DNA STRUCTURE and DNA REPLICATIONPAGDATOON, RIZHA C.PEÑAMANTE, KEYSEAN M.AAPD2F
Deoxyribose Nucleic Acid DNA 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
A nucleotide is a mono-, di-, ortriphosphate deoxyribonucleoside; that is, a deoxyribose sugar is attached to one, two, or three phosphates. Chemical interaction of these nucleotides forms phosphodiester linkages, creating the phosphate- deoxyribose backbone of the DNA double helix with the bases pointing inward.
Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adeninepairs with thymine and cytosine pairs with guanine
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.
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 synthesizedpartner strand.
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.
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".
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
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.
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 laggingstrand template
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.
RNA PRIMASE RNA Primase lays down the RNA primers so that the Polymerase III can get to work or can function.
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).
The leading strandrequires fewer stepsand therefore issynthesized thequickest. Once aRNA primer has beenlaid down byPrimase, the DNAPolymerase III canbuild the secondstrand continuouslyand in the samedirection that thedouble helix is beingopened. To completethe process, DNAPolymerase I replacesthe RNA Primer withDNA.
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.
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.
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.
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)
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.
The DNA Replication is not completed beforea mechanism of repair fixes possible errorscaused during the replication. Enzymeslike nucleases remove the wrong nucleotides andthe DNA Polymerase fills the gaps.
END OFPRESENTATION PAGDATOON, RIZHA C. PEÑAMANTE, KEYSEAN M. AAPD2F