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Cell Cycle
How do baby
elephants
grow up to be
big elephants?
Why do
organisms
need more
cells?
Why do organisms need new cells?
• When you are growing!
• When you need to replace damaged cells!
How do cells make new cells?
• All cells are created from other cells.
The only way to get more cells is for
cells to divide.
• Cell division is the process by
smaller
which a parent cell divides
into two or more daughter
cells (IDENTICAL CELLS!).
nutrients
grow
• The parent cell provides the genetic
code to each daughter cell.
Identical Daughter Cells
Two
identical
daughter
cells
Parent Cell
Cell Cycle
• In order for new
somatic (body)
cells to be
made, they
must go
interphase
through the cell
cycle:
• Interphase
• Mitosis
Interphase
• Before dividing, cells
spend most of their
time finding nutrients
and growing.
• This phase is called
interphase.
interphase
• During interphase,
there are two
growth phases.
These are known as
G1 and G2.
Interphase - DNA Replication
DNA replication
occurs during the
synthesis (S)
phase of
interphase.
Why Replicate?
Remember: ALL cells
need genetic
material….DNA!
 So….before a cell can
divide, the DNA must be
replicated (copied).
 If a cell doesn’t get a
copy of DNA, it is
USELESS and
essentially DEAD!

DNA Replication
 DNA
→ DNA (copy)
 DNA
is a HUGE
molecule, in fact it
is so HUGE it cannot
exit the nucleus EVER!
 Sooo…
DNA
Replication occurs in
the NUCLEUS!
3 EASY STEPS
 STEP
1:
◦DNA Unzips
5’ 3’
◦DNA Helicase unwinds
and separates the 2
DNA strands by breaking
the weak hydrogen bonds
3’ 5’
3 EASY STEPS
 STEP
1:
◦DNA Unzips
◦DNA Helicase unwinds
and separates the 2
DNA strands by breaking
the weak hydrogen bonds
3 EASY STEPS
 STEP
1:
◦DNA Unzips
◦DNA Helicase unwinds
and separates the 2
DNA strands by breaking
the weak hydrogen bonds
3 EASY STEPS
 STEP
1:
◦DNA Unzips
◦DNA Helicase unwinds
and separates the 2
DNA strands by breaking
the weak hydrogen bonds
3 EASY STEPS
 STEP
1:
◦DNA Unzips
◦DNA Helicase unwinds
and separates the 2
DNA strands by breaking
the weak hydrogen bonds
Replication Fork

The areas where
the double helix
separates are called
replication forks
because of their Y
shape. Once the 2
strands are
separated,
additional proteins
attach to each
strand, holding
them apart.
16
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS
 STEP
2:
◦ Complimentary
Base Pairing
5’
3’
5’
3’
3’
5’
3’
5’
◦ DNA
Polymerase
adds matching
nucleotides
3 EASY STEPS

STEP 3:
◦ DNA Ligase
Rezips DNA
5’ 3’
5’ 3’
3’ 5’
3’ 5’
DNA REPLICATED!

Two IDENTICAL copies
of DNA; one copy goes
into each new cell

Blue – Original DNA

Green – New DNA

DNA replication is a
semi-conservative
process; there is not 1
new set and 1 old set.
Each set of DNA
contains 1 old strand
and 1 new strand.
5’ 3’
5’ 3’
3’ 5’
3’ 5’
Checking for Errors

Errors sometimes occur and the
wrong nucleotide is added to the
new strand.

An important feature of DNA
replication is that DNA polymerases
have a “proofreading” role.

It can backtrack to remove the
incorrect nucleotide.

Reduces errors in DNA replication to
about one error per 1 billion
nucleotides.
31
Let’s recap
Interphase…
G1: The cell grows – makes more cytoplasm &
organelles; cell carries on normal metabolic activity
(respiration, cell transport, etc).
S: DNA is replicated.
G2: The cell continues to grow, preparing for mitosis.
Mitosis
•
•
Once the cell is ready to begin dividing, it
enters MITOSIS. The purpose of mitosis is
to separate the two sets of DNA into
TWO DIFFERENT NUCLEI – each having
it’s own complete set of chromosomes.
Cells are diploid (2n)
Mitosis
Mitosis has four phases:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
The DNA coils to form
chromosomes, the chromosomes
condense and become visible. The
nuclear membrane dissolves.
Special proteins
called spindle fibers line the
chromosomes up in a straight.
line.
The chromosomes
(which are doubled due to replication) divide
in half so that each complete and
identical set of chromosomes
move to opposite sides of the cell.
Cytokinesis
Once two nuclei have been formed, the cell
can divide its cytoplasm, which includes all its
organelles, into two new daughter cells.
This process is known as cytokinesis.
Cellular Division in Prokaryotes
Fission
• Mitosis occurs in
Eukaryotes.
• Why?
• Because they have a
nucleus!
• Binary Fission occurs
in Prokaryotes.
• Asexual reproduction
CheckpointsFissin
Ignoring Checkpointsn
Cancer