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DNA Synthesis, - Coggle Diagram
DNA Synthesis
Dna process:
At the origin sites, the DNA strands separate, forming a replication “bubble” with replication forks at each end, where the parental strands of DNA are being unwound.
The replication bubbles elongate as the DNA is replicated, and eventually fuse.
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Single-strand binding proteins bind to unpaired DNA strands, stabilizing them.
This untwisting causes tighter twisting ahead of the replication fork: topoisomerase helps relieve this strain.
Unwound sections of parental DNA strands are now available to serve as templates for the synthesis of new complementary DNA strands. However, the enzymes that synthesize DNA cannot initiate synthesis of a polynucleotide (DNA polymerase (III)) — They can only add nucleotides to the end of an existing chain that is base-paired with the template strand
The initial nucleotide chain is a short stretch of RNA called a primer, synthesized by the enzyme primase.
Primase starts a complementary RNA chain from a single RNA nucleotide, adding RNA nucleotides one at a time, using the parental DNA strand as a template.
The new dna starts from the 3’ end of the RNA primer. As nucleotides align with complementary bases along the template strand, they are added to the growing end of the new strand by the polymerase.
Enzymes called DNA polymerases catalyze the synthesis of new DNA by adding nucleotides to preexisting chain. Most DNA polymerases require a primer and a DNA template strand, along which complementary DNA nucleotides line up.
Each nucleotide added to a growing DNA strand comes from a nucleoside triphospate, a nucleoside ( a base and a sugar) with three phosphate groups (dATP)
Lagging strand:
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Unlike the leading strand, which elongates continuously, the lagging strand is synthesized as a series of short segments called Okazaki fragments.
Although only one primer is required for the leading strand, each okazaki fragment on the lagging strand must be primed separately
Another DNA polymerase, DNA polymerase 1, replaces the RNA nucleotides of the primers with DNA versions, adding them one by one to the 3’ end of the adjacent Okazaki fragments whose primer was just replaced.
DNA-ligase joins the sugar-phosphate backbones of all the Okazaki fragments into a continuous DNA strand.
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Leading strand:
DNA pol III remains te in the replication for on that template strand and continuously adds nucleotides to the new complementary strand as the fork progresses.
The dna stran made by this mechanism, is called leading strand and only requires a single primer.
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to summarize: at the replication fork, the leading strand is copied continuously into the fork from a single primer. The lagging strand is coped away from the fork in short segments, each requiring a new primers ( due to Okazaki fragments and inability of DNA polymerase to cade from 3’ to 5’ end)
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Okazaki fragments are initiated by creation of a new RNA primer by the primosome. To restart DNA synthesis, the DNA clamp loader releases the lagging strand from the sliding clamp, and then reattaches the clamp at the new RNA primer. Then DNA polymerase III can synthesize the segment of DNA.
A DNA fragment will be formed in the 5' to 3' direction because of the polarity of the DNA molecule. Adding nucleotides to the 3' end allows DNA polymerase to use the phosphate molecules as "fuel," and add a new nucleotide to the DNA strand.
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