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