21.10
DNA Replication
DNA replication involves the separation of the two strands of the double helix, with each strand serving as a template from which the new complementar…
DNA replication occurs by synthesizing new strands of DNA using existing strands as a template. The two new double helices each contain an original template strand and a newly synthesized daughter strand, which is why this process is known as semiconservative replication.
To begin replication, an enzyme, helicase, unwinds the DNA helix and breaks the hydrogen bonds between the two strands. It then separates the individual strands forming a Y-shaped structure, known as the replication fork, where the template strands can be accessed by additional enzymes.
Another enzyme, primase, adds short RNA fragments called primers onto each template strand. These primers are essential for the synthesis of DNA as DNA polymerase can only add nucleotides to an existing strand.
DNA polymerase adds to the growing daughter strands on both template DNA strands. The addition of nucleotides is guided by the sequence of the original DNA strand according to the DNA pairing rules.
Synthesis of one of the daughter strands, the leading strand, occurs in the direction of the replication fork movement. The other strand, the lagging strand, is synthesized in the opposite direction.
This leads to the leading strand being synthesized as a continuous polymer, whereas the lagging strand is synthesized as short fragments. This occurs because DNA can be synthesized only in the 5’ to 3’ direction.
Before their addition to the growing polymer, nucleotides exist as deoxyribonucleoside triphosphates, with three phosphates attached to the fifth carbon on the sugar.
This free nucleotide triphosphate reacts with the 3’ hydroxyl groups. This is the OH that is attached to the third carbon of the sugar at the end of the growing strand. The reaction causes the release of pyrophosphate and the formation of a phosphodiester bond between the two nucleotides.
After the synthesis of the new strands, RNase H or additional variants of DNA polymerase remove the primers and synthesize DNA in their place. The gaps between the fragments are then sealed by DNA ligase to generate a continuous strand.
The addition of nucleotides continues until two replication forks meet each other, resulting in the completed replication.
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Q1: Why is DNA replication called semiconservative?
DNA replication is semiconservative because each newly formed double helix contains one original template strand and one newly synthesized daughter strand. After replication, the two resulting DNA molecules each preserve one parental strand from the original double helix, ensuring genetic continuity while incorporating new genetic material.
Q2: What role does helicase play in DNA replication?
Helicase unwinds the DNA double helix by breaking hydrogen bonds between the two strands. This separation creates a Y-shaped structure called the replication fork, which exposes the template strands so that other enzymes can access and copy them during DNA synthesis.
Q3: Why are RNA primers necessary for DNA synthesis?
RNA primers are essential because DNA polymerase can only add nucleotides to an existing strand; it cannot initiate synthesis de novo. Primase adds short RNA fragments to each template strand, providing the 3'-OH group that DNA polymerase requires to begin extending the new DNA strand.
Q4: How does the directionality of DNA synthesis affect leading and lagging strand formation?
DNA polymerase synthesizes only in the 5' to 3' direction. The leading strand is synthesized continuously toward the replication fork. The lagging strand grows away from the fork, requiring DNA polymerase to repeatedly restart synthesis, producing short DNA fragments called Okazaki fragments separated by RNA primers.
Q5: What happens to nucleotide triphosphates when they are added to the growing DNA strand?
Nucleotide triphosphates react with the 3'-OH group at the end of the growing strand. This reaction releases pyrophosphate, and the energy from breaking phosphate bonds drives the formation of a phosphodiester bond between nucleotides, linking them together in the DNA backbone.
Q6: How are gaps sealed after DNA polymerase removes RNA primers?
After RNase H or DNA polymerase variants remove RNA primers and fill the gaps with DNA nucleotides, the enzyme DNA ligase seals the remaining nicks. DNA ligase catalyzes the formation of phosphodiester bonds between the 3'-OH end of one DNA fragment and the 5' phosphate end of the adjacent fragment.
Q7: How does eukaryotic DNA replication differ from prokaryotic replication?
Eukaryotic genomes are larger and more complex, with multiple origins of replication per chromosome, whereas prokaryotes have single origins. Eukaryotic replication occurs at approximately 100 nucleotides per second—ten times slower than prokaryotic replication. Despite these differences, the essential enzymatic steps and base pairing and its significance in DNA replication remain fundamentally similar.