8.3 DNA Replication
Essential Idea
• Genetic information in DNA can be accurately copied and
can be translated to make the proteins needed by the
cell.
•
8.3 DNA Replication
Understandings
• The replication of DNA is semi-conservative and
depends on complementary base pairing.
• Helicase unwinds the double helix and separates the two
strands by breaking hydrogen bonds.
• DNA polymerase links nucleotides together to form a
new strand, using the pre-existing strand as a template.
DNA Replication
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
Key features of DNA structure
5 end
Partial chemical structure
Space-filling model
DNA REPLICATION Introduction.
• A single strand of DNA serves as a template for a new
strand.
• The rules of base pairing direct
replication.
• DNA is replicated during the
S (synthesis) stage of the
cell cycle.
• Each body cell gets a
complete set of
identical DNA.
8.3 DNA Replication
IB Assessment Statement
3.4.1 Explain DNA replication in terms of unwinding the
double helix and separation by helicase followed by the
formation of the new complimentary strands by DNA
polymerase.
DNA REPLICATION INTRODUCTION
• DNA serves only as a template.
• Enzymes and other proteins do the actual work of
replication.
– Enzyme (Helicase) unzip the double helix.
– Free-floating nucleotides form hydrogen bonds
with the template strand.
nucleotide
The DNA molecule unzips
in both directions.
– DNA polymerase enzymes bond the nucleotides
together to form the double helix.
– Polymerase enzymes form covalent bonds between
nucleotides in the new strand.
new strand
nucleotide
DNA polymerase
DNA Replication
UNZIPPING DNA
How Replication Occurs
DNA replication is carried out by
enzymes that “unzip” a molecule of
DNA.
Hydrogen bonds between base pairs
are broken and the two strands of
DNA unwind.
Replication Fork
• The location where the DNA is unwinding is called the
replication fork
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication
Fork
Replication
Fork
DNA
Polymerase
Hydrogen bonds are broken
– Enzyme helicase aids in breaking bonds which allows
strands to separate.
– Single strand binding proteins (SSB's) keep strand
from joining back together.
– Replication Fork is Formed.
nucleotide
The DNA molecule unzips
in both directions.
Adding new nucleotides to the strands of DNA
• The principal enzyme involved in DNA replication is DNA
polymerase.
• DNA polymerase joins individual nucleotides to
produce a DNA molecule and then “proofreads” each
new DNA strand.
new strand
nucleotide
DNA polymerase
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication
Fork
Replication
Fork
DNA
Polymerase
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand.
• DNA replication is semiconservative.
• Semiconservative means that each DNA molecule contains
one new strand of DNA and one old strand of DNA.
original strand
Two molecules of DNA
new strand
8.3 DNA Replication
Nature of Science & SKILLS
• Obtaining evidence for scientific theories—Meselson and
Stahl obtained evidence for the semi-conservative
replication of DNA. (1.8)
• Skill: Analysis of Meselson and Stahl’s results to obtain
support for the theory of semi-conservative replication of
DNA.
8.3 DNA Replication
Skills and Applications
• Application: Use of Taq DNA polymerase to produce
multiple copies of DNA rapidly by the polymerase chain
reaction (PCR). (We will cover this in Unit 3.5
Genetics)
• Application: Production of human insulin in bacteria as an
example of the universality of the genetic code allowing
gene transfer between species. (We will cover this is
Unit 3.5 Genetics)