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DNA Replication Standards
5b. Students know how to apply base-pairing rules to explain precise copying of DNA during semiconservative replication and transcription of information from DNA into mRNA.
Enzymes initiate DNA replication by unwinding and unzipping,
the double helix to separate the two parental strands. Each
strand acts as a template to form a complementary daughter
strand of DNA. The new daughter strands are formed when
complementary new nucleotides are added to the bases of the
nucleotides on the parental strands. The nucleotide sequence of
the parental strand dictates the order of the nucleotides in the
daughter strands. One parental strand is conserved and joins a
newly synthesized complementary strand to form the new
double helix; this process is called semi-conservative replication.
DNA replication is usually initiated by the separation of DNA
strands in a small region to make a “replication bubble”. The
DNA strands progressively unwind and are replicated as the
replication bubble expands, and the two forks of replication move in opposite directions along the
chromosome. At each of the replication forks, the complementary strand that forms is either continuous
“leading” strand or “lagging” complementary strand is made as a series of short fragments that are ligated.
1. Students know that DNA makes a copy of itself through a process called DNA replication, regulated by a
set of enzymes. There are five major steps and five proteins (enzymes and binding) involved.
(1) ___________________________________ involving ____________________________ and
___________________________________ to keep DNA strands separate.
(2) _________________________ makes a primer of about 60 nucleotides of ___________ that is a
starting point for _______________________________.
(3) ___________________________________ elongates the DNA strands with the help of proteins that
make up the _________________________ that keeps the enzyme in place.
(4) _____________________ removes the RNA primers that will be replaced by ___________________
(5) The DNA fragments are bound together by the enzyme ______________________.
2. Students can explain why DNA Replication is described as “semiconservative.” The most significant
feature of DNA molecule is its capacity to make its own identical copies. This property is called replication.
It occurs regularly during the synthetic phase of the cell cycle, every time a cell divides.
During replication, the DNA molecule unwinds itself from one end. The two chains separate. With the help of
an enzyme called DNA polymerase free nucleotides of DNA become attached to the exposed nitrogen bases
of the two chains, as per the complementary base pairing. The uncoiling and complementary base pairing
continues in the 5'-3' direction till the other end of the DNA molecule is reached. Thus, each separated
chain develops a new complementary chain. Each of the resulting daughter molecules has one complete
strand received from the parent molecule while the other one is newly synthesised. Hence, replication is
described as semi-conservative type.
Standard 5b
1
Experiment to prove Semi-Conservative Mechanism of DNA Replication
The semi conservative DNA replication was confirmed by an elegant experiment conducted by M. Meselson
and F. W. Stahl in 1958. They grew the bacterium Escherichia coli in a medium containing nitrogen salts labeled
with the stable heavy isotope 15N. 15N was incorporated into both strands of DNA and this DNA was heavier
than the DNA obtained from E. coli grown in a 14N medium. Meselson and Stahl took E. coli cells grown in
15N and transferred them to a medium containing 14N. After one generation, when one bacterial cell had
multiplied into two, they isolated the DNA and evaluated its density by using a cesium chloride equilibrium
density gradient.
Its density was intermediate between that of the heavier 15N DNA and the lighter 14N DNA. This was
because during one generation, new DNA molecules with one 15N old strand and a complementary 14N new
strand were formed and this had an intermediate density between the DNA having 15N in both the strands and
the DNA having 14N in both the strands. This experiment provided evidence for the semi-conservative nature
of DNA replication.
As the two strands of the
parent DNA molecule separate,
each strand serves as a template
for the formation of a new
strand. Replication occurs in
opposite directions in the two
strands.
Explain how the scientists formed their conclusion.
Standard 5b
2
3. Students can describe the initial step in DNA replication as involving “unzipping” of the DNA to form two
separate strands.

The first major step for the DNA Replication to take place is the breaking of hydrogen bonds between
bases of the two antiparallel strands (
and
). The unwinding of the two strands is
the starting point. The splitting happens in places of the chains which are rich in A-T. That is because
there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between
Cytosine and Guanine).
Antiparallel DNA Double Helix – naming of orientation related to deoxyribose structures
Draw and label carbons in a deoxyribose sugar on antiparallel strands
4. Students can explain why a lagging and leading strand is produced during DNA replication.
_______________________ is the enzyme that splits the two strands. The
initiation point where the splitting starts is called "origin of replication".
The structure that is created is known as the _________________
_________. Once the double strand is unwound and separated special
proteins called ______________________
__________________________ bind to the individual strands to keep
them from reannealing - ____________________ . Label the diagram
Standard 5b
3
One of the most important steps of DNA Replication is the binding of RNA
____________ to the initiation point of the 3'-5' parent chain. RNA
____________ attracts RNA nucleotides which bind to the DNA nucleotides
of the 3'-5' strand due to the hydrogen bonds between the bases. RNA
nucleotides are the ______________(starters) for the binding of DNA
nucleotides.
The DNA elongation process requires the enzyme_________________________ and is different for the 5'-3'
and 3'-5' template.
