 Purpose of cell division › Unicellular organisms  Reproduction › Multicellular organisms  Development from a fertilized cell  Growth  Repair

Reproduction 100 µm Tissue renewal Growth and development 20 µm200 µm

 Genome › Complete set of genetic information  Chromosomes › DNA molecules packaged within cell  Chromatin › complex of DNA and protein that condenses during cell division (in eukaryotes)  Somatic cells › nonreproductive cells › have two sets of chromosomes  Gametes › reproductive cells  sperm and eggs  have half as many chromosomes as somatic cells

25 µm

Chromosome duplication (including DNA synthesis) 0.5 µm Centromere Sister chromatids Separation of sister chromatids CentromeresSister chromatids

 Eukaryotic cell division consists of: › Mitosis, the division of the nucleus › Cytokinesis, the division of the cytoplasm  Meiosis › yields nonidentical daughter cells › have only one set of chromosomes  half as many as the parent cell  Gametes

 The cell cycle consists of › Mitotic (M) phase  mitosis and cytokinesis › Interphase  cell growth and copying of chromosomes  preparation for cell division Interphase about 90% of the cell cycle divided into subphases:  G 1 phase (“first gap”)  G 0 (resting phase)  S phase (“synthesis”)  G 2 phase (“second gap”)

G1G1 G2G2 S (DNA synthesis) INTERPHASE Cytokinesis MITOTIC (M) PHASE Mitosis

 Mitosis is conventionally divided into five phases: › Prophase › Prometaphase › Metaphase › Anaphase › Telophase  Cytokinesis is well underway by late telophase

G 2 OF INTERPHASE PROPHASEPROMETAPHASE

METAPHASEANAPHASE TELOPHASE AND CYTOKINESIS 10 µm

Video: Animal Mitosis Video: Animal Mitosis Video: Sea Urchin (time lapse) Video: Sea Urchin (time lapse) Animation: Mitosis (All Phases) Animation: Mitosis (All Phases) Animation: Mitosis Overview Animation: Mitosis Overview Animation: Late Interphase Animation: Late Interphase Animation: Prophase Animation: Prophase Animation: Prometaphase Animation: Prometaphase Animation: Metaphase Animation: Metaphase Animation: Anaphase Animation: Anaphase Animation: Telophase Animation: Telophase

Includes microtubules, centrosomes, the spindle microtubules, and the asters Some microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate  Assembly of begins in the centrosome › microtubule organizing center  centrosome replicates › migrate to opposite ends of the cell, as spindle microtubules grow out from them  An aster (a radial array of short microtubules) extends from each centrosome

Microtubules Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto- chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore microtubules 1 µm Centrosome

 Anaphase › sister chromatids separate › move along the kinetochore microtubules toward opposite ends of the cell  microtubules shorten › depolymerizing at their kinetochore ends

Chromosome movement Microtubule Motor protein Chromosome Kinetochore Tubulin subunits

 Nonkinetochore microtubules › overlap and push against each other, elongating the cell  Telophase › genetically identical daughter nuclei form at opposite ends of the cell

 In animal cells › Process of cleavage  forms a cleavage furrow  In plant cells › a cell plate forms

Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells Cleavage of an animal cell (SEM)

1 µm Daughter cells Cell plate formation in a plant cell (TEM) New cell wall Cell plate Wall of parent cell Vesicles forming cell plate

Nucleus Cell plate Chromosomes Nucleolus Chromatin condensing 10 µm Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

 Reproduction by cell division  Prokaryotes › bacteria and archaea  In binary fission: › the chromosome replicates  beginning at the origin of replication › two daughter chromosomes actively move apart

Origin of replication Cell wall Plasma membrane Bacterial chromosome E. coli cell Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

Origin of replication Cell wall Plasma membrane Bacterial chromosome E. coli cell Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Origin

Origin of replication Cell wall Plasma membrane Bacterial chromosome E. coli cell Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Origin Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result.

G 1 checkpoint G1G1 S M M checkpoint G 2 checkpoint G2G2 Control system

G1G1 G 1 checkpoint G1G1 G0G0 If a cell receives a go-ahead signal at the G 1 checkpoint, the cell continues on in the cell cycle. If a cell does not receive a go-ahead signal at the G 1 checkpoint, the cell exits the cell cycle and goes into G 0, a nondividing state.

 regulatory proteins involved in cell cycle control: › cyclins › cyclin-dependent kinases (Cdks)

Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. Cancer cells 25 µm

 Cancer  Tumors

Cancer cell Blood vessel Lymph vessel Tumor Glandular tissue Metastatic tumor A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. A small percentage of cancer cells may survive and establish a new tumor in another part of the body.