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DNA Replication ModelBy: Katelyn Perry
Note: The DNA molecules are twisted
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
3’
5’
5’
3’
The DNA strands would be twisted around each other
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
The replication begins when helicase break the hydrogen bonds between the complementary strands and splits the molecule in half.
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
DNA polymerase can only add new nucleotides to a free 3’ end of the growing chain. The synthesis on the leading strand proceeds in the 5’ to 3’ direction
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
Leading strand
Lagging strand
Synthesis of the lagging strand is much more complex. DNA polymerase can add new deoxyribonucleotides only to a free 3’ OH
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
To provide a free 3’ OH starting point on the lagging strand, RNA primase attaches to the DNA and synthesizes a short RNA primer.
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase III adds deoxyribonucleotides to the 3’ end of the RNA primer
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
Okazaki fragment
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
T
G
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA polymerase I replaces DNA polymerase III, removes the RNA and replace it with DNA
Okazaki fragment
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA polymerase I replaces DNA polymerase III, removes the RNA and replace it with DNA
Okazaki fragment
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA polymerase I replaces DNA polymerase III, removes the RNA and replace it with DNA
Okazaki fragment
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
Finally, the enzyme DNA ligase forms a phosphodiester bond between the 3’ OH of the growing strand and the 5’ phosphate in front of it.
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
C
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
Now the process continues on the lagging strand until the whole strand it replicated.
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
A
C
T
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
T
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
T
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
T
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
T
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
A
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
A
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
A
A
A
A
A
T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
A
A
Key:
Phosphate
Sugar
Adenine
Thymine
Guanine
Cytosine
Helicase
DNA polymerase
RNA primer
RNA
primase
DNA polymerase III
DNA polymerase I
DNA ligase
The end product of DNA replication is having two identical DNA molecules.
A
A
A
A T
T
T
T
T
C
C
C
C
G
G
G
G
T
G
C
A
C
G
T
T
C
C
T
A
A
G
A
A
A
G
T
DNA Molecules are twisted
The Purpose of DNA Replication
• The primary purpose of DNA is to store information. The DNA molecule consist of bases adenine, thymine, guanine, and cytosine. These bases are attached to each other by the help of hydrogen bonds. The sequence of the bases determines the genetic code. Physical traits and characteristics are stored within the molecular formula of this macromolecule.
• The secondary purpose of DNA is for the synthesis of RNA. DNA direct the synthesis of RNA under a chemical process known as transcription. The cellular enzymes are directed by the genetic code to recreate strands of RNA in relation to the coded sequence stored in the DNA molecule.
Additional Information• A telomere is a region of repetitive DNA bases at the end of a
chromosome. The telomeric regions consist of telomeric repeat sequences. The exact sequence of the telomeric repeat can vary depending on the species.
• Telomerase is an enzyme that adds nucleotides to telomeres, especially in cancer cells. Telomerase is an unique enzyme because in addition to having DNA polymerase activity it as contains an RNA sequence that provides a template for the synthesis of telomeric repeat DNA.
• A cell transplant is the infusion, or injection, of healthy cells into the body to replace damaged or diseased cells. An example is a stem cell transplant.
• DNA cloning refers to the process of creating multiple copies of a DNA fragment. Two other types of cloning are reproductive and therapeutic cloning.
• Aging is when the body grows old, but there is a DNA damage theory of aging. This theory proposes that aging is a consequence of the accumulation of unrepaired DNA damages. The damage is a DNA alteration that has abnormal structure.
• Okazaki fragments are relatively short fragments of DNA synthesis on the lagging strand. The lagging strand is synthesized discontinuously in the form of short fragments. These fragments are Okazaki fragments and are later connected to covalently to form a continuous strand.
• DNA ligase is an enzyme that in the cell that repairs both complementary strands if there is a break in both of them. Purified DNA ligase is used in gene cloning in order to join DNA molecules together. A single-strand break, is fixed by a different type of DNA ligase using the complementary strand as a template.
• Normal cells become cancer cell due to DNA damage. In normal cells, the DNA would repair the damage, but if the damage is not repair and it is still reproducing then it turns into a cancer cell. The new cancer cell have the same abnormal DNA damage as the first. The damage in DNA can either be inherit or the bases can be matched up wrong to create a mutation in the DNA molecules.
• During interphase the cell grows, accumulating nutrients for mitosis preparing it for cell division and duplicating its DNA (DNA is split into two identical molecules). Next is the mitotic phase, during which the cell splits into two identical cells called “daughter cells”. Lastly, cytokinesis where the new cell is completely divided. Therefore, DNA replication occurs during interphase.
• A mutation in DNA replication can occur when the base pairs (adenine, thymine, guanine, and cytosine) are matched up wrong. Adenine is suppose to match up with thymine and guanine is suppose to match up with cytosine.
