DNA): transformation did occur, the bacteria did occur, the bacteria became lethal.became lethal.

This helped to determine that This helped to determine that DNA stores and transmits DNA stores and transmits genetic information.genetic information.

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Hershey and ChaseHershey and Chase Hershey and Chase (1950) Hershey and Chase (1950)

conducted experiments conducted experiments with with bacteriophagesbacteriophages (viruses that attack (viruses that attack bacteria) to determine if bacteria) to determine if genetic information is genetic information is carried on proteins or carried on proteins or DNA.DNA.

They used radioactive They used radioactive elements to ‘mark’ DNA elements to ‘mark’ DNA and protein.and protein.

Only the radioactively-Only the radioactively-labeled DNA was found in labeled DNA was found in bacteria cells.bacteria cells.

These findings further These findings further supported the conclusions supported the conclusions of Avery’s experiment & of Avery’s experiment & specified that specified that genetic genetic material is DNAmaterial is DNA and and NOT protein. NOT protein.

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Chargaff’s RuleChargaff’s Rule Chargaff determined that in any Chargaff determined that in any

sample of DNA:sample of DNA: The # of adenines (A) = the # of thymines The # of adenines (A) = the # of thymines

(T)(T) The # of cytosines (C) = the # of guanines The # of cytosines (C) = the # of guanines

(G)(G) Thus in DNA, the bases A and T pair Thus in DNA, the bases A and T pair

together, and C and G pair together.together, and C and G pair together.

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Rosalind FranklinRosalind Franklin . Franklin used x-. Franklin used x-

ray diffraction to ray diffraction to create pictures of create pictures of DNA’s molecular DNA’s molecular structurestructure

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Watson and CrickWatson and Crick James Watson and Frances James Watson and Frances

Crick determined the Crick determined the structure of DNA in 1953 structure of DNA in 1953 using their data and the using their data and the work of previous scientists. work of previous scientists.

Watson got a sneak peak at Watson got a sneak peak at Franklin’s X-ray images and Franklin’s X-ray images and used them with other used them with other evidence to determine evidence to determine DNA’s structure.DNA’s structure.

The structure of DNA was The structure of DNA was determined to be shaped determined to be shaped like a double helix, with like a double helix, with strands held together by strands held together by the weak hydrogen bonds the weak hydrogen bonds formed between the bases formed between the bases A-T and C-G.A-T and C-G.

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What is DNA?What is DNA? DNADNA = = Deoxyribose Deoxyribose

Nucleic AcidNucleic Acid Code of LifeCode of Life – codes for – codes for

proteinsproteins essential to essential to the health of the the health of the organism.organism.

Backbone made of Backbone made of alternating alternating Phosphorus and Phosphorus and SugarsSugars

Made of matching Made of matching Nitrogen basesNitrogen bases Adenine to TyrosineAdenine to Tyrosine Cytosine to GuanineCytosine to Guanine

What is the structure of What is the structure of DNA?DNA?

DNA is composed of DNA is composed of Nitrogen basesNitrogen bases, (ATGC) , (ATGC) anchored to a anchored to a deoxyribose sugardeoxyribose sugar along along a a Phosphate backbonePhosphate backbone..

NucleotidesNucleotides = 5 = 5 carbon sugar, carbon sugar, phosphate and a phosphate and a Nitrogen base.Nitrogen base.

Each nitrogen base is Each nitrogen base is bonded by weak bonded by weak Hydrogen BondsHydrogen Bonds.. Those bonds are weak for Those bonds are weak for

DNA to replicate or DNA to replicate or transcribe.transcribe.

The Structure of DNAThe Structure of DNA On the On the

diagram:diagram: Circle and label Circle and label

a a nucleotidenucleotide.. Label the Label the sugarsugar

and and phosphatephosphate molecules.molecules.

Label the Label the basesbases that are not that are not already labeledalready labeled

Label a Label a base base pairpair..

Label the Label the sugar-sugar-phosphate phosphate backbonesbackbones..

Label the Label the hydrogen hydrogen bondsbonds..

nucleotide

P

S

A

C

A

A

T

T

CG

G

Base pairSugar /P

backbone

G

Sugar /P

backbone

Hydrogen bonds

Base Pairing PracticeBase Pairing Practice For each example below, please give the For each example below, please give the

correct complementary strand of DNA.correct complementary strand of DNA. T T G C T A GT T G C T A G A A C G A T CA A C G A T C

T A G C G C TT A G C G C T A T C G C G AA T C G C G A

A C C G T C AA C C G T C A T G G C A G TT G G C A G T

G C T A T G T G C T A T G T C G A T A C A C G A T A C A

DNA ReplicationDNA Replication DNA must be replicated (copied) in order to insure DNA must be replicated (copied) in order to insure

that during cell division, each daughter cell that during cell division, each daughter cell receives a complete copy.receives a complete copy.

DNA replication occurs in the nucleus during S DNA replication occurs in the nucleus during S phase of the cell cycle, before chromatin (DNA phase of the cell cycle, before chromatin (DNA wrapped around proteins) condenses into wrapped around proteins) condenses into chromosomes.chromosomes.

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DNA Replication DNA Replication DNA Replication occurs in four basic steps:DNA Replication occurs in four basic steps:

Step 1 – Helicase unzips the strand of DNA by breaking the Step 1 – Helicase unzips the strand of DNA by breaking the weak hydrogen bonds between base pairs.weak hydrogen bonds between base pairs.

Step 2 – DNA polymerase inserts the appropriate bases.Step 2 – DNA polymerase inserts the appropriate bases. Step 3 – A new sugar-phosphate backbone is built.Step 3 – A new sugar-phosphate backbone is built. Step 4 – The sequence is proofread by DNA polymerase.Step 4 – The sequence is proofread by DNA polymerase.

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DNA replication is semi-conservative because each side of the parent strand serves as a template for the 2 new DNA strands.