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The Process of DNA Replication
Nicole M. Celani
ENG 202 C
Section 017
10/22/2012
Audience: Professor Mateer
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Audience and Scope of Description
The purpose of this description is to describe the process of how DNA, our genetic
information, is replicated and transferred among cells. This description lays outthe major steps that occur during DNA replication in Chronological order. After
reading this description the audience will have a better understanding of how all
cells in the body can contain the same DNA.
The anticipated audiences for this description are individuals with little to no
knowledge of how DNA is replicated. This process description can be placed
online and in textbooks for individuals to develop a more comprehensiveunderstanding of DNA replication.
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Introduction to DNA Replication
Did you know all the cells in your body contain the exact same DNA that was
deposited by your mother and father? Have you ever wondered how this couldeven be achieved? This phenomenon is made possible by the process of DNA
Replication. The following description will give you a better understanding of
how this process works. The replication of DNA is a biological process in whichDNA is copied and allows for inheritance of genetic information.
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The Structure of DNA
DNA, Deoxyribonucleic acid, is a
double stranded helix. A doublestranded helix is coiled structure with
the two strands linked with hydrogen
bonds. This creates a spiral structure
that can be best seen in Figure 1.
The inside of DNA is made up of four
nucleotides. The four nucleotides that
pair together in DNA are Adenine,
Guanine, Cytosine and Thymine. These
nucleotides can also be referred to as
base pairs. The nucleotides are heldtogether by hydrogen bonding.
The outside of the helix is connected
using a sugar-phosphate structure to
hold the molecule in place.
The two strands of DNA run anti-parallel, reverse order, to one another to
allow enzymes to synthesize two
strands of DNA from one strand of
DNA. Figure 2 indicates this anti-
parallel nature by showing one side
running 5-3 and the other side running
3-5.
One DNA molecule can be separated to
create two new DNA molecules. Thisdivision allows for the inheritance of
DNA from one cell to another. Figure 2
shows the overall structure of DNA.
Figure 1: The overall structure of
DNA replication
Figure 2:The four nucleotides of DNAand the outside held together using a
Sugar-Phosphate structure.
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DNA Replication
DNA replication is the process in which DNA can be copied. In order for genetic
information to be passed on to all cells, this process is essential. DNA replicationcan be broken down into the following four major steps.
STEP 1:Splitting the Two Strands of DNA
In order to split the DNA an enzyme called DNA Helicase is introduced
to the cell as seen in figure 3. This enzyme breaks the hydrogen bonds
between the nucleotides. Breaking the two sides of DNA apart allows for
the individual strands to be made into new DNA molecules.
The exact point on the DNA molecule where replication is occurring is
called the Replication Fork. The Origin of Replication is where the
replication process actually began.
Figure 3: Helicase is an enzyme that is used to
split the double helix of DNA. The Helicasemolecule is located at the Replication Fork.
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STEP 2: Starting the Polymerization of the new strands
To continue building the DNA molecules, modifications must be made toeach of the separated strands. These alterations are essential because
enzymes cannot simply attach and begin to polymerize without a starting
point.
The starting point is created by the addition ofRNA primers. RNA
Primase is the enzyme that attaches the RNA primers to each separated
strand and creates a start point for replication. Figure 4 shows the RNA
primer added to one strand of the separated DNA molecule.
Figure 4: The RNA primer has been addedto the strand by RNA Primase.
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STEP 3:Elongation of the New DNA Strands
The elongation of the new strands begins with an enzyme called DNApolymerase. DNA polymerase adds the complimentary nucleotides to
the original strands of DNA thus elongating the new DNA molecule as
seen in Figure 5.
The anti-parallel structure of DNA creates a leading and lagging strand
as seen in Figure 5. The leading strand can be synthesized by DNA
polymerase continuously, whereas the lagging strand must be
synthesized in pieces as seen in Figure 6. These pieces are called
Okazaki fragments.
DNA replication is a semi-conservative process because the new DNA
molecules contain one half from the original strand and the other half
will be newly synthesized.
Figure 5: The enzyme DNA polymerase is usedto synthesize the new strands by a semi-
conservative method.
Figure 6: Okazaki fragments originate on the
lagging strand due to the inability to continuouslyreplicate this strand.
