DNA is synthesized through a process called DNA replication. This process creates two identical DNA molecules from a single original DNA molecule.
Here's a breakdown of how DNA replication works:
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Initiation: The process begins at specific locations on the DNA called origins of replication. Enzymes called initiators bind to the origin and unwind the double helix, forming a replication bubble.
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Unwinding: The enzyme helicase unwinds the double helix at the replication fork, the Y-shaped region where replication is occurring. Single-strand binding proteins (SSBPs) bind to the single-stranded DNA to prevent it from re-annealing (coming back together).
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Primer Synthesis: DNA polymerase, the enzyme responsible for synthesizing new DNA strands, can only add nucleotides to an existing strand. Therefore, an enzyme called primase synthesizes short RNA primers complementary to the DNA template.
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Elongation: DNA polymerase adds nucleotides to the 3' end of the primer, creating a new DNA strand complementary to the template strand. DNA polymerase reads the template strand in the 3' to 5' direction and synthesizes the new strand in the 5' to 3' direction.
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Leading and Lagging Strands: Because DNA polymerase can only synthesize DNA in the 5' to 3' direction, replication occurs differently on the two template strands.
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Leading Strand: On one template strand, called the leading strand, DNA polymerase synthesizes a continuous strand of DNA.
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Lagging Strand: On the other template strand, called the lagging strand, DNA polymerase synthesizes DNA in short fragments called Okazaki fragments. Each Okazaki fragment requires a new RNA primer.
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Primer Removal: Once the Okazaki fragments are synthesized, another DNA polymerase removes the RNA primers.
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Gap Filling: The gaps left by the primer removal are filled in with DNA by DNA polymerase.
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Ligation: The enzyme DNA ligase seals the gaps between the Okazaki fragments, creating a continuous DNA strand.
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Proofreading and Error Correction: DNA polymerase has a proofreading function and can correct errors that occur during replication. Other enzymes also participate in DNA repair to correct any damage that might have occurred during replication.
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Termination: Replication continues until the entire DNA molecule is replicated. In prokaryotes, which have circular DNA, replication ends when the two replication forks meet. In eukaryotes, which have linear DNA, replication ends when the replication forks reach the end of the chromosome.
In summary, DNA synthesis via DNA replication is a complex but highly accurate process essential for cell division and inheritance of genetic information. Each daughter cell receives an accurate copy of the parent cell's genetic material, ensuring continuity of life.
