IB BIO I
DNA Replication
Van Roekel
1. Identify the phase of the cell cycle during which DNA replication occurs.
 S-Phase of interphase
2. State the function of DNA replication.
 To produce two identical strands of DNA so each daughter cell is identical to the parent cell.
3. Identify the end product of DNA replication in a human somatic cell. (do not worry about this for
quiz tomorrow)
A. 23 chromosomes
B. 46 chromosomes
C. 23 pairs of chromosomes
D. 23 pairs of pairs of sister chromatids
4. Describe why DNA replication is considered ‘semi-conservative’. Each new strand of DNA
created has one “old” strand, or the template strand, and one “new” strand, or the
complementary strand.
5. Some biochemists are making a mixture of enzymes for DNA replication in the lab. In each of
these cases, something was missing from the mixture. For each situation, deduce which one
enzyme was missing, with a reason:
a. The DNA produced came out as lots of short sections of DNA, a few hundred base-pairs
long, rather than one continuous strand.
o DNA Ligase – the short sections of DNA were the Okazaki Fragments, without ligase,
they would not be connected.
 The DNA produced had mostly DNA nucleotides, but 10 RNA nucleotides were located at the
start of each of the complementary strands.
o DNA polymerase I – since each new DNA molecule still had an RNA primers, this
means that that DNA polymerase I never removed the primer and replaced it with
DNA nucleotides.
b. No DNA was replicated. The original DNA remained untouched.
o Helicase – if the DNA strand is not separated, it cannot be copied. So it was never
separated in the first place.
IB BIO I
DNA Replication
Van Roekel
6. Answer the following questions to explain the process of DNA Replication in eukaryotes:
Process produces two identical strands of DNA. First, helicase separates the parent strand of
DNA by breaking hydrogen between nitrogenous bases. RNA primase then adds a primer to
prepare for the addition of DNA molecules. DNA polymerase III adds DNA nucleotides in a 5’-3’
direction based on complementary base pairing (A-T, C-G). DNA polymerase I then replaces the
RNA primer with DNA nucleotides. In the leading strand, this happens continuously, towards the
replication fork. In the lagging strand, this happens is fragments called Okazaki fragments, away
from the replication fork. Finally, DNA ligase will join the Okazaki fragments to make the lagging
strand a continuous molecule of DNA. This is a semi-conservative process, so each strand has
one template strand, and one complementary.
i.
Distinguish between the lead strand and the lagging strand.
o Lead strand: replication occurs continuously towards the replication fork, using the
enzymes listed above.
o Lagging strand: replication occurs using fragments called Okazaki fragments,
discontinuously, away from the replication fork.
ii.
Explain the importance of complementary base pairing in conserving the base-sequence
during DNA Replication.
Complementary base pairing means that Adenine always pairs with Thymine and
Cytosine always pairs with guanine. This is important because DNA polymerase III will
read the template strand of DNA and add nucleotides that match with their
complementary bases. So if the template sequence is ATGC, the complementary strand
created by polymerase III will be TACG.
iii.
Explain the process of DNA replication on the leading and lagging strands of DNA, with
reference to Helicase, RNA primase, primers, DNA polymerase III, Okazaki fragments,
DNA polymerase I and DNA ligase.
Process produces two identical strands of DNA. First, helicase separates the parent
strand of DNA by breaking hydrogen between nitrogenous bases. RNA primase then
adds a primer to prepare for the addition of DNA molecules. DNA polymerase III adds
DNA nucleotides in a 5’-3’ direction based on complementary base pairing (A-T, C-G).
DNA polymerase I then replaces the RNA primer with DNA nucleotides. In the leading
strand, this happens continuously, towards the replication fork. In the lagging strand,
this happens is fragments called Okazaki fragments, away from the replication fork.
Finally, DNA ligase will join the Okazaki fragments to make the lagging strand a
continuous molecule of DNA. This is a semi-conservative process, so each strand has one
template strand, and one complementary.
iv.
Explain ‘DNA replication occurs in a 5’ to 3’ direction’.
DNA nucleotides can only be added to the 3’ side of a DNA molecule because that is
where the hydroxyl group that is needed for condensation reactions is located. So,
DNA replications occurs by adding nucleotides to the 3’ side, or adding nucleotides
from the 5’ to 3’ side of the strand.
IB BIO I
DNA Replication
v.
Van Roekel
Summarize the roles of the enzymes of DNA Replication:
Enzyme
Function
DNA Helicase
Separates the parent strand of DNA by breaking hydrogen bonds between
nitrogenous bases.
DNA Polymerase III
Adds DNA nucleotides in a 5’-3’ direction using complementary base pairing (AT, C-G)
RNA Primase
Creates an RNA primer, which prepares for the addition of DNA nucleotides.
DNA Polymerase I
Replaces RNA primer with DNA nucleotides.
DNA Ligase
Connects the sugar-phosphate backbones of Okazaki Fragments on lagging
strand.