認識微生物.ppt

DNA Replication II 王之仰 The base-pairing template model theoretically could proceed either by a conservative or a semiconservative mechanism. In a conservative mechanism, the two daughter strands would form a new double-stranded DNA molecule, , and the parental strands would remain intact; in a semiconservative mechanism, the parental strands are permanently separated, and each forms a duplex molecule with the daughter strand base-paired to it. DNA polymerases require a primer to initiate replication; DNA synthesis always proceeds in the 5’to 3’direction because chain growth results from formation of a phosphodiester bond between the 3’ oxygen of a growing strand and the alpha phosphate of a dNTP. DNA polymerases cannot initiate chain synthesis de novo; instead, they require a short, preexisting RNA or DNA strand, called a primer, to begin chain growth. With a primer base-paired to the template strand, a DNA polymerase adds dNTPs to the free hydroxyl at the 3’ end of the primer as directed by the sequence of the template strand; when RNA is the primer, the daughter strand that is formed is RNA at the 5’ end and DNA at the 3’end. Duplex DNA is unwound or melted, to make the bases available for base pairing with the bases of the dNTPs that are polymerized into the newly synthesized daughter strands. The unwinding of the parental DNA strands is by specific helicases, beginning at unique segments at a DNA molecule called replication origins. The nucleotide sequences of the origins usually contain A-T rich sequences. A specialized RNA polymerase called primase forms a short RNA primer complementary to the unwound template strands. The primer, base-paired to its complementary DNA strand, is elongated by a DNA polymerase, forming a new daughter strand. The DNA region at which all these proteins come together to carry out synthesis of daughter strand is called the replication fork. In order for DNA polymerases to move along and copy a duplex DNA, helicase must s