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STEP 4: Termination of DNA Replication
The last step in DNA Replication is termination. Termination can be
broken down into three separate procedures.
I. Termination begins when the lagging strands Okazakifragments are glued together using an enzyme called DNA
ligase. This creates two separate functioning molecules of
DNA.
II. Next in termination, telomeres are added to the end of DNAmolecules. Every time DNA divides a portion of the telomere
is degraded and this functions to prevent unlimited divisions of
DNA. When the telomere is fully degraded, the DNA becomesnon functional. Telomerase is the enzyme that adds these
segments to the DNA.
III. Finally, mistakes are commonly made during DNA replicationby adding the wrong nucleotides. These errors must be repaired
by an enzyme called DNA nuclease. DNA nuclease serves to
cut out the wrong nucleotides and replace them with the correct
ones.
Finally, DNA replication is complete and two functioning DNA
molecules are created. The culminating products of DNA replication can
be seen in Figure 7.
Figure 7: Okazaki fragments are glued
together using DNA ligase and errors areremoved using DNA nuclease. This allows
for two complete strands of DNA.
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Conclusion of DNA Replication
DNA replication starts with a single molecule of DNA. A series of enzymes
catalyze reactions on a single DNA molecule to allow for the synthesis of twoindividual molecules of DNA. DNA Helicase separates the two strands and breaks
the hydrogen bonds between the base pairs. Next, RNA Primase adds RNA
primers to the separated DNA strands. This allows for DNA polymerase to
polymerize the new molecules of DNA. The leading strand is made continuously
and the lagging strand is made in fragments due to the anti-parallel structure of
DNA. DNA Ligase connects the lagging strands Okazaki fragments together and
creates two cohesive DNA molecules. The two strands are then checked for errorswith DNA nuclease and partitioned into two separate cells.
Replication of DNA allows for all organisms on earth to pass on their genetic code,
making life possible. DNA replication allows for humans to begin as one single
cell and develop into a trillion celled organism, all containing the same genetic
information.
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Glossary of Important Terms
DNA- Deoxyribonucleic acid is the genetic material that carries all the genes of all
organisms on earth.
DNA Helicase- An enzyme that splits the double strands of a DNA helix molecule.
DNA ligase- An enzyme that is used to glue the Okazaki fragments together. This
is done to make the lagging strand on continuous DNA molecule.
DNA Polymerase- DNA polymerase adds the complimentary nucleotides to theoriginal strand of DNA, thus elongating the new DNA molecule.
Double Helix- A double stranded helix is coiled structure with the two strandslinked by hydrogen bonds.
Enzymes- Enzymes are molecules that catalyze reactions within the cell. All
enzymes have the ending suffix -ase to designate that it is an enzyme.
Hydrogen Bonds- Hydrogen bonds are week bonds between any element and
hydrogen. These bonds are week and can be broken easily but in numerous
numbers can be very strong.
Lagging Strand of DNA- The lagging strand of DNA is one half of the separated
DNA molecule that must be re-primed continuously and synthesized in fragments.
Leading Strand of DNA- The leading strand is one half of a separated DNAmolecule that can by synthesized continuously by DNA polymerase.
Nucleotides- Structures that form the basic structural unit of nucleic acids such as
DNA. Nucleotides are located on the inside of DNA.
Okazaki Fragments- Fragments that result on the lagging stand of DNA due toDNAs anti-parallel structure.
Origin of Replication- The start position for replication on a DNA molecule
Replication Fork- The position on the molecule where DNA replication is
occurring at any given time.
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RNA Primase- RNA Primase attaches RNA primer to the individual strands of
DNA when the helix has been split into two halves.
RNA primer- RNA primers is a small attachment that is added to the individualhalves of a DNA molecule to allow for synthesis of new DNA.
Telomerase- An enzyme that adds Telomeres to the end of DNA molecules.
Telomeres- Molecular attachments present at the end of DNA molecules.
Telomeres function to prevent unlimited divisions of DNA molecules because they
are degraded continuously after each DNA division.
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List of Illustrations
FIGURE 1:
DNA Replication and Synthesis. 2011. Oracle Education Foundation Copyright Agent, Redwood
Shores. Web. 22 Oct 2012.
FIGURE 2:
File:DNA chemical structure.svg. 2011. Wikipedia,the free encyclopediaWeb. 22 Oct 2012.