What do we need to know about DNA replication to describe the mechanism of a drug that blocks it?

Recall that the opening story to this chapter described the discovery that platinum electrodes inhibit cell division, which lead researchers to wonder if they could invent a drug containing platinum that might inhibit the uncontrolled growth of cancer cells. The drug that Dr. Barnett Rosenberg developed, cisplatin, was remarkably efficient at halting certain kinds of tumor growth. Understanding how cisplatin works came with an understanding of the chemistry of DNA and the mechanism by which it replicates.

Investigating Life: The Meselson–Stahl Experiments discusses the experiments which revealed that DNA replicates semiconservatively—that separated strands of DNA serve as templates for new strands. We mentioned also the finding that semiconservative replication does not occur in the presence of cisplatin, suggesting that cisplatin interferes with strand separation, which is a prerequisite for replication. How does this come about, and what does it have to do with platinum?

Cisplatin contains a platinum atom bonded to two amino groups and two chlorine atoms (Figure 13.20A). In Rosenberg’s experiments, this compound was formed when the platinum electrode reacted with salts in the surrounding solution. The bonds between the platinum atom and the chlorine atoms are weak, and the latter can be displaced by electron-rich substances (you may know from chemistry that these are called nucleophiles). In DNA, one of the nitrogen atoms of guanine (Figure 13.20B) displaces one of the chlorines, forming a strong covalent bond. If there is a nearby guanine on the opposite DNA strand, the other chlorine of the cisplatin molecule can be displaced as well, so that the cisplatin is effectively bonded to both of the DNA strands. In short, the DNA strands become cross-linked and cannot separate as needed for DNA replication (Figure 13.20C). Without replication, the cell cannot divide, and it undergoes apoptosis. The type of DNA lesion caused by cisplatin cannot be repaired by any of the cell’s usual DNA repair mechanisms.

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Future directions

Eukaryotic chromosomes have long strands of DNA, each with up to tens of millions of base pairs. During S phase of the cell cycle, each chromosome must be replicated only once and completely. Since replication is bidirectional from an origin, and since S phase typically is not long enough to replicate an entire chromosome, there must be many starting points for replication. Understanding how this happens is a major challenge for biologists. A recent discovery may give a clue as to how a DNA replication complex arrives at each replication origin. While most RNA molecules transcribed from DNA leave their site of synthesis, some do not. These short RNAs, complementary to their DNA template, turn back instead and bind to the DNA by base pairing. Indeed, this rapid binding displaces the non-template strand of DNA, so it forms a loop. At these spots on the long chromosome, there are loops with a region of DNA that is unpaired with its complementary strand, because that strand at that location is bound up with RNA. Emerging evidence suggests that the “R-loops,” with their exposed DNA bases, act to recognize binding of the proteins of the DNA replication complex.