DNA REPLICATION
3.4
CORE
3.4.1 Explain DNA replication in terms of unwinding the double helix
an separation of the strands by helicase, followed by the formation
of the new complementary strands by DNA polymerase.
• Cells must prepare to divide by doubling the
DNA content of the cell in a process called
DNA replication. This process doubles the
quantity of DNA and also ensures that there
is an exact copy of each DNA molecule.
•
• DNA replicates during interphase of the cell
cycle.
Molecules needed for Replication
• Among the variety of molecules present in the
nucleoplasm are two types that are particularly
important for the process of DNA replication; they are
• Enzymes needed for replication- these include
helicase and group of enzymes collectively called DNA
polymerase.
• Free nucleotides – these are nucleotides that are not
yet bonded and are found floating freely in the
nucleoplasm, some contain adenine some thymine,
some cytosine and some guanine.
•
First step in Replication
• One of the early events of DNA replication is the
separation of the double helix into two single
strands. The double helix is held together by the
hydrogen bonds between complementary base
pairs.
•
• The enzyme helicase begins at a point in or at
the end of a DNA molecule and moves one
complementary base pair at a time, breaking the
hydrogen bond so the double stranded DNA
becomes 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. The unwounding
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). Helicase 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 "Replication Fork".
Template strand formation
• The unpaired nucleotide on each of these
single strand can now be used as a template
to help create two double stranded DNA
molecules identical to the original.
Formation of two Complementary
Strands
• Once DNA has become unzipped, the nitrogenous bases
on each of the single strands are unpaired. In the
nucleoplasm, there are many free-floating nucleotides.
These nucleotides are available to form complementary
pairs with the single-stranded nucleotides of the unzipped
molecule.
• This does not happen in a random fashion. A free
nucleotide locates on one opened strand at one end and
then a second nucleotide can come in to join the first. This
will require that these nucleotides become covalently
bonded together as they are beginning of a new strand.
The formation of a covalent bond between two adjoining
nucleotides is catalysed by one of the DNA polymerase
enzymes.
--• A third nucleotide then joins the first two and
the process continues in a repetitive way for
many nucleotides.
• The other unzipped strand also acts as a
template for the formation of another new
strand. This strand forms in a similar fashion,
but in the opposite direction to the first strand.
One strand is replicating in the same direction as
helicase is moving and the other strand is
replicating in the opposite direction.
3.4.2
Explain the significance of complementary base pairing in the
conservation of the base sequence of DNA.
• The pattern of DNA replication ensures that two
identical copies of DNA are produced from one.
• The area where replication has taken place, the
two strands are absolutely identical to each
other. This is because the original doublestranded molecule had complementary pairs of
nucleotides and it was the complementary
nucleotides that used the unzipped singlestranded areas as templates.
Replication is Semi onservative
• No DNA molecule is ever completely new. Every DNA
molecule after replication consists of a strand that was ‘old’
now paired with a strand that is ‘new’. DNA replication is
described as semiconservative process because half of preexisting DNA molecule is always conserved.
• http://www.wiley.com/college/pratt/0471393878/student/a
nimations/dna_replication/index.html
•
Diagram of DNA Replication
• Refer to page number
• IB Biology by C.J Clegg Page Number 67
DNA Replication Video (Cartoon Video,
a good one for review)
Replication
• To initiate DNA synthesis, a small segment of RNA,
called an RNA primer complementary to the template
DNA is synthesized by an unique RNA polymerase
known as primase. It is to this primer, that DNA
polymerase adds 5’ deoxyribonucleotides and extend
the DNA.
• DNA polymerase can function only in 5’->3’ direction.
While on the one strand, the DNA synthesis is
continuous and in 5’-> 3’ direction, on the other
strand, DNA is synthesized in small stretches resulting
in discontinuous DNA synthesis. This happens in the
opposite direction to the first strand . Such a process
is also referred to as semi-discontinuous replication.
• The short stretches of DNA, each primed by
RNA are called Okazaki fragments named after
the Japanese scientist who discovered them.
RNA primers are then removed, and the gap is
filled by DNA synthesis. Both steps are
performed by DNA polymerase. These
fragments are then sealed by the enzyme Ligase.
The strand which supports the continuous DNA
synthesis is the leading strand and the one
which is replicated in short stretches is called
the lagging strand.
Leading and Lagging Strand
Leading Strand
• The elongation process is different for the 5'3' and 3'-5' template. a)5'-3' Template: The
3'-5' proceeding daughter strand -that uses a
5'-3' template- is called leading strand
because DNA Polymerase ä can "read" the
template and continuously adds nucleotides
(complementary to the nucleotides of the
template, for example Adenine opposite to
Thymine etc).
Lagging Strand
• b)3'-5'Template: The 3'-5' template cannot be "read" by DNA
Polymerase ä. 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 å 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 å 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 Ligase (adds phosphate in the remaining
gaps of the phosphate - sugar backbone).
•
Each new double helix is consisted of one old and one new chain.
This is what we call semiconservative replication.
• If you need a simpler answer for DNA replication you can make
use of IB BIOLOGY By Andrew Allot (Oxford) Page number 16.
•
Discontinuous DNA Replication