cell reproduction and the cell cycle -...
TRANSCRIPT
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Chapter 12 – The Cell Cycle
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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.
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Cell division allows unicellular
organisms to 1reproduce and is
necessary to the 2growth and 3repair
of multicellular organisms.
Why do cells divide?
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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
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1. Binary Fission
2. Budding
3. Gemmules (Internal Buds)
4. Fragmentation
5. Regeneration
Types of Asexual Reproduction
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Binary Fission
A type of asexual reproduction in which a
prokaryote replicates DNA, and divides in
half, producing two identical daughter cells.
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Binary Fission
• 1 circular DNA molecule (nucleoid)
• Cell partitioned @ septum
• FtsZ molecule – similar to
eukaryotic tubulin
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Offspring grows out of body ofparent.
Budding
Yeast(Unicellular fungi)
Hydra(Multicellular cnidarian)
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• 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)
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Parent releases a specialized mass of
cells that can develop into offspring.
Freshwater sponges exhibit this type
of reproduction.
Gemmules (Internal Buds)
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Body of the parent breaks into
distinct pieces, each of which can
produce an offspring.
Planarians exhibit this type of
reproduction.
Fragmentation
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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
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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
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Cell Growth, Development & Repair
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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.
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Discovery of Chromosomes
• Walther Fleming – 1882
• Coined term “mitosis”
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Chromosome Number
• Varies from one species to another.
• Some have only 1 pair
• Most eukaryotes between 10-50
chromosomes.
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Chromosome Structure
• Composed of chromatin (40% DNA + 60%
protein)
• Human chromosome – 1.4108 nucleotides
• Single chromosome = 5 cm (2” long)
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Chromosomal Organization
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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
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The Cell Cycle
• Interphase
–G1 (Gap1)
–S (DNA Synthesis)
–G2 (Gap2)
• Mitotic (M) Phase
• Cytokinesis
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The Cell Cycle
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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
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Interphase
G1, S, G2
Preparing for Mitosis
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G1 and G2
• Periods of Active Growth
– Proteins synthesized
– Cell organelles produced
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S phase
Genetic material (chromosomes) duplicate)
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Homologous Chromosomesvs.
Sister Chromatids
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Kinetochores
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G2
• Condensation of chromosomes begins
• Motor protein production begins
• Centrioles replicate (animal cells only)
• Tubulin production increases
• Organelles reproduce
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Mitotic Phase
• Unique to eukaryotes
• Includes karyokinesis and cytokinesis
• Very accurate (error rate 1/100,000 cell divisions)
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Process by which the cytoplasm of a
eukaryotic cell is divided in two; usually
follows mitosis and meiosis.
Cytokinesis
Animation: Cytokinesis
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Process by which the nucleus of a cell is
divided in two.
Karyokinesis
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Actin filaments constrict
cell to create a cleavage
furrow
Cytokinesis in animal cells
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Cell wall too rigid for
actin filaments to
constrict cell.
Cell plate created
instead.
Cytokinesis in plant cells
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How cytokinesis differs in plants
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Mitosis in plants
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Nuclear membrane remains intact.
Mitosis occurs entirely within the
nucleus.
Nucleus and cell divides after mitosis.
Cytokinesis in Unicellular Protists(Dinoflagellates, Diatoms, Yeast)
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Cytokinesis in Unicellular Protists(Dinoflagellates, Diatoms, Yeast)
Dinoflagellates Diatoms & Yeast
EukaryotesBacteria
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Molecular Mechanisms of Cell Cycle Control
• Primary mechanism is phosyphorylation.
– kinases add phosphates
– phosphatases remove phosphates• Enzymes that accomplish are cyclin-dependent kinases
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Cyclin-Dependent Kinases
• Enzymatic subunit partnered with the protein
cyclin.
• Only active when “properly” phosphorylated”
– One site activates
– Other site inactivates
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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
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Cell Cycle Control
M
M (Spindle) checkpoint
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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
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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.
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M (Spindle) Checkpoint
• Insures connection of chromosomes to the spindle
• CDK’s are active here
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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.
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Multicellular Eukaryots
• Multiple CDK’s involved.
• Response from greater variety of external
signals.
• Cell proliferation must be limited (controlled)
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Growth Factors
• Trigger intracellular signaling systems.
• Override cellular controls that inhibit cell
division.
• > 50 identified (proteins)
• Specific cell surface receptor for each
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G0 Phase
• G0 = G1 checkpoint
• Results from lack of growth factors
• Accounts for diversity of cell cycle length.
• > 50 identified (proteins)
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Cancer
• Many different diseases
• Cancer – uncontrolled growth of cells.
• A failure of cell division control
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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
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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
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• 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
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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.
1 2 3 4
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