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4.3
| Question | Answer |
|---|---|
| Before a cell divides, it must duplicate its DNA so it can give complete and identical copies of all of its genes to each daughter cell. | Since DNA controls all cellular function, this replication process must be very exact. We now examine how it is accomplished and consider the consequences of mistakes. |
| The law of complementary base pairing shows that we can predict the base sequence of one DNA strand if we know the sequence of the other. | More importantly, it enables a cell to reproduce one strand based on information in the other. The fundamental steps of the replication process are as follows (fig. 4.13): 1. The double helix unwinds from the histones. |
| The law of complementary base pairing shows that we can predict the base sequence of one DNA strand if we know the sequence of the other. | 2. Like a zipper, an enzyme called DNA helicase opens up one short segment of the helix at a time, exposing its nitrogenous bases. The point where the DNA is opened up, like the two halves of a zipper separating, is called the replication fork. |
| The law of complementary base pairing shows that we can predict the base sequence of one DNA strand if we know the sequence of the other. | Molecules of the enzyme DNA polymerase move along each strand, read the exposed bases, and like a matchmaker, arrange “marriages" with complementary free nucleotides. |
| If the ----- finds the sequence TCG, for example, it assembles AGC across from it. The two separated strands of DNA are copied by separate polymerase molecules, proceeding in opposite directions. | polymerase |
| On one strand (top of figure 4.13), the polymerase moves toward the replication fork and makes a long, continuous, new strand of DNA to complement the old one. | On the other strand (bottom of the figure), another polymerase moves away from the replication fork and copies only a short segment of DNA at a time. The segments are then joined together by another enzyme called DNA ligase. |
| Ultimately, from the old parental DNA molecule, two new daughter DNA molecules are made. Each daughter | DNA consists of one new helix synthesized from free nucleotides and one old helix conserved from the parental DNA. The process is therefore called semiconservative replication. |
| While DNA is synthesized in the nucleus, new histones are synthesized in the cytoplasm. | Millions of histones are transported into the nucleus within a few minutes after DNA replication, and each new DNA helix wraps around them to make new nucleosomes. |
| Despite the complexity of this process, each DNA polymerase works at an impressive rate of about 100 base pairs per second. Even at this rate, however, it would take weeks for one polymerase molecule to replicate even one chromosome. | But in reality, thousands of polymerase molecules work simultaneously on each DNA molecule, and all 46 chromosomes are replicated in a mere 6 to 8 hours. |
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Russells3709