Essential Cell Biology 5th edition

CHAPTER SIX
6
DNA Replication and Repair
For a cell to survive and proliferate in a chaotic environment, it must be
able to accurately copy the vast quantity of genetic information carried in
its DNA. This fundamental process, called DNA replication, must occur
before a cell can divide to produce two genetically identical daughter
cells. In addition to carrying out this painstaking task with stunning accuracy
and efficiency, a cell must also continuously monitor and repair its
genetic material, as DNA is subjected to unavoidable damage by chemicals
and radiation in the environment and by reactive molecules that are
generated inside the cell.
DNA REPLICATION
DNA REPAIR
Yet despite the molecular safeguards that have evolved to protect a cell’s
DNA from copying errors and accidental damage, permanent changes—
or mutations—sometimes do occur. Although most mutations do not
affect the organism in any noticeable way, some have profound consequences.
Occasionally, these changes can benefit the organism: for
example, mutations can make bacteria resistant to antibiotics that are
used to kill them. What is more, changes in DNA sequence can produce
small variations that underlie the differences between individuals of the
same species (Figure 6–1); such changes, when they accumulate over
hundreds of millions of years, provide the variety in genetic material that
makes one species distinct from another, as we discuss in Chapter 9.
Unfortunately, as mutations occur randomly, they are more likely to be
detrimental than beneficial: they are responsible for thousands of human
diseases, including cancer. The survival of a cell or organism, therefore,
depends on keeping the changes in its DNA to a minimum. Without the
systems that are continually inspecting and repairing damage to DNA, it
is questionable whether life could exist at all. In this chapter, we describe
the protein machines that replicate and repair the cell’s DNA. These