• 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