DNA polymerase can add free nucleotides to only the 3' end of the newly-forming strand. This results
in elongation of the new strand in a 5'-3' direction. No known DNA polymerase is able to begin a new
chain (de novo). DNA polymerase can add a nucleotide onto only a
preexisting 3'-OH group, and, therefore, needs a primer at which it can
add the first nucleotide.
5'-3' Template: The 3'-5' proceeding daughter strand -that uses a 5'-3'
template- is called leading strand because DNA Polymerase can "read"
the template and continuously adds nucleotides (complementary to the
nucleotides of the template, for example Adenine opposite to Thymine
etc). Label the new DNA orientation.
3'-5'Template: The 3'-5' template cannot be "read" continuously by DNA Polymerase. The replication of this
template is complicated and the new strand is called lagging strand. In the lagging strand the RNA Primase
adds more RNA Primers. DNA polymerase reads the template and lengthens the bursts. The gap between two
RNA
primers
is
called
"Okazaki
Fragments".
The RNA Primers are necessary for DNA Polymerase to bind Nucleotides to the 3' end of them. The daughter
strand is elongated with the binding of more DNA nucleotides.
In the lagging strand the DNA Pol I –exonuclease _________________reads the
fragments and removes the RNA Primers. The gaps are closed with the action of DNA
Polymerase (adds complementary nucleotides to the gaps) and DNA
_________________________ adds phosphate in the remaining gaps of the phosphate sugar backbone to connect fragments.
The last step of DNA Replication is the Termination. This process happens when the DNA Polymerase
reaches to an end of the strands. We can easily understand that in the last section of the lagging strand, when
the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no
primer). So, the end of the parental strand where the last primer binds isn't replicated. These ends of linear
(chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres. As
a result, a part of the telomere is removed in every cycle of DNA Replication.
5. Students can describe the idea of “complementary base pairing” and how this allows for ‘faithful’
replication and transcription.
Finally, DNA Replication is not completed before a mechanism of repair fixes possible errors caused during
the replication. Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.
Mutations, permanent changes occur when repair does not correctly occur.
Standard 5b
4
Vocabulary
1. Helicase
2. DNA Polymerase
3. Ligase
4. Nucleotide sequence
5. Complementary Base
6. Pairing
7. DNA Replication
8. Semi-conservative
9. Replication bubble
10. Okazaki fragments
11. Continuous vs. Discontinuous synthesis
12. Lagging vs. Leading strand
13. Ligated
14. Anti-parallel orientation
15. Directionality
Practice Questions
1. Which of the following enzymes is the first to attach to the DNA molecule to begin DNA replication?
a. RNA polymerase
c. DNA polymerase
b. DNA helicase
d. RNA helicase
2. This enzyme is responsible for attaching nucleotides to the exposed strand of a DNA molecule during the process
of replication.
a. DNA replicase
c. DNA helicase
b. DNA nuclease
d. DNA polymerase
3. DNA replication results in two DNA molecules,
a. each with two new strands.
b. each with one new strand and one original strand.
c. each with two original strands.
d. one with two new strands and the other with two original strands.
Standard 5b
5
4. A scientist analyzed the base composition present in a certain cell. He generated the incomplete data shown on the
table below:
Base Percent
What would be the expected % Thymine content of the cell?
a. 13%
c. 28%
b. 14%
d. 56%
5. In the figure, the area X highlights the structure of a(an)
a. chromosome
b. amino acid
c. RNA molecule
d. DNA nucleotide
6. In figure, what do the rings labeled A,T G and C represent?
a. deoxyribose sugar
c. phosphate group
b. nitrogen base
d. amino acid
7. Before a cell goes through either mitosis or meiosis, which process must be carried out by the DNA in the
nucleus?
c. transcription
a. translation
d. nondisjunction
b. replication
8. What is the base-pairing rule for purines and pyrimidines in the DNA molecule?
a. C—A and G—T
c. A—G and T—C
b. C—U and A—G
d. T—A and G—C
9. DNA is copied during a process called
a. transcription.
b. replication.
c. transformation.
d. translation.
10. Which of the following statements best describes a DNA molecule?
a. It contains the sugar ribose.
b. It is composed of amino acids.
c. It is a double helix.
d. It contains the nitrogenous base uracil.
11. Which of the following is part of a DNA nucleotide?
a. sucrose
c. ribose
b. deoxyribose
d. lactose
12. Because of base pairing in DNA, the percentage of
a. purines in DNA is much greater than the percentage of pyrimidines.
b. adenine molecules in DNA is about equal to the percentage of guanine molecules.
c. cytosine molecules in DNA is much greater than the percentage of guanine molecules.
d. pyrimidines in DNA is about equal to the percentage of purines.
13. A segment of a DNA strand has the following bases: TAC GAT
What is the complementary strand of DNA?
a. TAG CAT
c. UAG CAU
b. AUG CUA
d. ATG CTA
14. During DNA replication, a DNA strand that has the bases CTAGGT produces a strand with the bases
a. AGCTTG.
c. GATCCA.
b. GAUCCA.
d. TCGAAC.
15. An analysis of the DNA from a particular species of yeast shows that 20% of the nucleotides in the
organism consist of cytosine. What percentage of the organism’s DNA nucleotides will consist of
guanine?
a. 40%
b. 20%
Standard 5b
c. 30%
d. 50%
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