Section 8.3: DNA Replication

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Transcript Section 8.3: DNA Replication

Chapter 8
Section 8.3: DNA Replication
• SWBAT summarize the process of DNA
• SWBAT describe the role of enzymes in DNA
DNA Replication
DNA polymerase
Review base pairing rules
S-Phase of Interphase
• What is a template?
• How are templates used?
• What would a DNA template be and how
would it be used?
DNA Replication
• Recall: the nitrogen containing base (known as
bases from here on) can pair only in one way.
– Adenine and Thymine (A-T) and
– Guanine and Cytosine (G-C)
– A always has to pair with T (A-T or T-A) and
– C always has to pair with G (C-G or G-C).
– If the sequence of one strand of DNA is known,
then automatically you should be able to create
the other strand.
DNA Replication
• A single strand can serve as a template
(pattern) for a new strand.
– In other words, a new strand of DNA is made
using an already existing strand.
– The order of the bases are preserved (which is
really good since they are your genes)
• The process of creating a new strand of DNA
from an already existing strand is known as
DNA replication.
DNA Replication
The double helix is
unzipped and
each existing
strand acts as a
template to create
a new strand and
restore the double
Replication takes place during S-Phase of cell cycle
DNA replication
assures that every
cell has a complete
set of identical
DNA is replicated in
each round of the
cell cycle.
DNA Polymerases (protein enzymes)
• We might say that DNA replicates itself but
that really isn’t true (DNA is only a medium
for storing information).
• DNA polymerases (a class of
proteins/enzymes) do the actual replicating.
• Some enzymes start the process by unzipping
the DNA double helix creating two template
DNA Polymerases (protein enzymes)
• Other proteins hold the strands of DNA apart
while they serve as templates.
• The DNA polymerases take nucleotides
floating free in the nucleus and bind them
• When the DNA polymerases are done, the
result is two new, identical, DNA molecules.
DNA Polymerases (protein enzymes)
DNA is
unzipped by
proteins at
places along the
These places
are called the
origins of
The DNA molecule unzips
in both directions.
Unzipping means that the hydrogen bonds that hold the
complementary base pairs together are broken. The bases on
each strand are exposed.
DNA Polymerases (protein enzymes)
Free floating
nucleotides in the
nucleus pair, oneby-one, with the
bases on the
template strands
(forming hydrogen
DNA polymerases bond the nucleotides together to form new
strands that are complementary to the template strand.
DNA Polymerases (protein enzymes)
original strand
Two identical
result from the
new strand
Two molecules of DNA
Each new double helix has a strand from the original molecule and one
that is new. Therefore, we call the DNA replication process semiconservative.
• It is fast, accurate, and occurs in every living
– In human cells, the replication of a chromosome
takes just a few hours.
• As DNA is being replicated, DNA polymerase
“proofreads the copy.” Generally errors are
caught and corrected.
– Copying errors in germ cells are mutations.
– They are relatively rare – one error per 1 billion
Final Thought
• Your DNA has replicated trillions of times since
you grew from a single cell.