Marwan Alhalabi Professor of Reproductive Medicine and Infertility,
Damascus University
Head of Assisted Reproduction Unit, Orient Hospital
President of Middle East Fertility SocietyPresident of Syrian Society of Obstetricians and Gynecologists
• Theprogenyoforganismdevelopscharacterssimilartothat
organism
• Theresemblanceofoffspringtotheirparentsdependson
theprecisetransmissionofprinciplecomponentfromone
generationtothenext
• Thatcomponentis-
TheGeneticMaterial
Thegeneticmaterialofacelloranorganismreferstothosematerialsfoundinthenucleus,mitochondriaandcytoplasm,whichplayafundamentalroleindeterminingthestructureandnatureofcellsubstances,andcapableofself-propagatingandvariation.
DNAProtein
RNA
Protein,RNA and DNA were thought as genetic material.
But many experiments
suggest DNA as genetic material
rather than protein and RNA
•Mustcarryinformation– Crackingthegeneticcode
•Mustselfreplicate– DNAreplication
•Mustallowforinformationtochange
– Mutation
•Mustgoverntheexpressionofthe
phenotype– Genefunction
RNA
DNA
PROTEINDNA
• Theprocessofidentificationofgeneticmaterialbeganin
1928withexperimentsofGriffithandconcludedin1952
withthestudiesofHersheyandChase.
• Betweenthesetwoexperimentsotherthreescientists,
Avery,Macloed andMcCartyweredidanexperimentto
identifythegeneticmaterial.
• 1952AlfredHersheyandMarthaChase
• NewYorkscientists
• Performedanexperimentthatsettledthecontroversy
• ProvedthatDNAcarriesthegeneticmaterial
• Bacteriophage: a virus thatinfects bacteria.
• When Bacteriophages infectbacterial cells they producemore viruses.
• The viruses are releasedwhen the bacterial cellsrupture.
• Howdoesthebacteriophagereprogramthebacterialcelltomakemoreviruses?
• DoesthebacteriophageDNA,theprotein,orbothgiveinstructionstothebacteria?
• BacteriophageDNAwaslabeledwithradioactivephosphorus(32P)
• Bacteriophageproteinwaslabeledwithradioactivesulphur(35S)
• onlythebacteriophageDNA(asindicatedbythe32P)enteredthebacteriaandwasusedtoproducemorebacteriophage
• Conclusion:DNAisthegeneticmaterial
• The final evidence that DNA transmits genetic informationwas provided by Hershey and Chase in 1952
• They experimented with T2 bacteriophages, viruses thatattack bacteria.
• 1928
• FredrickGriffith
• Bacteriologist
• Tryingtoprepareavaccineagainstpneumonia
Controls
• Two types, or strains, of S. pneumoniae
• First strain is enclosed in a capsule composed ofpolysaccharides.• Capsule protects the bacterium from the body’ sdefense system.
• Forms smooth-edges (S) when grown in a petri dish
• Helps make the microorganism virulent, or able tocause disease.
• Secondstrainlacksthepolysaccharidecapsuleanddoesnotcausedisease.• Formsrough-edges(R)whengrowninapetridish
• The harmless R bacteria had changed and
became virulent S bacteria.
• Transformation is a change in genotype
caused when cells take up foreign material.
• BasedontheseobservationsheconcludedthatsomeofthecellsoftypeIIRhadchangedintotypeIIISduetoinfluenceofdeadtypeIIIScells
• Hecalledthisphenomenonastransformation
• PrincipleComponentoftypeIIIScellswhichinducedtheconversionoftypeIIRcellsintotypeIIISwasnamedtransformingprinciple.
TheStructureofDNA
• DoubleHelix
• Nucleotide
• Deoxyribose
• Base-pairingRules
• ComplementaryBasePair
• 1953—JamesWatsonandFrancisCrickdeterminedthestructureoftheDNAmoleculetobeadoublehelix• 2strandsofnucleotidestwistedaround
eachother
• They proposed that DNA
as a right handed double
helix with two poly
nucleotide chains are
coiled about one another
in a spiral.
