DNA Structure DNA Structure and Functionand Function

Chapter 13Chapter 13

Miescher Discovered Miescher Discovered DNADNA

18681868 Johann Miescher investigated the Johann Miescher investigated the

chemical composition of the nucleuschemical composition of the nucleus Isolated an organic acid that was high in Isolated an organic acid that was high in

phosphorusphosphorus He called it nucleinHe called it nuclein We call it DNA (deoxyribonucleic acid)We call it DNA (deoxyribonucleic acid)

Mystery of the Mystery of the Hereditary MaterialHereditary Material

Originally believed to be an Originally believed to be an unknown class of proteinsunknown class of proteins

Thinking wasThinking was Heritable traits are diverseHeritable traits are diverse Molecules encoding traits must be Molecules encoding traits must be

diversediverse Proteins are made of 20 amino acids Proteins are made of 20 amino acids

and are structurally diverseand are structurally diverse

Structure of Structure of the the

Hereditary Hereditary MaterialMaterial Experiments in the 1950s Experiments in the 1950s

showed that DNA is the showed that DNA is the hereditary materialhereditary material

Scientists raced to Scientists raced to determine the structure of determine the structure of DNADNA

1953 - Watson and Crick 1953 - Watson and Crick proposed that DNA is a proposed that DNA is a double helixdouble helix

Figure 13.2Page 217

Griffith Discovers Griffith Discovers TransformationTransformation

19281928 Attempting to develop a vaccineAttempting to develop a vaccine Isolated two strains of Isolated two strains of Streptococcus Streptococcus

pneumoniaepneumoniae Rough strain was harmlessRough strain was harmless Smooth strain was pathogenicSmooth strain was pathogenic

Griffith Discovers Griffith Discovers TransformationTransformation

1. Mice injected with live cells of harmless strain R.

2. Mice injected with live cells of killer strain S.

3. Mice injected with heat-killed S cells.

4. Mice injected with live R cells plus heat-killed S cells.

Mice die. Live S cells in their blood.

Mice live. No live R cells in their blood.

Mice die. Live S cells in their blood.

Mice live. No live S cells in their blood.

Figure 13.3Page 218

TransformationTransformation

What happened in the fourth What happened in the fourth experiment?experiment?

The harmless R cells had been The harmless R cells had been transformedtransformed by material from the by material from the dead S cellsdead S cells

Descendents of the transformed Descendents of the transformed cells were also pathogeniccells were also pathogenic

Oswald & AveryOswald & Avery

What is the transforming material?What is the transforming material? Cell extracts treated with protein-Cell extracts treated with protein-

digesting enzymes could still digesting enzymes could still transform bacteriatransform bacteria

Cell extracts treated with DNA-Cell extracts treated with DNA-digesting enzymes lost their digesting enzymes lost their transforming abilitytransforming ability

Concluded that DNA, not protein, Concluded that DNA, not protein, transforms bacteriatransforms bacteria

BacteriophagesBacteriophages

Viruses that Viruses that infect bacteriainfect bacteria

Consist of Consist of protein and DNAprotein and DNA

Inject their Inject their hereditary hereditary material into material into bacteriabacteria

cytoplasm

bacterial cell wall plasma

membrane

Figure 13.4bPage 219

Hershey & Chase’s Hershey & Chase’s ExperimentsExperiments

Created labeled bacteriophagesCreated labeled bacteriophages Radioactive sulfur Radioactive sulfur Radioactive phosphorus Radioactive phosphorus

Allowed labeled viruses to infect Allowed labeled viruses to infect bacteriabacteria

Asked: Where are the radioactive Asked: Where are the radioactive labels after infection?labels after infection?

virus particle labeled with 35S

virus particle labeled with 32P

bacterial cell (cutaway view)

label outside cell

label inside cell

HershHershey and ey and Chase Chase ResultResult

ss

Figure 13.5Page 219

Structure of Nucleotides Structure of Nucleotides in DNAin DNA

Each nucleotide consists ofEach nucleotide consists of Deoxyribose (5-carbon sugar) Deoxyribose (5-carbon sugar)

