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Biology 102Biology 102

Lecture 11: DNALecture 11: DNA

DNA and Information StorageDNA and Information Storage

•• DNA stores a vast amount of information in a DNA stores a vast amount of information in a cellcell

•• By far the most compact, efficient method By far the most compact, efficient method knownknown

•• Virtually all cell function is a result of proteins Virtually all cell function is a result of proteins and their interactionsand their interactions

•• Information for every protein is stored in DNAInformation for every protein is stored in DNA

•• Used to be thought that DNA was too simple to Used to be thought that DNA was too simple to be importantbe important

•• Two experiments disproved this…Two experiments disproved this…

Cell PhysiologyCell Physiology

•• Change was termed Change was termed transformationtransformation

•• 1944 1944 –– proved that the transforming agent is proved that the transforming agent is DNADNA

•• Results from bacteria’s ability to take up DNA Results from bacteria’s ability to take up DNA from environment and replicate itfrom environment and replicate it

•• We now exploit this to generate DNA for We now exploit this to generate DNA for experimentsexperiments

DNADNA

•• Virus = protein coat with RNA or DNA insideVirus = protein coat with RNA or DNA inside

•• DNA contains phosphorusDNA contains phosphorus

•• Grow viruses in radioactive Grow viruses in radioactive

phosphorus = viruses with phosphorus = viruses with

radioactive DNAradioactive DNA

•• Protein contains sulfurProtein contains sulfur

•• Grow viruses in radioactive Grow viruses in radioactive

sulfur = viruses with sulfur = viruses with

radioactive protein coatradioactive protein coat

DNADNA

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•• Proved that only DNA is transferred from virus Proved that only DNA is transferred from virus to infected cellto infected cell

•• Despite this, new viruses were made in infected Despite this, new viruses were made in infected cells that contained both DNA and protein coatscells that contained both DNA and protein coats

•• Protein is not required for information transferProtein is not required for information transfer

•• DNA must beDNA must be

DNADNA

•• Considered one of the most significant Considered one of the most significant discoveries in the history of sciencediscoveries in the history of science

•• Credited to James Watson and Francis Crick in Credited to James Watson and Francis Crick in 19531953

Structure of DNAStructure of DNA

•• Their discovery was only possible because of Their discovery was only possible because of XX--ray reflection images by Rosalind Franklin ray reflection images by Rosalind Franklin

(and some unauthorized access)(and some unauthorized access)

Structure of DNAStructure of DNA

•• These images led to the discovery of the doubleThese images led to the discovery of the double--helical structure of DNAhelical structure of DNA

Structure of DNAStructure of DNA

•• Recall: DNA strands are long chains of Recall: DNA strands are long chains of nucleotide monomersnucleotide monomers

•• All DNA nucleotides contain…All DNA nucleotides contain…

•• PhosphatePhosphate

•• DeoxyriboseDeoxyribose (sugar)(sugar)

•• Nitrogenous base (1 of 5)Nitrogenous base (1 of 5)

NucleotidesNucleotides

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Adenine nucleotideAdenine nucleotide

•• DNA = A G C TDNA = A G C T

•• RNA = A G C URNA = A G C U

•• GoutGout

Nucleotide BasesNucleotide Bases

•• The phosphate of one nucleotide binds to the The phosphate of one nucleotide binds to the sugar of another nucleotidesugar of another nucleotide

•• SugarSugar--phosphate backbonephosphate backbone

•• Bases project out at right angleBases project out at right angle

Structure of Nucleic AcidsStructure of Nucleic Acids

•• RNA is singleRNA is single--strandedstranded

•• DNA made of two antiDNA made of two anti--

parallel strandsparallel strands

•• Complimentary baseComplimentary base--

pairing with hydrogen bondspairing with hydrogen bonds

holds the strands togetherholds the strands together

•• Twisted into a double helixTwisted into a double helix

Structure of DNAStructure of DNA

•• Only Only purine:pyramidinepurine:pyramidine pairs fit inside the pairs fit inside the double helixdouble helix

Structure of DNAStructure of DNA Structure of DNAStructure of DNA

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•• Hydrogen bonds have to line up tooHydrogen bonds have to line up too

•• Let’s see how some combinations might bind to Let’s see how some combinations might bind to each othereach other

Structure of DNAStructure of DNA

•• Number of hydrogen bonds? Number of hydrogen bonds? NoneNone

•• Width of base pair? Width of base pair? Too wideToo wide

Can A Bind to A? NO!!Can A Bind to A? NO!!

•• Number of hydrogen bonds? NoneNumber of hydrogen bonds? None

•• Width of base pair? CorrectWidth of base pair? Correct

Can C Bind to A? NO!!Can C Bind to A? NO!!

•• Number of hydrogen bonds? Number of hydrogen bonds? 22

•• Width of base pair? Width of base pair? CorrectCorrect

Can A Bind to T? YES!!Can A Bind to T? YES!!

•• Number of hydrogen bonds? Number of hydrogen bonds? NoneNone

•• Width of base pair? Width of base pair? Too wideToo wide

Can G Bind to G? NO!!Can G Bind to G? NO!!

•• Number of hydrogen bonds? Number of hydrogen bonds? 33

•• Width of base pair? Width of base pair? CorrectCorrect

Can G Bind to C? YES!!Can G Bind to C? YES!!

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•• Number of hydrogen bonds? Number of hydrogen bonds? 2?2?

•• Width of base pair? Width of base pair? Too narrowToo narrow

Can T Bind to T? NO!!Can T Bind to T? NO!!

•• Shapes have to complimentaryShapes have to complimentary

•• Hydrogen bonds have to line upHydrogen bonds have to line up

•• Means only 2 combinations are possibleMeans only 2 combinations are possible

•• A binds to TA binds to T

•• C binds to GC binds to G

Structure of DNAStructure of DNA

Structure of DNAStructure of DNA Reconstruct The Missing StrandReconstruct The Missing Strand(Or DNA Matchmaking)(Or DNA Matchmaking)

Reconstruct The Missing StrandReconstruct The Missing Strand(Or DNA Matchmaking)(Or DNA Matchmaking)

•• DNA strand is very long and delicateDNA strand is very long and delicate

•• PRECIOUS PRECIOUS –– must be protectedmust be protected

•• Associated with organizing proteins called Associated with organizing proteins called

histoneshistones

•• HistonesHistones + DNA = + DNA = chromatinchromatin

•• Subdivided into smaller portions called Subdivided into smaller portions called chromosomeschromosomes

DNA OrganizationDNA Organization

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•• Most human cells have 46 chromosomesMost human cells have 46 chromosomes

•• If all were of equal length, each would contain 65 If all were of equal length, each would contain 65 million base pairsmillion base pairs

•• Actual length varies considerablyActual length varies considerably

DNA OrganizationDNA Organization

•• Each chromosome carries information to Each chromosome carries information to synthesize several proteinssynthesize several proteins

•• Portion of DNA that codes for a single protein is Portion of DNA that codes for a single protein is called a called a genegene

•• Physical location of a gene on a chromosome is Physical location of a gene on a chromosome is called a called a locuslocus

DNA OrganizationDNA Organization

•• Massive international project to map the human Massive international project to map the human genomegenome

•• 25,000 genes25,000 genes

•• 3 billion base pairs3 billion base pairs

•• 22--3% code for proteins (mostly the same)3% code for proteins (mostly the same)

•• The rest = ? (DNA “fingerprint”)The rest = ? (DNA “fingerprint”)

Human Genome Project: 1990Human Genome Project: 1990--20032003