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Chapter 12 – The Cell Cycle
Chapter 12 – The Cell Cycle
Objectives
• Describe how cell reproduction contributes
to repair and growth.
• Compare and contrast prokaryotic and
eukaryotic cell division.
• Compare and contrast asexual and sexual
reproduction.
Cell division allows unicellular
organisms to 1reproduce and is
necessary to the 2growth and 3repair
of multicellular organisms.
Why do cells divide?
Process in which a single cell
or set of cells produces
offspring that inherit all their
genetic material from one
parent.
Offspring are not diverse.
Asexual reproduction
1. Binary Fission
2. Budding
3. Gemmules (Internal Buds)
4. Fragmentation
5. Regeneration
Types of Asexual Reproduction
Binary Fission
A type of asexual reproduction in which a
prokaryote replicates DNA, and divides in
half, producing two identical daughter cells.
Binary Fission
• 1 circular DNA molecule (nucleoid)
• Cell partitioned @ septum
• FtsZ molecule – similar to
eukaryotic tubulin
Offspring grows out of body ofparent.
Budding
Yeast(Unicellular fungi)
Hydra(Multicellular cnidarian)
• Ring of chitin develops.• Enzymatic activity and turgor pressure weakenand extrude cell wall.
• Cell contents are forced into the progeny cell•Mitosis ends and cell plate forms.
Budding (Yeast)
Parent releases a specialized mass of
cells that can develop into offspring.
Freshwater sponges exhibit this type
of reproduction.
Gemmules (Internal Buds)
Body of the parent breaks into
distinct pieces, each of which can
produce an offspring.
Planarians exhibit this type of
reproduction.
Fragmentation
In this form, if a piece of a parent is
detached, it can grow and develop
into a completely new individual.
Echinoderms exhibit this type of
reproduction.
Regeneration
Process in which genetic material from two
parents combines and produces offspring
that differ genetically from either parent.
Leads to diversity in offspring.
Sexual reproduction
Cell Growth, Development & Repair
How Eukaryotic Cells Divide
Objectives
• Describe the structure of a chromosome.
• Name the stages of the cell cycle and explain
what happens during each stage.
Discovery of Chromosomes
• Walther Fleming – 1882
• Coined term “mitosis”
Chromosome Number
• Varies from one species to another.
• Some have only 1 pair
• Most eukaryotes between 10-50
chromosomes.
Chromosome Structure
• Composed of chromatin (40% DNA + 60%
protein)
• Human chromosome – 1.4108 nucleotides
• Single chromosome = 5 cm (2” long)
Chromosomal Organization
How often a cell divides depends on the
type of cell.
Eukaryotic cells that do divide undergo an
orderly sequence of events known as the
cell cycle.
The Cell Cycle
The Cell Cycle
• Interphase
–G1 (Gap1)
–S (DNA Synthesis)
–G2 (Gap2)
• Mitotic (M) Phase
• Cytokinesis
The Cell Cycle
Cell Cycle Duration
• Varies among organisms and by type of
cells within organisms.
• Tissues/cells
– Liver > 1 year
– Nerve and Muscle Stuck in G0
• Organisms
– Fruit Fly = 8 minutes
– Mammals = 24 hours
Interphase
G1, S, G2
Preparing for Mitosis
G1 and G2
• Periods of Active Growth
– Proteins synthesized
– Cell organelles produced
S phase
Genetic material (chromosomes) duplicate)
Homologous Chromosomesvs.
Sister Chromatids
Kinetochores
G2
• Condensation of chromosomes begins
• Motor protein production begins
• Centrioles replicate (animal cells only)
• Tubulin production increases
• Organelles reproduce
Mitotic Phase
• Unique to eukaryotes
• Includes karyokinesis and cytokinesis
• Very accurate (error rate 1/100,000 cell divisions)
Process by which the cytoplasm of a
eukaryotic cell is divided in two; usually
follows mitosis and meiosis.
Cytokinesis
Animation: Cytokinesis
Process by which the nucleus of a cell is
divided in two.
Karyokinesis
Actin filaments constrict
cell to create a cleavage
furrow
Cytokinesis in animal cells
Cell wall too rigid for
actin filaments to
constrict cell.
