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Session Length
~17 min
Adaptive Checks
15 questions
Transfer Probes
8
DNA replication is the biological process by which a cell duplicates its entire genome before division, ensuring each daughter cell receives a complete and accurate copy of the genetic instructions. The process is semiconservative: each new double helix contains one original parent strand and one newly synthesized strand, as demonstrated by the landmark Meselson-Stahl experiment. Replication begins at specific origin sequences where helicase unwinds the double helix, creating Y-shaped replication forks where synthesis occurs.
The central enzyme, DNA polymerase, reads the template strand in the 3-prime to 5-prime direction and synthesizes the new strand in the 5-prime to 3-prime direction. Because the two template strands run antiparallel, one new strand (the leading strand) is synthesized continuously toward the fork, while the other (the lagging strand) must be synthesized discontinuously as short Okazaki fragments that are later joined by DNA ligase. Additional enzymes -- primase, topoisomerase, single-strand binding proteins -- coordinate to keep the process efficient and accurate.
Fidelity is achieved through three layers of error correction: base-pair geometry during nucleotide selection, proofreading by the 3-prime to 5-prime exonuclease activity of DNA polymerase, and post-replication mismatch repair systems. Together, these mechanisms reduce the error rate to approximately one mistake per billion nucleotides copied. In eukaryotes, the end-replication problem causes chromosomes to shorten with each division, a challenge addressed by the enzyme telomerase in stem cells and germ cells. Understanding DNA replication is fundamental to genetics, cancer biology, forensic science, and biotechnology.
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DNA replication produces two identical double helices, each containing one original (parent) strand and one newly synthesized strand. This was demonstrated by the Meselson-Stahl experiment.
Example: After one round of replication, a DNA molecule with two heavy nitrogen-labeled strands produces two molecules, each with one heavy and one light strand.
Helicase is the enzyme that unwinds the double helix by breaking hydrogen bonds between base pairs, creating a Y-shaped replication fork where new strands are synthesized.
Example: At the replication fork, helicase separates the two strands like unzipping a zipper, allowing DNA polymerase to access each strand as a template.
The primary enzyme that synthesizes new DNA strands by adding complementary nucleotides to the 3' end of a growing strand, reading the template in the 3' to 5' direction. It requires an RNA primer to begin synthesis.
Example: DNA polymerase III in E. coli adds about 1000 nucleotides per second, making it one of the fastest and most accurate enzymes in the cell.
The leading strand is synthesized continuously in the 5' to 3' direction toward the replication fork. The lagging strand is synthesized discontinuously as Okazaki fragments in the 5' to 3' direction away from the fork, then joined by DNA ligase.
Example: On the lagging strand, each Okazaki fragment (~1000-2000 nucleotides in prokaryotes) requires its own RNA primer and is later connected to adjacent fragments by ligase.
DNA polymerase has 3' to 5' exonuclease activity that allows it to detect and remove mismatched nucleotides immediately after they are added. Additional mismatch repair systems correct errors that escape proofreading.
Example: The error rate of DNA replication is approximately 1 mistake per billion nucleotides copied, thanks to polymerase proofreading and post-replication mismatch repair.
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