Transcript Cell Reproduction 2

Figure 12.0 Mitosis
Living things must be able to reproduce.
Reproduction of cells is done via cell division.
The division occurs as part of the cell cycle.
Figure 12.1a The functions of cell division: Reproduction
Purposes:
Reproduction of cells
Repair of tissue
Growth
Pass on DNA
Figure 12.1b The functions of cell division: Growth and development
Cell division is also central to the
development of a multicellular organism that
begins as a fertilized egg or zygote.
Figure 12.1c The functions of cell division: Tissue renewal
•Multicellular organisms also use cell division to
repair and renew cells that die from normal wear
and tear or accidents.
•A cell’s genetic information, packaged as DNA, is called its
genome.
•In prokaryotes, the genome is often a single long DNA molecule.
•In eukaryotes, the genome consists of several DNA molecules.
Figure 12.2 Eukaryotic chomosomes
•DNA molecules are packaged into chromosomes.
•Every eukaryotic species has a characteristic number of
chromosomes in the nucleus.
•Human somatic cells (body cells) =46 chromosomes.
•Human gametes (sperm or eggs) = 23 chromosomes
•Fertilization fuses two gametes together and doubles
the number of chromosomes to 46 again.
Figure 12.3 Chromosome duplication and distribution during mitosis
•Chromosomes have a
DNA and protein
complex, chromatin,
organized into a long
thin fiber.
Figure 12.4 The cell cycle
Interphase: [90%] G1 (growth 1), S (synthesis) and G2
(growth 2)
M Phase [10%] – mitosis and cytokinesis
Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase
•cell completes
preparations for
cell division.
•chromosomes are tightly
•the nuclear envelope fragments
coiled
and microtubules from the
•The nucleoli disappear. spindle interact with the
chromosomes.
•The mitotic spindle begins
•Microtubules from one poles
to form
attach the kinetochores,
Figure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and
cytokinesis.
• The cell continues to elongate
•The spindle fibers push •At anaphase, the
the sister chromatids until centromeres divide, •Two nuclei begin to form
they are all arranged at separating the sister •Cytokinesis, division
the metaphase plate,
chromatids. (Pulled of the cytoplasm,
toward the poles)
begins.
Figure 12.5x Mitosis
Figure 12.6 The mitotic spindle at metaphase
•
Assembly of the spindle microtubules starts in the centrosome.
• The centrosome (microtubule-organizing center) of animals has a pair of
centrioles at the center, but the function of the centrioles is somewhat
undefined.
Figure 12.7 Testing a hypothesis for chromosome migration during anaphase
•One hypothesis for the movement of
chromosomes in anaphase is that
motor proteins at the kinetochore
“walk” the attached chromosome
along the microtubule toward the
opposite pole.
•The excess microtubule sections
depolymerize.
•Experiments support
the hypothesis that spindle fibers
shorten during anaphase from the end
attached to the chromosome, not
the centrosome.
•Nonkinetichore microtubules are
responsible for lengthening the cell
along the axis defined by the poles.
Figure 12.8 Cytokinesis in animal and plant cells
Figure 12.9 Mitosis in a plant cell
Figure 12-09x Mitosis in an onion root
Figure 12.10 Bacterial cell division (binary fission) (Layer 1)
Figure 12.10 Bacterial cell division (binary fission) (Layer 2)
Figure 12.10 Bacterial cell division (binary fission) (Layer 3)
Figure 12.11 A hypothesis for the evolution of mitosis
Figure 12.12 Evidence for cytoplasmic chemical signals in cell cycle regulation
•The cell cycle appears to be driven by specific chemical
signals in the cytoplasm.
•Fusion of an S phase cell and a G1 phase cell induces the G1
nucleus to start S phase.
•Fusion of a cell in mitosis with one in interphase induces the
second cell to enter mitosis.
Figure 12.13 Mechanical analogy for the cell cycle control system
•The distinct events of the cell cycle are directed by a
distinct cell cycle control system.
•These molecules trigger and coordinate key events in the cell
cycle.
•The control cycle has
a built-in clock, but it
is also regulated by
external adjustments
and internal controls
•For many cells, the G1 checkpoint, the restriction point in
mammalian cells, is the most important.(go ahead or G0
phase.
Figure 12.14 Molecular control of the cell cycle at the G2 checkpoint
Peaks in the activity of
one cyclin-Cdk complex,
MPF, correspond to peaks
in cyclin concentration.
•MPF (“maturation-promoting factor” or
“M-phase-promoting-factor”) triggers the
cell’s passage past the G2 checkpoint to the
M phase.
•MPF promotes mitosis by
phosphorylating a variety of other protein
kinases.
•MPF stimulates fragmentation of the
nuclear envelope.
•It also triggers the breakdown of cyclin,
dropping cyclin and MPF levels during
mitosis and inactivating MPF
• A variety of external chemical and physical factors
can influence cell division.
• Particularly important for mammalian cells are
growth factors, proteins released by one group of
cells that stimulate other cells to divide.
• For example, platelet-derived growth factors (PDGF),
produced by platelet blood cells, bind to tyrosine-kinase
receptors of fibroblasts, a type of connective tissue cell.
• This triggers a signal-transduction pathway that leads to
cell division.
• Each cell type probably responds specifically to a
certain growth factor or combination of factors.
Figure 12.15 The effect of a growth factor on cell division
•The role of PDGF is easily seen in cell culture.
•Fibroblasts in culture will only divide in the presence of a medium that also contains
PDGF.
•In a living organism, platelets release PDGF in the vicinity of an injury.
•The resulting proliferation of fibroblasts helps heal the wound.
Figure 12.15x Fibroblast growth
Figure 12.16 Density-dependent inhibition of cell division
•Growth
factors appear
to be
important in
densitydependent
inhibition of
cell division.
Figure 12.17 The growth and metastasis of a malignant breast tumor
•If the abnormal cells remain at the originating site, the lump is called a benign tumor.
•Most do not cause serious problems and can be removed by surgery.
•In a malignant tumor, the cells leave the original site to impair the functions of one or
more organs.
•This typically fits the colloquial definition of cancer.
•In addition to chromosomal and metabolic abnormalities, cancer cells often lose
attachment to nearby cells, are carried by the blood and lymph system to other tissues,
and start more tumors in a event called metastasis.
Figure 12-17x1 Breast cancer cell
Figure 12-17x2 Mammogram: normal (left) and cancerous (right)