The Cell Cycle

Chapter 12p. 218-228

Cell Division

An important part of a cell’s life cycle is reproduction

Cell Division: division of an existing cell into 2 genetically identical daughter cells3 functions:

1) reproduction, 2) growth and

development3) tissue repair/renewal

Reproduction, Growth & Repair

1) ReproductionAsexual: produces 2 identical cells (Mitosis)Sexual: combines DNA from 2 parents to form

new offspring (Meiosis – Ch 13)2) Growth

Development of organism from 1 cell3) Repair

Occurs throughout life i.e. RBC’s

Distribution of Chromosomes

For most of cell cycle, DNA is packaged w/protein in chromatin (long, thin fiber)

During Mitosis:1) Identical copy of

DNA is made2) DNA condenses into

chromosomes (46)Composed of 2

identical sister chromatids, connected by centromere

3) Sister chromatids pull apart to opposite sides of cell

4) Cell splits, forming 2 new identical daughter cells (46 chromos. = diploid)

During Meiosis, each parent supplies1/2 chromos. needed (23 chromos. = haploid))Occurs in reproductive organs (ovaries/testes)Cells must divide twice to cut chromosome number

in half

The Cell Cycle: an overview

Most of cell’s life spent in Interphase (mitosis very brief)Grow, produce proteins

& organelles3 Phases:

1) G1: grows to full size2) S: replicates DNA3) G2: prepares for

mitosis

Phases of Mitosis: an overview

1) ProphaseDNA condensesNucleolus disappearsSpindle begins to form

2) PrometaphaseNuclear envelope

breaks apartSpindles attach to

kinetochore

3) MetaphaseChromosomes line up

on metaphase plate4) Anaphase

Sister chromatids pulled apart to opposite sides of cell

5) Telophase2 new nuclear

envelopes formCytoplasm divides

(cytokinesis), forming 2 new daughter cells

Mitosis in action!!!

The Mitotic Spindle

Consists of fibers of microtubules & proteins

Assembled in centrosome at either end (pole) of cell

Attach to kinetochore of each sister chromatid

Align chromosomes along metaphase plate down center of cell

Cytokinesis

“Division of cytoplasm”

Cleavage: “pinching in” of plasma membrane along cleavage furrowPlants develop cell

plate → new cell walls

Mitosis in Plant Cells

Binary Fission

Prokaryotic cell division

DNA replicates, each copy moves to cell poles

Plasma membrane grows inward & cell divides

Evolution of Mitosis

Regulation of the Cell Cycle

Chapter 12p. 228-235

Regulation of Cell Cycle: an overview

Frequency of cell division depends on type of celli.e.: skin (fast), nerve (almost never)

Cell Cycle Control System: directs events of cell cycleBuilt-in molecular “clock” that triggers each

phaseRegulated at internal/external checkpoints

Cell Cycle Checkpoints

Occur at G1, G2, M phasesStop cycle until

requirements for next phase are met

If not, cell exits cycle & enters G0 phase (non-dividing state)

G1 Checkpoint: “restriction point” if clear, cell will usually complete cycle

Cell Cycle ClockCycle is controlled by bonding

of certain protein molecules in cell to each other 1) Cyclin-dependent kinases

(Cdks) 2) Cyclin – conc. fluctuates

Rises until M phase, then drops off

Together, make MPF complex Initiates mitosis by

phosphorylating certain proteins

Turns off by destroying cyclin

Internal Signals

Occur at anaphaseWill not proceed until all kinetochores are

properly attached to spindlesAnaphase Promoting Complex (APC):

becomes active, cyclin breaks down, chromatids separate

External Signals

Most cells depend on external signals to control cell division 1) Growth Factors: proteins

that stimulate other cells to dividei.e.: PDGF: stimulates

platelet growth after injury (signal-transduction pthwy)

2) Density-Dependent Inhibition: cells stop dividing if become crowded

3) Anchorage Dependence: animal cells must be attached to substratum to divide

Cancer Cells Escape Cycle Controls

Not affected by growth factors, density, or anchorage 1) Normal cell undergoes

transformation → cancer cell

2) Cancerous cell reproduces to form tumor

3) If remains in place → benign

If spreads & impairs organs → malignantMay enter bloodstream, have

abnormal # chromosomes, affect metabolism

Spread quickly (“metastasis”); may require radiation or chemotherapy