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866 Nucleic Acid Synthesis

mismatch repair process is initiated, which causes removal
of a stretch of the newly synthesized strand spanning
the mismatch, followed by resynthesis of the segment, as
described later.

H. Replication of Telomeres—The End Game
Because DNA synthesis proceeds unidirectionally from

5 → 3 withrespecttodeoxyribose,bysequentialaddition

of deoxynucleotides to the 3 terminus of the deoxynu-
cleotide added last, chain elongation can proceed to the

terminus of the template strand oriented in the 3 to 5 di-

rection. But how about synthesis of the terminus of the
complementary strand ? Because synthesis of this discon-
tinuous(lagging)strandoccursintheoppositedirectionby
repeated synthesis of a primer, the terminus could not be
replicated. This problem of end replication is eliminated
in the circular genomes of bacteria and the small genomes
of plasmids and viruses. However, in the case of linear eu-
karyotic chromosome, the problem is solved by a special-
ized mechanism of telomere replication. Telomeres are
repeats of short G-rich sequences found at both ends of the
chromosomes (Fig. 6). In the human genome, the telomere
repeat unit is 5 (T/A)m Gn 3 , where n > 1 and 1 < m < 4.

Telomerase is a special DNA polymerase (reverse tran-
scriptase) containing an oligoribonucleotide template 5
Cn(A/T)m3 (which is complementary to the telomere re-

peat sequence) as an integral part of the enzyme (Fig. 6). In
the presence of other accessory proteins, telomerase uti- FIGURE 6 A schematic description of the role of telomerase in
the maintenance of telomeres at chromosome termini. The dou-
lizes its own template to generate the telomeric repeat unit
ble lines with break represent one telomere terminus of a chro-
and, by “slippage,” utilizes the same oligoribonucleotide mosome in which the 5 terminal region of the lagging strand is

template repeatedly to generate thousands of repeats of unreplicated (as in Fig. 4), resulting in an overhanging 3 terminal
the same hexanucleotide unit sequence. Because the lag- region. In order to avoid shortening of this telomere sequence dur-
ging strand terminal region does not require an external ing successive rounds of replication, DNA template-independent

telomerase extends the 3 overhang by adding the telomere repeat
DNA template, the newly synthesized DNA is present in
sequence TTGGGG as shown in (C). The template for the repeat
an extended single-stranded region. Telomeres provide a is an RNA present in the telomerase complex. The extended 3
critical protective function to the chromosome by their single-strand region then allows de novo initiation and filling in of
unique structures and prevent their abnormal fusion. the 5 strand (E). Finally, the 3 overhang loops to anneal with an

internal sequence mediated by the telomere repeat factor (TRF2)
in order to protect the terminus from degradation by nonspecific
I. Telomere Shortening: Linkage Between nucleases (F).
Telomere Length and Limited Life Span
is generally believed that cells will senesce if the telomere
One profound implication of the specialized telomere
length is reduced below a critical level after repeated repli-
structure and its synthesis is that in the absence of telom-
cation of the genome.
erase, the repeat length of telomeres could not be main-
tained. Telomerase is active in neonatal cells and also in
some immortal tumor cells, but is barely detectable in IV. MAINTENANCE OF GENOME INTEGRITY
diploid, terminally differentiated mammalian cells. Most
such diploid cells can multiply in vitro in specialized cul- The integrity of the genome, both in regard to sequence
ture medium, but have a limited life span. Loss of replica- and to size, is essential for perpetuation of species. This in-
tive capacity is associated with shortening of telomere re- tegrity can be threatened in two ways. The first is by errors
peat lengths. Furthermore, ectopic and stable expression in DNA replication, as discussed earlier. A second inex-
of telomerase in human diploid cells by introduction of its orable process of DNA alteration occurs due to chemical
gene confer an indefinite reproductive life on such cells. It reactions which can be either endogenous or induced by