Phosphate groupPhosphate group

A nitrogen-containing baseA nitrogen-containing base

Four basesFour bases Adenine, Guanine, Thymine, CytosineAdenine, Guanine, Thymine, Cytosine

Nucleotide BasesNucleotide Bases

phosphate group

deoxyribose

ADENINE (A)

THYMINE (T)

CYTOSINE (C)

GUANINE (G)

Figure Figure 13.613.6

Page 220Page 220

Composition of DNAComposition of DNA

Chargaff showed:Chargaff showed: Amount of adenine relative to guanine Amount of adenine relative to guanine

differs among speciesdiffers among species Amount of adenine always equals Amount of adenine always equals

amount of thymine and amount of amount of thymine and amount of

guanine always equals amount of guanine always equals amount of

cytosinecytosine

A=T and G=CA=T and G=C

Rosalind Franklin’s WorkRosalind Franklin’s Work

Was an expert in X-ray Was an expert in X-ray crystallographycrystallography

Used this technique to examine DNA Used this technique to examine DNA fibers fibers

Concluded that DNA was some sort Concluded that DNA was some sort of helixof helix

Watson-Crick ModelWatson-Crick Model

DNA consists of two nucleotide strandsDNA consists of two nucleotide strands

Strands run in opposite directionsStrands run in opposite directions

Strands are held together by hydrogen Strands are held together by hydrogen

bonds between basesbonds between bases

A binds with T and C with GA binds with T and C with G

Molecule is a double helixMolecule is a double helix

Watson-Watson-Crick ModelCrick Model

Figure Figure 13.713.7

Page 221Page 221

DNA Structure Helps DNA Structure Helps Explain How It Explain How It

DuplicatesDuplicates

DNA is two nucleotide strands DNA is two nucleotide strands

held together by hydrogen bondsheld together by hydrogen bonds

Hydrogen bonds between two Hydrogen bonds between two

strands are easily brokenstrands are easily broken

Each single strand then serves as Each single strand then serves as

template for new strandtemplate for new strand

DNA DNA ReplicatiReplicati

onon

newnew old old

Each parent Each parent

strand remains strand remains

intactintact

Every DNA Every DNA

molecule is half molecule is half

“old” and half “old” and half

“new”“new”Figure Figure 13.913.9

Page 222Page 222

Base Base Pairing Pairing during during

ReplicationReplication

Each old Each old strand serves strand serves as the as the template for template for complementacomplementary new ry new strandstrand

Figure 13.10Page 223

Enzymes in ReplicationEnzymes in Replication

Enzymes unwind the two strandsEnzymes unwind the two strands

DNA polymerase attaches DNA polymerase attaches

complementary nucleotides complementary nucleotides

DNA ligase fills in gaps DNA ligase fills in gaps

Enzymes wind two strands togetherEnzymes wind two strands together

Continuous and Continuous and Discontinuous AssemblyDiscontinuous Assembly

Strands can only be assembled in the 5’ to 3’ direction

Figure Figure 13.1013.10

Page 223Page 223

DNA RepairDNA Repair

Mistakes can occur during Mistakes can occur during

replicationreplication

DNA polymerase can read correct DNA polymerase can read correct

sequence from complementary sequence from complementary

strand and, together with DNA strand and, together with DNA

ligase, can repair mistakes in ligase, can repair mistakes in

incorrect strandincorrect strand

CloningCloning

Making a genetically identical copy Making a genetically identical copy

of an individualof an individual

Researchers have been creating Researchers have been creating

clones for decadesclones for decades

These clones were created by These clones were created by

embryo splittingembryo splitting

Showed that differentiated cells Showed that differentiated cells

could be used to create clonescould be used to create clones

Sheep udder cell was combined Sheep udder cell was combined

with enucleated egg cellwith enucleated egg cell

Dolly is genetically identical to the Dolly is genetically identical to the

sheep that donated the udder cellsheep that donated the udder cell

Dolly: Dolly: Cloned from an Adult Cloned from an Adult

CellCell

More ClonesMore Clones

MiceMice CowsCows PigsPigs GoatsGoats Guar (endangered Guar (endangered

species)species)