(WatsonandCrick,1953)
• RosalindFranklincontributedtothisdiscoverybyproducinganX-raycrystallographicpictureofDNA• Determinedhelixwasauniformdiameterandcomposed
of2strandsofstackednucleotides
DNA=tightlywoundhelix
• NucleotidesarethemonomericunitsthatmakeupDNAØ 3mainparts:
Ø5carbonsugar—deoxyribose
ØPhosphategroup
ØNitrogenousbase
NitrogenousbasesPyrimidines:single-ringstructures
Thymine(T)
Cytosine(C)
Purines:larger,double-ringstructures
Adenine(A)
Guanine(G)
• Generate and store energy in ATP form
(eukaryote cells).
• Have DNA and mtDNA is double
stranded circular molecule.
• Circular mtDNA has 37 genes. 1 part (D-
loop) does’ t contain genes. Why?
Becomes displaced during replication.
Notice:
DNAstrandsruninoppositedirections=ANTI-PARALLEL
P
S
S
S
P P
P
SGC
AT
Chargaff’sbasepairingrule:
Percentofadenine=percentofthymine(A=T)
Percentofcytosine=percentofguanine(C=G)
A+G=T+C(orpurines=pyrimidines)
(Chargaffetal.,1950)
ErwinChargaffshowedtheamountsofthefourbasesonDNA(A,T,C,G)Inabodyorsomaticcell:
A=30.3%T=30.3%G=19.5%C=19.9%
T A
G C
Ø Three major forms:
ü B-DNA
ü A-DNA
ü Z-DNA
v B-DNAis biologically THE MOST COMMON
ü It is a helix meaning that it has a Right handed, or clockwise, spiral.
ü Ideal B-DNA has 10 base pair per turn
ü So each base is twisted 36o relative to adjacent bases.
ü Base pair are 0.34 nm apart.
ü So complete rotation ofmolecule is 3.4 nm.
ü Axis passes throughmiddle of each basepairs.
ü MinorGrooveisNarrow,Shallow.ü Major GrooveisWide,Deep.ü Thisstructureexistswhen plenty
ofwatersurroundsmoleculeandthereisnounusualbasesequenceinDNA-Conditionthatarelikelytobepresentinthecells.
ü B-DNA structureismoststableconfiguration forarandomsequenceofnucleotidesunderphysiologicalcondition.
A-DNAü Right-handedhelixü WiderandflatterthanB-DNAü 11bpperturnü Itsbasesaretiltedawayfrom
mainaxisofmoleculeü NarrowDeep majorGrooveand
Broad,ShallowminorGroove.ü Observedwhen lesswateris
present.i.e.Dehydratingcondition.ü A-DNAhasbeenobservedin
twocontext:• ActivesiteofDNApolymerase
(~3bp)• Gram(+)bacteriaundergoing
sporulation
Z-DNA• Aleft-handed helix• SeeninConditionofHigh saltconcentration.• Inthisformsugar-phosphatebackboneszigzag back• andforth,givingrisetothenameZ-DNA(forzigzag).• 12 basepairsperturn.• A deepMinorGroove.• No DiscernibleMajorGroove.• Partofsomeactivegenesform
Z-DNA, suggestingthatZ-DNAmayplayaroleinregulatinggenetranscription.
Property B-DNA A-DNA Z-DNA
Strand Antiparallel Antiparallel Antiparallel
TypeofHelix Right-handed Right-handed Left-handed
Overallshape Longandnarrow
Shortandwide Elongatedandnarrow
Basepairperturn 10 11 12
Distancebetweenadjacentbases
0.34nm 0.23nm 0.38nm
Pitch/turnofhelix 3.40nm 2.82nm 4.56nm
HelicalDiameter 2.0nm 2.3nm 1.8nm
Tilt/inclination ofbptoaxis 10 200 90
Property B-DNA A-DNA Z-DNAMajorGroove Wide& Deep Narrow&Deep Nodiscrenible
MinorGroove Narrow, shallow Broad, Shallow Narrow,Deep
• Two strands wind about each otherin a right-handed manner
• Diameter: ~20Å• Bases per turn: 10 (~34Å)• A major and a minor groove
20Å
Major
Minor
• OnestrandofDNAgoesfrom5’to3’(sugars)
• Theotherstrandisoppositeindirectiongoing3’to5’(sugars)
• DNAiswrappedaroundabundantnuclearproteinscalledHistones
• ThisformsacomplexcalledaNucleosome
• HistonesareH1,H2A,H2B,H3,H4