DNA STRUCTURE and DNA REPLICATION

DNA STRUCTURE and DNA REPLICATIONPAGDATOON, RIZHA C.PEAMANTE, KEYSEAN M.

AAPD2FDeoxyribose Nucleic AcidDNA STRUCTURE

DNA usually exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix.

Each single strand of DNA is a chain of four types of nucleotides having the bases: AdenineCytosineGuanineThymine

A nucleotide is a mono-, di-, or triphosphate 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. DNA BACKBONE

Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine and cytosine pairs with guanine

DNA strands have a directionality, and the different 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 5 prime to the 3 prime region.

The strands of the helix are anti-parallel with one being 5 prime to 3 then the oppositestrand 3 prime to 5.

These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3' end of a DNA strand.

The pairing of bases in DNA through hydrogen bonding means that the information contained within each strand is redundant. The nucleotides on a single strand can be used to reconstructnucleotides on a newly synthesized partner strand.

DNA Replication

DNA REPLICATIONDNA 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 theDNA Replicationto take place is the breaking of hydrogen bonds between bases of the two antiparallel strands.The unwounding of the two strands is the starting point. The splitting happens in places of the chains which are rich in A-T. That is because there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between Cytosine and Guanine).

Helicaseis the enzyme that splits the two strands. The structure that is created is known as "Replication Fork".

In order for DNA replication to begin, the double stranded DNA helix must must first be opened. The sites where this process first occurs are called replication origins. Helicase unwinds the two single strandsSingle-Strand Binding ProteinsSingle-Strand DNA Binding Proteins, SSB for short, work to bind individuals strands in a DNA double stranded helix and aid the helicases in opening it up into single strands. They are particularly useful in stabilizing the unwound single-stranded formation.Replication ForkThe replication fork is a structure that forms within the nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA.

These two strands serve as the template for the leading and lagging strands, which will be created as DNA polymerase matches complementary nucleotides to the templates; The templates may be properly referred to as the leading strand template and the lagging strand templateOne of the most importantsteps of DNA Replicationis the binding ofRNA Primasein the initiation point of the 3'-5' parent chain.

RNA Primasecan attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases. RNA nucleotides are the primers (starters) for the binding of DNA nucleotides.RNA PRIMASE

RNA Primase lays down the RNA primers so that the Polymerase III can get to work or can function.Theelongationprocess is different for the 5'-3' and 3'-5' template. a)5'-3' Template: The 3'-5' proceeding daughter strand -that uses a5'-3' template- is calledleading strandbecauseDNA Polymerase IIIcan "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc).

The leading strand requires fewer steps and therefore is synthesized the quickest. Once a RNA primer has been laid down by Primase, the DNA Polymerase III can build the second strand continuously and in the same direction that the double helix is being opened. To complete the process, DNA Polymerase I replaces the RNA Primer with DNA.3'-5'Template: The3'-5' templatecannot be "read" by DNA Polymerase III. The replication of this template is complicated and the new strand is calledlagging strand. In the lagging strand the RNA Primase adds more RNA Primers.DNA polymerase IIIreads the template and lengthens the bursts. The gap between two RNA primers is called "Okazaki Fragments".

The RNA Primers are necessary for DNA Polymerase III to bind Nucleotides to the 3' end of them. The daughter strand is elongated with the binding of more DNA nucleotides.

In the synthesis of the lagging strand, the helix uncoiling occurs in the opposite direction to w/c Polymerase III works. The process therefore has to be done in pieces, called Okazaki Fragments. In the lagging strand theDNA Pol I-exonuclease- reads the fragments and removes the RNA Primers. The gaps are closed with the action of DNA Polymerase which adds complementary nucleotides to the gaps and DNA Ligase which acts as a glue to attach the phosphate to the sugar by forming phosphodiester bond.Each new double helix is consisted of one old and one new chain. This is what we callsemiconservative replication.

The total mechanism requires a cycle of repeating steps that include:Creation of RNA Primers (Primase)Synthesizing a short segment of DNA between the primers (Polymerase III)Replacing the RNA primer with DNA (Polymerase I) and finallyThe binding of these pieces (Ligase)The laststep of DNA Replicationis theTermination. This process happens when the DNA Polymerase reaches to an end of the strands. We can easily understand that in the last section of the lagging strand, when the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer). So, the end of the parental strand where the last primer binds isn't replicated. These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are calledtelomeres. As a result, a part of the telomere is removed in every cycle of DNA Replication.

The DNA Replication is not completed before amechanism of repairfixes possible errors caused during the replication. Enzymes likenucleasesremove the wrong nucleotides and the DNA Polymerase fills the gaps.END OF PRESENTATIONPAGDATOON, RIZHA C.PEAMANTE, KEYSEAN M.

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