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CASE FILES: BIOCHEMISTRY
serve to introduce DNA double-strand breaks (DSBs) as well as thymine-
thymine dimers into the DNA. DSBs may generate deletion or insertion
mutations and could alter the reading frame of the genetic code, an event that
can easily lead to the malfunction of a protein. UV-induced dimers may generate
point mutations that also alter the reading frame.
To respond to the various forms of DNA damage, cells have evolved a host
of DNA repair mechanisms that serve to restore the genetic material. Depending
on the nature of the DNA damage (DSB versus ultraviolet [UV]-induced
thymine dimers, etc.), the cell will invoke a different mechanism of repair. In
addition, the stage of the cell cycle at which the lesion is detected and processed
can activate independent DNA repair pathways. For example, if a daughter
chromatid template is present in S or G2 phase of the cell cycle, the cell will
use this unperturbed partner molecule to fix a DSB. This process is referred
to as homologous recombination (HR) and represents a major branch of the
DNA repair process. However, if the damage occurs during the G1
phase of the
cell cycle, a period devoid of an existing chromatid template that can be
used for repair, a general end joining process will be used. This process,
referred to as nonhomologous end joining (NHEJ), ligates the broken ends
together with little to no regard for the loss of intervening sequences. Therefore,
NHEJ is considered an error prone process, but given the large size of the
genome and the presence of many forms of “junk DNA,” the NHEJ process
may not necessarily disrupt DNA sequences that encode proteins. In contrast to
NHEJ, HR is a process that is error free by virtue of the fact that the daugh-
ter chromatid is used as a template for repair.
Recombination and transposition of genes are two processes that mutate
the genetic material. As discussed above, recombination is integral to the
process of DNA repair. When DNA recombination is impaired through muta-
tion of specific genes (like
BRCA1
or
Rad51),
aberrant recombination takes
place, generating abnormal chromosomes that possess translocations from two
or more chromosomes. Translocations transpose genes from one chromoso-
mal environment to another and often this leads to a disruption in gene expres-
sion. Such events are known to cause gene amplification, a phenomenon often
associated with cancer. They may also generate chromosomes that possess two
centromeres (dicentrics), leading to a variety of cellular defects in mitosis.
DNA replication is tightly controlled by a variety of proteins that act to pro-
mote the process (i.e., DNA polymerases and ds-acting elements that bind to
DNA and recruit factors involved in the process) as well as ones that inhibit
the synthesis of DNA, either directly or indirectly. One factor that indirectly
inhibits DNA synthesis is the p53 tumor suppressor. Tumor suppressors
refer to a general class of proteins that function to slow and alter cell
growth and development through a variety of mechanisms. In the absence of
these factors, cells will have a reduced capacity to perform a variety of func-
tions essential to maintain genomic stability. The p53 functions to control the
Gj S boundary of the cell cycle, and if the cell is not prepared to enter
S phase, then DNA synthesis will be negatively regulated.
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