- 1. DNA STRUCTURE and DNA REPLICATIONPAGDATOON, RIZHA
C.PEAMANTE, KEYSEAN M.AAPD2F
2. Deoxyribose Nucleic AcidDNA STRUCTUREDNA usually exists as a
double-strandedstructure, with both strands coiled togetherto form
the characteristic double-helix.Each single strand of DNA is a
chain offour types of nucleotides having the bases: Adenine
Cytosine Guanine Thymine 3. A nucleotide is a mono-, di-,
ortriphosphate deoxyribonucleoside; that is, a deoxyribose sugar is
attached to one, two, or three phosphates. Chemical interaction of
thesenucleotides forms phosphodiesterlinkages, creating the
phosphate-deoxyribose backbone of the DNA double helix with the
bases pointing inward. 4. DNA BACKBONE 5. Nucleotides (bases) are
matched between strands through hydrogen bonds to form base pairs.
Adeninepairs with thymine and cytosine pairs with guanine 6. DNA
strands have a directionality, and thedifferent ends of a single
strand are called the"3 (three-prime) end" and the "5
(five-prime)end" with the direction of the naming going 5prime to
the 3 prime region.The strands of the helix are anti-parallel
withone being 5 prime to 3 then the oppositestrand 3 prime to
5.These terms refer to the carbon atom indeoxyribose to which the
next phosphate in thechain attaches. Directionality hasconsequences
in DNA synthesis, because DNApolymerase can synthesize DNA in only
onedirection by adding nucleotides to the 3 end ofa DNA strand. 7.
The pairing of bases in DNA throughhydrogen bonding means that
theinformation contained within each strandis redundant. The
nucleotides on asingle strand can be used to reconstructnucleotides
on a newly synthesizedpartnerstrand. 8. DNA Replication 9. DNA
REPLICATION DNA replication is a biological process that occurs in
all living organisms and copies their exact DNA. It is the basis
for biological inheritance. 10. The first major step for the DNA
Replication totake place is the breaking of hydrogen bondsbetween
bases of the two antiparallel strands.The unwounding of the two
strands is thestarting point. The splitting happens in placesof the
chains which are rich in A-T. That isbecause there are only two
bonds betweenAdenine and Thymine (there are threehydrogen bonds
between Cytosine andGuanine).Helicase is the enzyme that splits the
twostrands. The structure that is created is knownas "Replication
Fork". 11. In order for DNA replication to begin, thedouble
stranded DNA helix must must first beopened. The sites where this
process firstoccurs are called replication origins. Helicaseunwinds
the two single strands 12. Single-Strand Binding
ProteinsSingle-Strand DNA Binding Proteins,SSB for short, work to
bind individualsstrands in a DNA double stranded helixand aid the
helicases in opening it upinto single strands. They are
particularlyuseful in stabilizing the unwoundsingle-stranded
formation. 13. Replication ForkThe replication fork is a structure
that formswithin the nucleus during DNA replication. It iscreated
by helicases, which break thehydrogen bonds holding the two DNA
strandstogether. The resulting structure has twobranching "prongs",
each one made up of asingle strand of DNA.These two strands serve
as the template forthe leading and lagging strands, which will
becreated as DNA polymerase matchescomplementary nucleotides to the
templates;The templates may be properly referred to asthe leading
strand template and the lagging 14. One of the most important steps
of DNAReplication is the binding of RNAPrimase in the initiation
point of the 3-5parent chain.RNA Primase can attract RNA
nucleotideswhich bind to the DNA nucleotides of the3-5 strand due
to the hydrogen bondsbetween the bases. RNA nucleotides arethe
primers (starters) for the binding ofDNA nucleotides. 15. RNA
PRIMASE RNA Primase lays down the RNA primers so that the
Polymerase III can get to work or can function. 16. The elongation
process is different forthe 5-3 and 3-5 template. a)5-3Template:
The 3-5 proceedingdaughter strand -that uses a 5-3template- is
called leadingstrand because DNA PolymeraseIII can "read" the
template andcontinuously adds nucleotides(complementary to the
nucleotides ofthe template, for example Adenineopposite to Thymine
etc). 17. The leading strandrequires fewer
stepsandthereforeissynthesized thequickest. Once aRNA primer has
beenlaid downbyPrimase, the DNAPolymerase III canbuild the
secondstrand continuouslyand in the samedirection that thedouble
helix is beingopened. To completethe process, DNAPolymerase I
replacesthe RNA Primer withDNA. 18. 3-5Template: The 3-5 template
cannot be"read" by DNA Polymerase III. The replicationof this
template is complicated and the newstrand is called lagging strand.
In the laggingstrand the RNA Primase adds more RNAPrimers. DNA
polymerase III reads thetemplate and lengthens the bursts. The
gapbetween two RNA primers is called "OkazakiFragments".The RNA
Primers are necessary for DNAPolymerase III to bind Nucleotides to
the 3end of them. The daughter strand is elongatedwith the binding
of more DNA nucleotides. 19. In the synthesis of the lagging
strand, thehelix uncoiling occurs in the oppositedirection to w/c
Polymerase III works. Theprocess therefore has to be done inpieces,
called Okazaki Fragments. 20. In the lagging strand the DNA Pol I
-exonuclease- reads the fragments andremoves the RNA Primers. The
gapsare closed with the action of DNAPolymerase which adds
complementarynucleotides to the gaps and DNALigase which acts as a
glue to attachthe phosphate to the sugar by formingphosphodiester
bond. 21. Each new double helix is consisted of one old and one
newchain. This is what we call semiconservative replication. The
total mechanism requires a cycle of repeating steps that include:
1) Creation of RNA Primers (Primase) 2) Synthesizing a short
segment of DNA between the primers(Polymerase III) 3) Replacing the
RNA primer with DNA (Polymerase I) and finally 4) The binding of
these pieces (Ligase) 22. The last step of DNA Replication isthe
Termination. This process happens whenthe DNA Polymerase reaches to
an end of thestrands. We can easily understand that in the
lastsection of the lagging strand, when the RNAprimer is removed,
it is not possible for the DNAPolymerase to seal the gap (because
there is noprimer). So, the end of the parental strand wherethe
last primer binds isnt replicated. These endsof linear
(chromosomal) DNA consists ofnoncoding DNA that contains repeat
sequencesand are called telomeres. As a result, a part ofthe
telomere is removed in every cycle of DNAReplication. 23. The DNA
Replication is not completed beforea mechanism of repair fixes
possible errorscaused duringthereplication. Enzymeslike nucleases
remove the wrong nucleotides andthe DNA Polymerase fills the gaps.
24. END OFPRESENTATIONPAGDATOON, RIZHA C. PEAMANTE, KEYSEAN
M.AAPD2F
