DNA Replication

DNA replication is a fundamental biological process that ensures each cell has a complete set of DNA. It occurs in the S phase of the cell cycle, before a cell divides, allowing each new cell to have an accurate copy of the DNA. Here’s a simplified overview of how DNA replication works: Initiation: Replication begins at specific locations in the DNA, known as origins of replication. Proteins bind to the DNA at these origins, separating the two strands of the DNA helix, creating a 'replication fork.' Unwinding: The enzyme helicase unwinds the DNA helix at the replication fork, separating the two strands. Single-strand binding proteins then bind to the separated strands to keep them apart and prevent them from re-annealing. Primer Synthesis: DNA polymerases, the enzymes responsible for synthesizing new DNA strands, can only add new nucleotides to an existing strand. Therefore, an RNA primer (a short nucleic acid sequence) is synthesized by primase on the DNA template strand to provide a starting point for DNA polymerase. Elongation: DNA polymerase III adds new complementary DNA nucleotides to the 3' end of the RNA primer, synthesizing the new DNA strand in a 5' to 3' direction. Because DNA polymerases can only synthesize DNA in this direction, one new strand (the leading strand) is synthesized continuously towards the replication fork, while the other new strand (the lagging strand) is synthesized discontinuously in short segments called Okazaki fragments, away from the replication fork. Primer Removal and Replacement: The RNA primers are removed by DNA polymerase I, which replaces the RNA nucleotides with DNA nucleotides. Ligation: The enzyme DNA ligase seals the gaps between the Okazaki fragments on the lagging strand, creating a continuous DNA strand. Termination: Replication continues until the entire DNA molecule is copied, including the meeting point of replication forks, which eventually leads to the separation of the two new DNA molecules. DNA replication is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. This process is highly accurate due to the proofreading function of DNA polymerases, which corrects errors during synthesis, ensuring fidelity in the replication process. Problem: In DNA replication: A) Both strands replicate in the same direction B) Each strand replicates in a different direction C) Only one strand of DNA is used as a template D) A single strand of DNA is copied to make two single strands of DNA