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Cell Replication
Cell Replication
Overview
Need for Cell Replication
Cell Cycle
Controls & Regulators of Cell Replication
G0 phase
Animal vs Plant Cell Division
Cell Death
Cell Replication
Why Should Cells
Replicate?
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Growth
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Repair
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Reproduction
Cell Replication
Cell Cycle
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Cells grow but eventually their
larger size makes it difficult for a
cell to maintain itself metabolically
When a cell divides the “parent” cell
produces two genetically identical
“daughter” cells, each of which are
more capable of maintaining
themselves metabolically
The cell cycle also allows for
growth and reproduction of the
individual
Controlled by chemical signals
Cell Replication
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Cell Cycle
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Interphase
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Day-to-day cell processes
take place
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G1
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S
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Early preparation for cell
division
Centrioles replicate
Chromosomes replicate
G2
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Remaining cell organelles
replicate
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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Deoxyribonucleic Acid
● Nucleotides
● Deoxyribose sugar
● phosphate group and
a base
● Bases
● Pyrimidines
● Cytosine
● Thymine
● Purines
● Guanine
● Adenine
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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Double stranded molecule
(double helix)
Sugar and phosphate
backbone
Bases of the two strands form
hydrogen bonds according to
the law of complementary
base paring
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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DNA wraps around
proteins (histones) to
form nucleosomes
Long fibers of DNA
wrapped around the
histones represent the
chromatin
Chromatin condenses,
forming chromosomes,
to carefully organize the
nucleosomes
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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DNA is the primary component
of a cell's chromosomes
Genes-small sections of the
chromosome that contain the
instructions for synthesizing a
specific kind of protein
Genome-all the genes in a cell
Prior to a cell dividing, each
chromosome of the cell
replicates to produce two
identical chromosomes (sister
chromatids)
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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Helicase (enzyme) breaks hydrogen bonds between bases
of the two strands of a DNA molecule
DNA polymerase (enzyme) binds to each of the free strands
of the original DNA molecule & makes a new complimentary
strand using base pairing rules
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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One of the original free strands of the DNA molecule is
replicated in a continuous fashion
The other free strand is replicated in small segments call
Okazaki fragments which are later spliced together
Cell Replication
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Cell Cycle
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S
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DNA Synthesis
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Each of the two new doublestranded DNA molecules
produced is composed of 1
newly synthesized strand &
1 old strand from the original
DNA molecule.
Thus, DNA replication is
considered semiconservative
replication
Original DNA sequence is
preserved
Cell Replication
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Cell Cycle
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Interphase
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Karyokinesis
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G1
S
G2
Division of the nucleus
Occurs by mitosis
Cytokinesis
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Division of the cytoplasm
Cell Replication
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Cell Cycle
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Karyokinesis
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Division of the nucleus
Occurs by mitosis
Stages of mitosis
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Prophase
Metaphase
Anaphase
Telophase
Cell Replication
Cell Cycle
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Karyokinesis
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Prophase
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Nuclear envelope
breaks down
Spindle fibers form
between centrioles
Chromosomes
(duplicated)
condense and
attach to the
spindle fibers via
centromeres
Cell Replication
Cell Cycle
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Karyokinesis
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Metaphase
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Chromosomes are
carried by the spindle
fibers toward the
center of the cell
Chromosomes align at
the equator of the cell
Cell Replication
Cell Cycle
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Karyokinesis
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Anaphase
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Centromeres divide
Spindle fibers begin to
shorten
Chromosomes are
pulled toward the
opposite sides of the
cell
Cell Replication
Cell Cycle
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Karyokinesis
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Telophase
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Chromosomes reach
opposite poles of the
cell
Spindle fibers begin to
dissolve
Nuclear envelope
begins to reform
around each new set
of chromosomes
Cytokinesis begins
Cell Replication
Cell Cycle
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Cytokinesis
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Cytoplasm divides
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Proceeds by
shortening of protein
microtubules on the
plasma membrane
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Cleavage furrow
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Begins concurrently
during the end of
mitosis
Cell Replication
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Cell Cycle (Summary)
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Interphase
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S
G2
Karyokinesis
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G1
Division of the nucleus
Occurs by mitosis
Cytokinesis
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Division of the cytoplasm
Cell Replication
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Cell Cycle Controls
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3 Cell Cycle Checkpoints
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G1 Checkpoint
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Aka, “restriction checkpoint”
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No DNA damage
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Adequate cell resources
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Appropriate cell size
G2 Checkpoint
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Correct cell size & protein reserves
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Chromosomes replicated without damage
M Checkpoint
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Aka, “spindle checkpoint”
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Sister chromotids attached to spindle fibers
Prevent genetically mutated cells from
replicating
Cell Replication
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Cancer & Cell Cycle
Regulators
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Normal cell cycles are regulated
by proteins produced from
normal positive cell cycle
regulatory genes call protooncogenes
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Oncogenes = mutated protooncogenes
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When oncogenes occur
cancerous cells result
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Cancer: uncontrolled cell
divisions
Cell Replication
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Cancer & Cell Cycle
Regulators
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Negative cell cycle regulatory
genes like the tumor suppressor
gene can produce proteins that
prevent cancerous cells from
replicating
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A cell that carries a mutated
form of a negative regulator
gene might not be able to halt
the cell cycle and thus allow
uncontrolled cell divisions
Cell Replication
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G0 Phase
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An “inactive” phase
which some cells enter
after mitosis rather than
going into interphase
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External chemical
signals trigger some
cells to leave G0 for G1
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Others, like nerve &
heart cells, never exit G0
Cell Replication
Animal vs Plant Cell Division
Animal Cell Division is characterized by a
cleavage furrow created when actin
proteins at the cell’s equator shorten
resulting in a pinching of the cell
membrane in two
Plant Cell Division is characterized by
Golgi bodies merging together at the
former metaphase plate forming the cell
plate. The cell plate grows from the center
outwards eventually dividing the cell. New
cells walls are made from the merged
Golgi body contents
Cell Death
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Cells replication is finite
Each cell cycle caused the some of
the tips of DNA molecules
(telomeres) to be lost to replication
When the last of the telomere is
lost lysosomes auto-digest the cell
and it dies
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Called apoptosis (normal cell
death)
Cells which die before their time
result in cell death called necrosis