Cell plate created
instead.
Cytokinesis in plant cells
How cytokinesis differs in plants
Mitosis in plants
Nuclear membrane remains intact.
Mitosis occurs entirely within the
nucleus.
Nucleus and cell divides after mitosis.
Cytokinesis in Unicellular Protists(Dinoflagellates, Diatoms, Yeast)
Cytokinesis in Unicellular Protists(Dinoflagellates, Diatoms, Yeast)
Dinoflagellates Diatoms & Yeast
EukaryotesBacteria
Molecular Mechanisms of Cell Cycle Control
• Primary mechanism is phosyphorylation.
– kinases add phosphates
– phosphatases remove phosphates• Enzymes that accomplish are cyclin-dependent kinases
Cyclin-Dependent Kinases
• Enzymatic subunit partnered with the protein
cyclin.
• Only active when “properly” phosphorylated”
– One site activates
– Other site inactivates
General Cell Cycle Control
• 2 irreversible points in cell cycle
– Replication of genetic material
– Separation of sister chromatids
• Can be put on hold at specific checkpoints
– G1/S (aka Restriction Point)
– G2/M
– M (Spindle) Checkpoint
Cell Cycle Control
M
M (Spindle) checkpoint
G1/S Checkpoint Factors
• To divide or not to divide – that is the question. . .
• Influenced by Internal & External Factors
– Internal
Nutritional state of cell
Cell size
Intact genome
– External
growth factors
G2/M Checkpoint Factors
• Assesses success of DNA replication
– Sensitive to disruption or delay of DNA
replication or DNA damage
• CDK’s active here (cyclin-dependent kinases)
– *MPF
* MPF (M-phase-promoting-factor) – the CDK that acts here.
M (Spindle) Checkpoint
• Insures connection of chromosomes to the spindle
• CDK’s are active here
M Checkpoint(Spindle Checkpoint)
• Before second irreversible step in cell cycle
(anaphase separation of sister chromatids)
• Ensures all chromosomes are
– Present at metaphase plate
– Aligned properly
– Connected to spindle
• Seems to involve “tension” between opposite poles
• APC → protease → destroys cohesin complex of
sister chromatids.
Multicellular Eukaryots
• Multiple CDK’s involved.
• Response from greater variety of external
signals.
• Cell proliferation must be limited (controlled)
Growth Factors
• Trigger intracellular signaling systems.
• Override cellular controls that inhibit cell
division.
• > 50 identified (proteins)
• Specific cell surface receptor for each
G0 Phase
• G0 = G1 checkpoint
• Results from lack of growth factors
• Accounts for diversity of cell cycle length.
• > 50 identified (proteins)
Cancer
• Many different diseases
• Cancer – uncontrolled growth of cells.
• A failure of cell division control
Cancer and Control of Cell Proliferation
• p53 gene (involved in G1 checkpoint) product, p53 protein
– Monitors integrity of DNA
– Halts cell division, directs DNA repair or apoptosis.
• Cigarette smoking causes mutation in p53gene
Cancer - Loss of Cell Cycle Control
• Cancer cells do not respond normally to the body’s
control mechanisms
• Cancer cells may not need growth factors to grow
and divide:
– May make their own growth factor
– May convey a growth factor’s signal
without the presence of the growth factor
– May have an abnormal cell cycle control
system
• Transformation – Process by which a normal cell
is converted to a cancerous cell
• Tumor - mass abnormal cells within otherwise
normal tissue
• Benign tumor - Abnormal cells remain at the
original site
• Malignant tumors – cells invade surrounding
tissues and spread to other parts of body.
• Metastasize - exporting of cancer cells to other
parts of the body where they may form secondary
tumors
Cancer Terms
Fig. 12-20
Tumor
A tumor grows
from a single
cancer cell.
Glandulartissue
Lymphvessel
Bloodvessel
Metastatictumor
Cancercell
Cancer cells
invade neigh-
boring tissue.
Cancer cells spreadto other parts ofthe body.
Cancer cells maysurvive and
establish a new
tumor in another
part of the body.
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