how is Dna copied?
DNA is copied through a process called DNA replication. This process occurs during the cell division called S phase of the cell cycle. DNA replication is crucial for cell growth and reproduction, as it ensures that each newly formed cell receives an identical set of genetic information.
Here's a step-by-step explanation of how DNA is copied:
1. Initiation: DNA replication starts at specific sites on the DNA molecule called origins of replication. Proteins called initiator proteins bind to these origins and separate the two DNA strands, forming a replication bubble.
2. Unwinding: An enzyme called DNA helicase attaches to the replication bubble and moves along the DNA molecule, breaking the hydrogen bonds between the base pairs. This action results in the separation of the two DNA strands and the creation of a replication fork.
3. Priming: Enzymes called primases synthesize short RNA primers complementary to the DNA template. These primers provide a starting point for DNA synthesis.
4. Elongation: The enzyme DNA polymerase III adds nucleotides to the growing DNA strand, following the complementary base pairing rule (A-T and G-C). The enzyme moves in the 3' to 5' direction along the template strand, synthesizing a new DNA strand in the 5' to 3' direction. Because the two DNA strands have antiparallel orientations, the DNA polymerase synthesizes the new strand discontinuously as short fragments called Okazaki fragments on the lagging strand.
5. Proofreading and correction: DNA polymerases have a built-in proofreading function that allows them to detect and correct errors in DNA replication. If a mismatched pair occurs, the incorrect nucleotide is removed, and the correct one is added.
6. Termination: DNA replication continues until the replication forks from both directions meet and merge, resulting in the complete duplication of the DNA molecule. At this point, the DNA has been accurately copied, and each new DNA molecule consists of one original strand (the template strand) and one newly synthesized complementary strand.
Overall, DNA replication is a highly precise and tightly regulated process involving multiple enzymes and proteins working together to ensure the accurate duplication of the entire DNA molecule.