AP Biology: Notes and Practice
DNA Replication
Model 1
DNA Replication is a process that occurs during synthesis of the cell cycle. It results in two identical sets of
DNA, one for each of the daughter cells that will be produced in mitosis. During replication, the two nucleic
acid strands of DNA separate and each serves as a template for building a new nucleic acid polymer. Four
major events must take place in order for replication to proceed correctly:
1. Helicase, an enzyme, breaks hydrogen bonds between nucleotides
2. Strands of DNA separate
3. Free nucleotides are attracted to exposed bases on the loose strands of DNA
4. Hydrogen bonds between nucleotides form
A model displaying these events is shown below.
1. This type of replication is called semi-conservative replication. Considering the meaning of these words
(semi—half; conserve—to keep), explain why DNA replication is called semi-conservative.
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AP Biology: Notes and Practice
The image used in model 1 to describe DNA replication is very simplified. In reality, this process requires
many enzymes in order to facilitate many steps in this process. In the models that follow, we will explore a
comprehensive model of replication.
Model 2
Helicase
5’
3’
2. Model 2 displays a double stranded molecule of DNA (solid lines) that has been partially separated by
helicase. The bottom strand has been labeled 5’ and 3’ for you. Label the 5’ and 3’ ends for the top strand.
Explain your labeling.
3. The bottom strand has a molecule attached to it called DNA polymerase III.
a. What type of biological molecule is DNA polymerase III?
b. Predict the function of DNA polymerase III based on its name and position in the model.
4. The gray dotted line indicates a newly synthesized strand of DNA that is made during replication. Label the
5’ and 3’ ends for the new DNA strand and explain your labeling.
5. DNA polymerase III is limited in that it can only synthesize DNA in the 5’ to 3’ direction. Draw an arrow
to indicate the direction DNA polymerase III will have to move in order to synthesize the strand of DNA
indicated by the gray dotted line.
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AP Biology: Notes and Practice
6. The black dotted line indicated the second newly synthesized strand of DNA that is made during replication.
Label the 5’ and 3’ ends for the new DNA strand and explain your labeling.
7. Based on the new information provided in question 5, draw an arrow to indicate the direction DNA
polymerase III will have to move in order to synthesize the strand of DNA indicated by the black dotted
line.
8. Note that the helicase shown in model 2 is moving to the left. Paying attention to the arrows that you drew
displaying the direction of synthesis for the new DNA strands, which strand do you think is more easily
synthesized: the black dotted line or the gray dotted line? Explain your answer.
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AP Biology: Notes and Practice
Model 3
9. Label the leading and lagging strands of newly synthesized DNA in model 3 and explain your labeling.
10. Besides being able to only build DNA in the 5’ to 3’ direction, DNA polymerase III can only add
nucleotides to an existing nucleic acid strand. The enzyme primase builds a short, starter nucleic acid
sequence so that DNA polymerase III can attach and build DNA strands. What type of molecule does
primase synthesize?
11. Observing model 3, you see that the leading and lagging strands contain different numbers of RNA strands.
a. How many RNA strands does the leading strand contain?
b. How many RNA strands does the lagging strand contain?
c. Why do the leading and lagging strands contain different numbers of RNA strands?
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AP Biology: Notes and Practice
Model 4
In order to remove the RNA from the newly synthesized strands, cells use two enzymes: ligase and DNA
polymerase I. Ligase connects the Okazaki fragments in the lagging strand together to form a continuous strand
of DNA. DNA polymerase I replaces RNA primers with DNA so that the newly synthesized strand consists of
only DNA.
12. DNA polymerase I, just like DNA polymerase III, moves along a strand in the 5’ to 3’ direction and can
only build onto an existing DNA strand. Based on this information, circle any RNA strands in Model 4 that
you predict can be replaced with DNA using DNA polymerase I. Explain your answer.
13. Based on the new information in question 17, draw a star next to any RNA strands in Model 4 that you
predict cannot be replaced with DNA using DNA polymerase I. Explain your answer.
14. If RNA cannot be replaced with DNA, the RNA will eventually degrade. What does this suggest will
happen to the length of chromosomes over time?
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