DNA, Gene Structure, and DNA Replication - Insect Molecular Genetics - page 19

pre-mRNA. The pre-mRNA must undergo processing (splicing) in the nucleus
to remove the introns before it can travel to the cytoplasm for translation into
proteins. This process is described in Chapter 3, but first DNA replication is
reviewed.
1.13 Efficient DNA Replication is Essential
Every living organism must make a copy of its genes in each cell each time
the cell divides. Such replication ideally is both rapid and accurate. If not, the
organism's survival and integrity are jeopardized. Even a very small error rate
of 0.001% (one mistake/100,000nt) can lead to detrimental changes or muta-
tions. Although many mutations are detrimental, many apparently are neutral
or nearly so, and a few are beneficial. The intrinsic structure of DNA helps to
ensure that replication is accurate most of the time.
1.14 DNA Replication is Semiconservative
DNA replication is semiconservative , i.e., the daughter molecules each contain
one polynucleotide derived from the original DNA molecule and one newly syn-
thesized strand ( Figure 1.10 , Box 1.1 ). Semiconservative DNA replication requires
the hydrogen bonds that hold the two strands together be broken so that syn-
thesis of new complementary strands can occur. Semiconservative replication of
DNA increases the likelihood that replication error rates are very low.
1.15 Replication Begins at Replication Origins
During the replication of long DNA molecules, only a limited region of the DNA
molecule is in an unpaired form at any one time. Replication occurs after the
two strands separate; separation involves breaking the weak hydrogen bonds
holding the bases of the opposite strands together. The separation of the two
strands starts at specific multiple positions in the chromosome called origins of
replication and moves along the molecule. Replication sites in Drosophila , for
example, occur at thousands of sites throughout the genome ( Eaton et al. 2011 ).
Synthesis of the new complementary polynucleotides occurs as the double helix
“unzips.” The region at which the base pairs of the parent molecule are broken
and the new polynucleotides are synthesized is the replication fork ( Figure 1.11 ).
The two strands of the parent DNA molecule are broken apart by enzymes
called helicases . Once the helicase has separated the two strands, single-strand
binding proteins attach to the single strands to prevent them from immedi-
ately reannealing to each other ( Figure 1.11 ). This attachment makes it possi-
ble for DNA polymerase to synthesize new complementary DNA strands. DNA