• Multicellular organisms depend on cell division for:– Development from a fertilized cell– Growth– Repair
Fig. 12-2
100 µm 200 µm 20 µm
(a) Reproduction (b) Growth and development
(c) Tissue renewal
Concept 12.1: Cell division results in genetically identical daughter cells
• Most cell division results in daughter cells with identical genetic information, DNA
• Somatic cells (body cells -nonreproductive cells) have two sets of chromosomes
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Fig. 12-3
20 µm
• Eukaryotic cell division consists of:
Interphase, stage prior to mitosis, cell growth
Mitosis, the division of the nucleus
Cytokinesis, the division of the cytoplasm
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• Interphase (about 90% of the cell cycle) can be divided into subphases:
– G1 phase (“first gap”) – cell growth
– S phase (“synthesis”)- DNA replicates
– G2 phase (“second gap”)- cell growth
• The cell grows during all three phases, but chromosomes are duplicated only during the S phase
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Fig. 12-5
S(DNA synthesis)
MITOTIC(M) PHASE
Mitosis
Cytokinesis
G1
G2
• Mitosis is conventionally divided into four (five) phases:– Prophase– (Prometaphase)– Metaphase– Anaphase– Telophase
• Cytokinesis is well underway by late telophaseBioFlix: MitosisBioFlix: Mitosis
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Prophase
• Chromosomes are visible form sisters
• Nucleus and nucleolus disappears
• Centrosomes/ Centrioles appear, spindles form from centrosome
Fig. 12-10a
Nucleus
Prophase1
NucleolusChromatincondensing
Metaphase
• Movement of centrosomes to the poles
• Movement of spindle fibers to reach the chromatids
• Movement of chromatids to the equator
Fig. 12-10c
Metaphase3
Anaphase
• Spindle fibers attach to kinetochores on the sisters chromatids
• Sister chromatids are pulled apart
• Chromosomes move as equal sets to the poles
Fig. 12-10d
Anaphase4
Telophase
• Nuclear membrane and nucleolus reappear
• Spindles/centrosomes disappear
• Cytokinesis occurs
Fig. 12-10e
Telophase5
Cell plate 10 µm
Cytokinesis: A Closer Look
• Cyto – cytoplasmic area separated
• In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow made from microfilaments
• In plant cells, a cell plate forms during cytokinesis made from the golgi vesicles forms a cell wall
Animation: CytokinesisAnimation: Cytokinesis
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Video: Sea Urchin (Time Lapse)Video: Sea Urchin (Time Lapse)
Video: Animal MitosisVideo: Animal Mitosis
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Cleavage furrow
Fig. 12-9a
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Contractile ring ofmicrofilaments
Fig. 12-9b
Daughter cells
(b) Cell plate formation in a plant cell (TEM)
Vesiclesformingcell plate
Wall ofparent cell
New cell wallCell plate
1 µm
Fig. 12-10
Chromatincondensing
Metaphase Anaphase TelophasePrometaphase
Nucleus
Prophase1 2 3 54
Nucleolus Chromosomes Cell plate 10 µm
Cell Growth
• Limits
• surface area / volume = ratio
• Lab we did showed as volume of a cell increase the ratio decrease
• Therefore there is a need to increase protein production to build more membranes for transportation of compounds around the cell
Rate of cell growth
• Bacteria replicate every ?
• Heart and nerve cells divide ?
• Skin and Digestive cells ?
Cells depend on …..
• Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing
• Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide
• Growth factors nutrients that help cells through the cell cycle
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Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
(a) Normal mammalian cells (b) Cancer cells25 µm25 µm
• Cancer cells exhibit neither density-dependent inhibition , anchorage dependence or a need for growth factors
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Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells do not respond normally to the body’s control mechanisms
• Cancer cells may not need growth factors to grow and divide:– They may make their own growth factor
– They may convey a growth factor’s signal without the presence of the growth factor
– They may have an abnormal cell cycle control system
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• Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
• If abnormal cells remain at the original site, the lump is called a benign tumor
• Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors
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Cancer spreads
• Most commonly by the lymphatic system
• Local invasion
• Blood stream
Fig. 12-UN2
Fig. 12-UN3
Fig. 12-UN5