DNA Synthesis

DNA replication in plant cells occurs in three basic steps, each catalyzed by at least one enzyme. First, the two original, or parental, DNA strands of the double helix unwind and separate. Then each parental strand is used as a template for the formation of a new daughter strand. Finally, one parental strand and its newly synthesized daughter strand wind together into one double helix, while the other parental strand and its daughter strand wind together into a second double helix.

The process begins with the separation and unwinding of segments of the parental double helix. To accomplish this, an enzyme named DNA helicase, powered by adenosine triphosphate (ATP), works its way between the two strands. As this enzyme "plows" its way through the double helix, it breaks the hydrogen bonds that hold together the "rungs" of the ladder, formed by the base pairs. DNA helicase then "walks" along one strand, nudging the other strand out of its way as it goes. The result is that the two DNA strands separate, thus exposing their bases, as though the ladder had been split vetically, down through the rungs.

The second step of DNA synthesis requires the enzyme DNA polymerase, which performs a dual function during the replication. First, it recognizes bases exposed in a parental strand and matches them up with free nucleotides that have complementary bases. Second, DNApolymerasebonds to-

Stages in DNA Replication a

At left, a double-stranded DNA molecule, with the sides made of sugar-phosphate molecules and the "rungs" of base pairs. Replication begins at a point (a), with the separation of a base pair, as a result of the action of special initiator proteins (b). The molecule splits, or "unzips," in opposite directions (c) as each parental strand is used as a template for the formation of a new daughter strand (new bases pair with their appropriate "mate" bases to form new ladder "rungs"). Finally (right), one parental strand and its newly synthesized daughter strand form a new double helix, while the other parental strand and its daughter strand form a second double helix.

a a gether the sugars and phosphates of the complementary nucleotides to form the backbone of the daughter strand. Because the DNA polymerase can travel in only one direction on a DNA strand, from the 3' end to 5' end, the two DNA polymerase molecules, one on each parental strand, move in opposite directions. Only one daughter strand is synthesized continuously, while the duplication of another is done piece by piece. As the DNA helicase molecule continues to separate the parental strands, one polymerase simply follows behind it, synthesizing a long, continuous complementary daughter strand as it goes. The polymerase on the second parental strand travels away from the DNA helicase. As the helicase continues to separate the parental strands, this polymerase cannot reach the newly separated segment of the second strand. Hence a new DNA polymerase attaches to the second strand close behind the helicase to synthesize another small piece of DNA. These pieces are "sewn" together by another enzyme called DNA ligase. This process is repeated many times until the copying of a second parental strand is completed.

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