Replication of DNA
DNA is a unique molecule because it is able to direct its own
replication and reproduce itself EXACTLY.
What I need to know:1.
2.
3.
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DNA can make an exact copy of itself
The stages of DNA replication
The requirements for DNA replication
The use of a primer and DNA polymerase
DNA replication of the lagging strand
DNA replication
DNA can reproduce itself exactly and
this is called REPLICATION.
Full replication is complete in 6 stages:
1. DNA double helix unwinds
2. Weak hydrogen bonds break causing the 2 strands to
separate
3. Free DNA nucleotide joins complimentary pair on open
strand
4. Weak hydrogen bonds reform between base pairs
5. Strong chemical bond forms between both nucleotides
(controlled by enzyme called DNA POLYMERASE)
6. Newly formed daughter DNA (identical to original)
begins to wind into double helix
Stage 1 – DNA double helix unwinds
Stage 2 – Weak hydrogen bonds break
causing the
2 strands to separate
Stage 3 - Free DNA nucleotide joins
complimentary pair on open strand
Stage 4 – Weak hydrogen bonds reform between base p
Stage 5 – Strong chemical bond forms between
both nucleotides
(controlled by enzyme called DNA
POLYMERASE)
Stage 6 – Newly formed daughter DNA
(identical to original) begins to wind into
double helix
This process can only
take place if the nucleus
contains:
• DNA (acts as
template for new
molecule)
• Supply of DNA
nucleotides
• Appropriate
enzymes
• Supply of ATP
Semi-conservative
replication
Replication results in two new
DNA molecules. It is said to be
SEMI-CONSERVATIVE
because each new DNA
molecules receives one strand of
the original parent molecule.
Enzyme control
DNA is a complex process that involves many
enzymes
The section of DNA that is to be replicated
unwinds and unzips creating a y-shaped replication
fork.
On the leading strand
The enzyme DNA polymerase controls the sugar-phosphate
bonding between nucleotides on the new DNA strand. This
will only work if it has two nucleotides to join.
Therefore a primer is needed to initiate DNA replication.
A primer is a short strand of complementary nucleotides
formed at the 3’ end of the parental DNA strand.
DNA replication on this strand is continuous
On the lagging strand
DNA polymerase is only able to add nucleotides to
the free 3’ end of a growing strand.
Therefore the DNA on the parental strand that has
the 5’ end exposed is replicated in fragments
starting at the 3’ end of a primer.
As the replicated strand is discontinuous, the
enzyme LIGASE joins the fragments together
http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter14/dna_replication.html
What is required for DNA
replication?
• DNA
• Primers
• Free DNA nucleotides
• Enzymes
• ATP
DNA and the bigger picture
Once the two identical daughter DNA strands
form, they coil up and become IDENTICAL
CHROMATIDS (held together by a centromere).
Following DNA replication (and before cell
division) the cells DNA content is at its maximum.
DNA replication ensures that an exact copy of a
species genetic information is passed on.
If DNA failed to replicate itself, mitosis and
meiosis would be unable to take place.
DNA is essential for life to exist.
Meselson and Stahl Experiment
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites
/dl/free/0072437316/120076/bio22.swf::Meselson%20an
d%20Stahl%20Experiment
Now attempt the following questions
1) Decide whether the following statements are true or false.
When the statement is false, give the word that should have
been used in place of the word in bold print
A – during DNA replication , each DNA parental strand acts as a
template.
B – A guanine base can only pair up with a thymine base
C – An adenine base can only pair with a cytosine base.
D – Complementary base pairs are held together be weak antiparallel bonds
E – each new DNA molecule formed by replication contains one
parental and one new strand.
2) Describe the stage of DNA replication on the leading strand
3a) name four requirements needed for DNA replication to occur.
3b) briefly explain why DNA replication is important
Glossary
• ANTIPARALLEL – describes the orientation of the strands in a DNA duplex; these run in
opposite directions with respect to their 5’-3’ polarity
• CENTROMERE – region of a replicated chromosome where 2 chromatids are held
together
• CHROMATID – refers to the copy of a chromosome after DNA replication and prior to
cell division. Each replicated chromosome is composed of 2 chromatids, joined at the
centromere
• DNA POLYMERASE – an enzyme that creates DNA molecules by assembling
neucleotides
• LAGGING STRAND – the DNA strand that is replicated discontinuously from the 5' to
the 3' direction
• LEADING STRAND – the DNA strand that is synthesised continuously during replication
• PRIMER – short fragment of DNA that serve as a starting point for DNA replication
• REPLICATION FORK – The point at which the two strands of DNA are separated to
allow replication of each strand
• SEMICONSERVATIVE REPLICATION the normal process of DNA synthesis, in which the two original strands of the molecule
separate, and each acts as a template on which a new complementary strand is laid
down