DNA
Replication
DNA Function
• Recall:
• DNA is the carrier of hereditary information for all
living organisms.
• DNA controls the activity of all cells by coding for
proteins that are produced.
• DNA contains the full ‘blue-print’ for an individual of
a species, including the instructions for replication or
making more DNA.
DNA Replication
• When DNA is being replicated it is described as
being ‘Semi-Conservative’.
• ‘Some of the DNA is kept in the new strand.’
• What this means is that each parent strand of a
DNA molecule acts as a template for the formation
of a new complimentary strand.
• Both strands are conserved (used) but end up as
part of two different DNA molecules.
Mechanisms of DNA
Replication
• DNA is a polymer (combination of molecules) of
nucleotides.
• Each parent strand acts as a template for a new
complimentary strand.
Challenges that are found with DNA
Replication
• DNA is very long (6 feet long in humans cells).
• DNA must be unwrapped from it’s double helix
shape before replication can take place.
• Once unwrapped, each strand has a different
polarity.
The 5’ to 3’ Direction Rule
• Polarity of DNA strands can change depending on
whether they end/begin in a phosphate or a
deoxyribose sugar.
• The 5’ (5 prime) end of the DNA ends in a carbon
from the deoxyribose sugar part of the DNA.
• Remember the sugar is a 5 carbon sugar.
• The 3’ (3 prime) end of the DNA ends in a
phosphate from the phosphate group (PO4) part of
the DNA.
Enzymes Involved
• In DNA replication, the process is controlled by 6
enzymes.
• These enzymes each have a specific job involved in
creating the DNA molecule.
• They include:
o DNA POLYMERASE
o RNA PRIMER
o PRIMASE
o TOPOISOMERASE
o HELICASE
o LIGASE
DNA Polymerase
• Primary enzyme involved in DNA synthesis.
• Using a single parent DNA strand as a template to
make a new complimentary strand.
• Adds nitrogenous bases (A,T,C,G) to the
complimentary strand.
LIMITATIONS
1) Can only add nucleotides to the 3’ end of an
growing DNA chain.
• Remember the Rules! – DNA can only be replicated
in a 5’ to 3’ direction.
2) Cannot add the first 2 nucleotides.
RNA Primer
• Creates the starting point for the first 2 strands to be
added.
• Does not actually add the bases, only starts the
process.
Primase
• Adds the first 2 nucleotides after RNA primer has
prepped the site.
• Once the first 2 are added, DNA polymerase can
add continually along the strand.
Topoisomerase
• Binds to the double
helix of the DNA.
• Clips the ‘backbone’ of
the DNA to allow it to
unravel.
• Continually unwinds all
parts until the DNA is a
flat ladder and is no
longer twisted into a
double helix.
Helicase
• Think scissors!
• Helicase cuts along all
the hydrogen bounds
between the 2 parent
strands.
• This exposes them to
allow more bases to be
added to create the
complimentary strand.
Ligase
• Think glue!
• This sticks the hydrogen
bonds back together
to create the 2 new
DNA strands.
2 Types of Strands
• When complimentary strands are being created for the
parent strands, there are two types:
• 1) Leading strand
o Easier to figure out.
o Goes in the 5’-3’ direction easily, therefore is
continuously synthesized.
• 2) Lagging strand
o More complicated
o Same steps as leading strand but goes against 5’-3’
direction rule
o Additional steps must be taken to replicate this
strand, as it cannot be continuously synthesized.
Leading Strand
Steps to Replication:
Leading Strand
• 1) DNA is unwound by Topoisomerase.
• 2) Helicase cuts the hydrogen bounds, continuously,
between the nitrogenous bases to open up the 2
parent strands.
• 3) RNA primer is created at the 3’ end of leading
strand. This is at the end of the strand, away from
the ‘fork’ of the DNA molecule. This forms the spot
for the first 2 nucleotides.
• 4) Primase places the first 2 nucleotides down.
• 5) DNA polymersase adds the rest of the bases, in a
5’-3’ direction, creating the complimentary strand.
• 6) Bases are added continually until the full strand is
complete.
• 7) Ligase then attaches all hydrogen bonds back
together.
Lagging Strand
Steps to Replication:
Lagging Strand
• 1) DNA is unwound by Topoisomerase.
• 2) Helicase cuts the hydrogen bounds, continuously,
between the nitrogenous bases to open up the 2
parent strands.
• 3) RNA primer is created at the 3’ end of lagging
strand. This is at the middle of the strand, at the
opening of the ‘fork’ of the DNA molecule. This
forms the spot for the first 2 nucleotides.
• 4) Primase places the first 2 nucleotides in a short
piece of DNA called an OKAZAKI FRAGMENT.
• 5) DNA polymerase adds the rest of the bases to
the fragment, in a 5’-3’ direction.
• 6) The OKAZAKI FRAGMENT is placed down beside
the parent strand.
• 7) OKAZAKI FRAGMENTS are continually made until
the entire strand is complete.
• 8) Ligase not only attaches the hydrogens back
together, its also glues all the OKAZAKI FRAGMENTS
together between bases.
Animation
http://www.wiley.com/legacy/college/boyer/0470
003790/animations/animations.htm
Directions of DNA
• Replication does not begin at one end of the strand
to another.
• Replication begins at many points along the strand,
using Replication bubbles.
• Replication is bidirectional as well, decreasing the
time it takes to make a full, new DNA molecule.