IMED1002 Study Guide - Midterm Guide: Okazaki Fragments, Dna Replication, Semiconservative Replication
LECTURE 15 – DNA REPLICATION
DNA replication is when the DNA helix unwinds and separates into 2 single strands which form the
template for new strands to be synthesised. To make RNA, only 1 DNA strand is copied whereas with
replicating DNA, both strands are copied meaning the replication is referred to as semiconservative.
Each daughter cell will contain one DNA strand from the parent cell and one strand which is newly
synthesized. DNA replication takes place in the S phase of the cell cycle.
Explain how semiconservative replication was shown experimentally
Meselson and Stahl showed that DNA replication was semiconservative by growing bacteria on
media containing 15N and then transferring them to media containing 14N. The 15N isotope is
heavier than the 14N isotope, therefore in the parental generation, all the DNA (only containing
15N) was found towards the bottom of the tube. In the first generation, all the DNA was found in the
middle of the tube (14N/15N hybrid) whereas in the second generation, half was found in the
middle (14N/15N) and half at the top (14N/14N). This meant that a new strand must have been
synthesised during each replication which incorporated 14N from the media while one strand from
the parent was also retained, hence, semiconservative replication.
The first step of DNA replication is to unwind the helix. This is done by an enzyme called helicase
which breaks H bonds between the complementary strands. Next, the new strand needs to be
synthesised. The enzyme responsible for doing this is DNA Polymerase and its substrate is dNTPs; it
also requires Mg ions. Synthesis of the new strands is in the 5’ to 3’ direction, meaning new base
pairs are added to the 3’ end. A template strand is required to base pair with and the reaction also
requires an RNA primer which is a short piece of nucleic acid that binds complementarily to the start
of the template strand.
How are Mg2+ ions involved in DNA replication?
They function as co-receptors for DNA Polymerase – ultimately, the polymerase ca’t futio
ithout the. Oe io ids ol to the ioig dNTP hile the other ids to the 3’O of the
growing strands as well as the incoming dNTP.
What are the functions of:
DNA Pol. α – Initiation of nuclear DNA synthesis (has primase activity – ie: lays down primers), also
involved in DNA repair
DNA Pol. – Leading & lagging strand synthesis of nuclear DNA, DNA repair
DNA Pol. – Leading strand synthesis
What is 3’-5’ eoulease atiit ad hih oe/s of the aoe polerases has this fution?
The ability of polymerases to proof-read and excise out nucleotides that have been laid down
wrongly – oes akards 3’ to 5’, hops rog oe out, the otiues oig 5’ to 3’ to put the
correct one in. Both ad hae this futio.
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