Chapter 16

Molecular Basis of Inheritance

Deciphering DNA

The Search for Genetic MaterialKnown

Genes on chromosomesChromosomes made of DNA and protein

UnknownWhich chromosomal component was the genetic

materialProtein

Heterogeneous class of macromolecules with specific functions

Case stronger initiallyNucleic acids

Physical and chemical properties too uniform for amount of variation

Experimentation gradually changed perceptions

DNA’s role clarified by studying bacteria and their viruses

Frederick Griffitho Streptococcus pneumoniae modelo S encapsulated and virulent; R non-encapsulated and non-virulento Heat killed S cells mixed with R cells created S cellso Concluded that S cells have a chemical component that can transform other cells

Oswald AveryIdentified the transforming substance from

Griffith’s work as DNAFocused on DNA, RNA, and protein

Extract components from pathogenic bacteriaEach individually inactivated and tested for

transformation abilityDegradation of DNA only substance to prevent

Not uniformly supportedProteins better candidatesDoubted bacterial DNA similar to that of complex

organismsLittle still known about DNA

Alfred Hershey and Martha Chase

o Tracked protein and DNA of E. coli phage T2o Bacteriophag

e is a virus that infects bacteria

o Radioactive isotopes to label cells

o Determined that DNA entered bacteria and directed virus reproduction not protein

Existing Knowledge of DNAPolymer of nucleotides

with 3 componentsPentose sugar

(deoxyribose) and a phosphate group

Purines = two ringsAdenine (A)Guanine (G)

Pyrimidines = one ring Thymine (T)Cytosine (C)

Erwin ChargaffThe amount of A, T, G, and C in the DNA vary from

species to speciesEvidence of molecular diversity to increase DNA credibility

Chargaff’s rulesIn each species, the amount of A = T while the amount of

C = GImportance unknown until discovery of double helix

Organism A T C GHuman 30.3% 30.3% 19.9% 19.5%

Chicken 28.8% 29.2% 20.5% 21.5%

Grasshopper 29.3%  29.3% 20.5% 20.7%

Sea Urchin 32.8% 32.1% 17.7% 17.3%

Wheat 27.3% 27.1% 22.7% 22.8%

Yeast 31.3% 32.9% 18.7% 17.1%

E. coli 24.7% 23.6% 26.0% 25.7%

Rosalind FranklinX-ray diffraction image

of DNADNA is helical in

structureUniform in width and

spacing between basesSuggested that there were

2 strands = double helix

Concluded that sugar-phosphate backbones were on the outside

Evidence was groundwork for Watson and Crick

James Watson and Francis Crick

Double helix with anti-parallel strandsSugar-phosphate backbone on outsidePaired nitrogenous bases on inside

Complimentary hydrogen binding of a purine and a pyrimidineA with T form 2 bonds, G with C form 3 bonds

Consistent with Chargoff and FranklinAwarded the Nobel Prize

DNA Double-Helix Structure

DNA Replication

DNA Replication

Each strand of original DNA serves as a templateNucleotides match to template according to base

pairing rules1 ‘parent’ DNA strand produces 2 new ‘daughter’

strands

A) Two parent strands eventually come back together

B) Watson and Crick: each daughter strand with 1 old parent strand

C) All four strands have a mixture of new and old DNA

Matthew Meselson and Franklin Stahl’s work confirmed the semi-conservative model

DNA Replication Models

Replication Efficiency

E. coli with 4.6 million nucleotide pairs replicates in less than an hour

Humans with 6 billion pairs a few hours, with only about 1 error every 10 billion nucleotides

Enzymes and proteins are responsibleBetter understood in prokaryotes than

eukaryotesProcess is fundamentally similar

Origins of ReplicationShort specific

nucleotides sequencesProkaryotes with

1, eukaryotes with multiple

Proteins recognize and attach

Separates strands and opens them up to form a replication bubble

Proceeds in both directions until fully copied

Overall DNA ReplicationCOMPONENTS (Table 16.1)• Helicase• Single-strand binding protein• Topoisomerase • Primer and primase• DNA pol III and I• Leading and lagging strands• Okazaki fragments• DNA ligase

KEY POINTS:• DNA pol binds to 3’ end• Strands grow 5’ 3’ only

Proofreading and Repairing DNA

DNA polymerases also proofread each nucleotideIncorrect pairs are removed

Mismatch pairs result from those that evaded the polymerasesAlternate enzymes remove and

replaceNucleases cut out damaged DNA

Polymerases and ligases fill gap with nucleotidesSkin cell repair from UV light damage