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DNA Replication
How does each cell have the same DNA?
How is a prokaryote different than a
eukaryote?
Prokaryotic Chromosome Structure
Chromosome
E. coli bacterium
Bases on the chromosome
Go to
Section:
Eukaryotic DNA
• Eukaryotes have 1000 times the amount of
DNA as prokaryotes, and its more complex
• DNA is found in the nucleus in chromosomes
(the number of chromosomes varies widely of
different species)
• DNA is very long!... but it is highly folded
packed tightly to fit into the cell!
– For example, a human cell contains more than 1
meter of DNA made of more than 30 million base
pairs!
Eukaryotic Chromosomes
• Contain DNA and proteins called histones
• Tightly packed DNA and proteins form
chromatin
• During mitosis, the chromatin condenses to
form tightly packed chromosomes
Figure 12-10 Chromosome
Structure of Eukaryotes
Section 12-2
Nucleosome
Chromosome
DNA
double
helix
Coils
Supercoils
Histones
Go to
Section:
Watson & Crick again
• Earlier: Discovered double helix of DNA
• Then: DNA can be copied or replicated,
because each strand of the DNA double helix
has all the information needed to reconstruct
the other half by way of base pairing
– The strands are complementary!
DNA Replication
• The process of making a copy of the DNA
• Occurs inside the nucleus of the cell
• Occurs when the cell is going to divide so
each resulting cell will have a complete set
of DNA
• During DNA replication, the DNA
separates into two strands, then produces
two new complementary strands following
the rules of base pairing.
• Each strand serves as a template, or
model, for the new strand.
• Replication occurs in opposite directions
(bidirectional DNA replication)
• The site where separation occurs is called
the replication fork or replication origins
DNA Replication
• The two strands of DNA unwind or “unzip”
breaking the hydrogen bonds and separating.
Then each strand becomes the guide or
“template” for the making of a new strand.
• A protein called an enzyme called DNA
polymerase breaks the nitrogen base bonds and
the two strands of DNA separate, polymerizes
individual nucleotides to produce DNA and
“proof reads” the new DNA.
• The bases on each strand pair up with new
bases found in the cytoplasm
• Then the sugar and phosphate groups form the
sides of each new DNA strand
• Each new DNA molecule contains an original
DNA strand and a new DNA strand (semiconservative)
Figure 12–11 DNA Replication
Section 12-2
New strand
Original
strand
DNA
polymerase
Growth
DNA polymerase
Growth
Replication fork
Replication fork
New strand
Go to
Section:
Original
strand
Nitrogenous bases
DNA Polymerases
• There are different
forms of DNA
polymerases that have
different functions
during DNA replication
• DNA helicase is an
enzyme that opens up
the double helix
• Limitations:
– DNA polymerases can
only add to an existing
nucleotide chain
– DNA polymerases
synthesize chains in the
5’ to 3’ direction, adding
onto the 3’ end of the
chain
– DNA polymerases
require single stranded
DNA as a template, but
can not open up DNA
5’ to 3’
• The leading strand is
synthesized as a
continuous
polynucleotide
• The lagging strand is
synthesized as smaller
pieces called Okazaki
fragments are added
• Ligase, another enzyme,
joins together the
Okazaki fragments to
make a complete DNA
STEP 1
Replication fork
• Two original strands of DNA
separates by unwinding
• This occurs through the
action of an enzyme that
breaks the hydrogen bonds
between the strands
• The two areas on either end
where the DNA separates is
the REPLICATION FORK
STEP 2
• At the replication fork,
primase (another enzyme)
synthesizes RNA primers that
are later replaced
• Then, DNA polymerases
move along the DNA strands
adding nucleotides
• As DNA polymerase moves
along two strands of DNA
form
Step 3
• DNA Polymerase
continues until all the
nucleotides have
been added
• Two new identical
molecules of DNA are
formed
• Another enzyme
connects all the
pieces of DNA
together
Proofreading
• At the end of
Replication, DNA
polymerase goes
through the DNA to
make sure there are no
errors
• This prevents mistakes
in the DNA sequences
• Mutations- Change in
the sequence of the
DNA
Like a Zipper….
Figure 12–11 DNA Replication
Section 12-2
New strand
Original
strand
DNA
polymerase
Growth
DNA polymerase
Growth
Replication fork
Replication fork
New strand
Go to
Section:
Original
strand
Nitrogenous bases