Post on 01-Jan-2016
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