genetic material

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