89–90), it does not occur in DNA synthesis no 3′-to-5′ DNA polymerase has ever been found.Īn incorrect model for DNA replication. Although head-growth polymerization occurs elsewhere in biochemistry (see pp. The other would move in the 3′-to-5′ direction and work by so-called “head growth,” in which the end of the growing DNA chain carried the triphosphate activation required for the addition of each subsequent nucleotide ( Figure 5-7). One would polymerize in the 5′-to-3′ direction, where each incoming deoxyribonucleoside triphosphate carried the triphosphate activation needed for its own addition. Such a replication fork would require two different DNA polymerase enzymes. But because of the antiparallel orientation of the two DNA strands in the DNA double helix (see Figure 5-2), this mechanism would require one daughter strand to polymerize in the 5′-to-3′ direction and the other in the 3′-to-5′ direction.

Initially, the simplest mechanism of DNA replication seemed to be the continuous growth of both new strands, nucleotide by nucleotide, at the replication fork as it moves from one end of a DNA molecule to the other. An active zone of DNA replication moves progressively along a replicating DNA molecule, creating a Y-shaped DNA structure known as a replication fork: the two arms of each Y (more.) Two replication forks moving in opposite directions on a circular chromosome.