Lagging-strand DNA synthesis is delayed because it must wait for the leading strand to expose the template strand on which each Okazaki fragment is synthesized. For the lagging strand, the direction of nucleotide polymerization is opposite to the overall direction of DNA chain growth. Its synthesis slightly precedes the synthesis of the daughter strand that is synthesized discontinuously, known as the lagging strand. The DNA daughter strand that is synthesized continuously is known as the leading strand. (Similar replication intermediates were later found in eucaryotes, where they are only 100–200 nucleotides long.) The Okazaki fragments were shown to be polymerized only in the 5′-to-3′chain direction and to be joined together after their synthesis to create long DNA chains.Ī replication fork therefore has an asymmetric structure ( Figure 5-8). This experiment revealed the transient existence of pieces of DNA that were 1000–2000 nucleotides long, now commonly known as Okazaki fragments, at the growing replication fork. Researchers added highly radioactive 3H-thymidine to dividing bacteria for a few seconds, so that only the most recently replicated DNA-that just behind the replication fork-became radiolabeled. How, then, is overall 3′-to-5′ DNA chain growth achieved? The answer was first suggested by the results of experiments in the late 1960s. In this scheme, both daughter DNA strands would grow continuously, using (more.) Although it might seem to be the simplest possible model for DNA replication, the mechanism illustrated here is not the one that cells use. 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.) _ and _ proofread the newlyformed strand and remove and replace any incorrectly pairednucleotides.Two replication forks moving in opposite directions on a circular chromosome. _ joins the segments togetherby forming a _ bond between the nucleotides on theends of the segments. _ removes the RNA nucleotides and replaces themwith DNA nucleotides. Replication can proceed continuously with the _strand while the _ strand must be made in discontinuoussegments called _. Using the template strand as a guide, _ addsDNA nucleotides to the 3' end of the growing strand.į. The enzyme _ lays down RNA nucleotides that will beused by _ as a starting point to begin synthesis of thenew DNA strand.Į. _ attach themselves to the individual DNAstrands to prevent them from reforming hydrogen bonds.ĭ. The enzyme _ breaks the hydrogen bonds between thetwo DNA strands, resulting in an unzipped helix that terminates atthe _.Ĭ. The enzyme _ removes the tension by unwinding thedouble helix.ī. Fill in the missing terms to complete thesentences.Ī. The following sentences summarize the process of DNAreplication.
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