FREE ESSAY ON DNA REPLICATION

DNA REPLICATION

DNA replication is a complex cellular function that is necessary in order to sustain life
and achieve growth. Many enzymes, proteins, and other molecules work together to ensure
that genetic information is replicated efficiently, quickly, and accurately. Without any
one of these components, replication would be very limited in its efficacy.
DNA is comprised of two strands of complementary nitrogenous bases (adenine & thymine,
guanine & cytosine), five-carbon sugars (either ribose or deoxyribose), and phosphate
groups. The strands of DNA are arranged in a double-helix array and are held together
with hydrogen bonds. The semiconservative replication model is used to depict
replication. In this model, each new double helix has one old strand and one new strand.
This is yet another way in which accuracy is ensured. 
Because the shape of the DNA molecule is extremely important to its functionality, care
must be taken to ensure that all parts of the molecule remain in their appropriate space
during replication, and that no part of the strand is broken. To replicate DNA, the two
strands must first be separated from one another. The first enzyme used in this process
is called helicase. Helicases use the energy from ATP molecules to unwind the
three-dimensional double helix. While the strand is unwinding, topoisomerase enzymes
(such as gyrase) prevent the strands from being winded into a supercoil due to the torque
produced by the separating action. Since each strand is comprised of complementary base
pairs that have a high affinity to hydrogen-bond with one another, single-stranded
binding proteins (SSBs) are attached to the strands to keep them from reattaching to one
another. 
Once the strands are separated, work can begin to construct two new complementary strands
that will ultimately attach to the existing DNA strands to form new complete DNA
sequences. DNA polymerase III is the active enzyme that builds the new complementary
strands. DNA polymerase III is a DNA-dependent enzyme. As such, a template (the existing
separated strand) must be present to generate the new strand. DNA polymerase III requires
a primer to begin its action. The primer used is a short RNA sequence with a 3' hydroxyl
group that is formed by an enzyme known as primase. This primer is usually about ten
nucleotides in length and is complementary to the existing DNA strand. DNA polymerase
always works in the same direction: from the 5' end to the 3' end. 
Since DNA polymerase III always works in the 5' to 3' direction, and DNA strands are
complementary, this gives rise to a few minor issues that must be dealt with. The strand
in which DNA polymerase can move in the same direction as gyrase (with the replication
fork) is known as the leading strand. As the strand is unwound, DNA polymerase III can
easily begin to replicate the strand, as the replication fork is already moving in the 5'
to 3' direction. The complementary strand is known as the lagging strand. The replication
fork is necessarily moving in the 3' to 5' direction on this strand. On this strand,
numerous primer sequences are inserted so that the DNA polymerase III can backtrack to
build the new sequence as the strand is unwound. The DNA sequences between these primers,
which are 1000 to 2000 nucleotides long, are known as Okazaki fragments. 
Once DNA polymerase III has replicated the fragments, the need arises to remove the RNA
primer sequences and fuse the portions of the new strand together. The first critical
enzyme used to do this is known as DNA polymerase I. This enzyme removes the primer
sequence with the crucial 3' hydroxyl group and synthesizes complementary DNA to fill in
the gaps left by the primers. After this is completed, yet another enzyme known as ligase
is used to join the fragments. This enzyme works by forming a phosphodiester bond between
the 3' hydroxyl of the new strand and the 5' phosphate group found on the Okazaki
fragment. Using each enzyme to perform a specific function, DNA is successfully
replicated.