原核生物の複製

Initiator proteins bind to the origin of replication, a small DNA region with a specific base sequence, creating a complex that initially separates the DNA strands. This complex prepares the DNA for unwinding by helicase and subsequent replication machinery to begin synthesizing new DNA strands.

DNA polymerase synthesizes DNA only in the 5' to 3' direction. The leading strand has the same orientation as the replication fork, allowing continuous synthesis. The lagging strand has opposite orientation, requiring synthesis in short pieces called Okazaki fragments that are later joined together by DNA ligase.

Single-stranded DNA binding proteins stabilize the newly separated DNA strands after helicase unwinds the double helix by breaking hydrogen bonds between complementary bases. These proteins prevent the single strands from re-annealing or forming secondary structures, keeping them available as templates for DNA polymerase.

As helicase unwinds the DNA double helix, it creates torsional strain ahead of the replication fork. Topoisomerase enzymes bind to the DNA and break one side of the phosphate-sugar backbone, allowing the helix to unwind more rapidly and relieving this tension during the replication process.

Primase synthesizes short RNA primers complementary to the DNA template sequence. These primers provide the 3'-OH group that DNA polymerase requires to begin adding nucleotides. After replication, RNA primers are removed and replaced with DNA by enzymes like DNA polymerase I, then joined by DNA ligase.

During semiconservative replication, the original double-stranded DNA molecule unwinds, and each original strand serves as a template for a new complementary strand. The result is two DNA molecules, each containing one original strand and one newly synthesized strand from the replication process.

E. coli initiates a new round of DNA replication from the origin of replication before the previous round completes. Daughter cells receive chromosomes already undergoing replication in eukaryotes, allowing them to divide again quickly without waiting for complete genome duplication to finish.