AP Biology, Chapter 12
The Cell Cycle

Summary
Introduction

THE KEY ROLES OF CELL DIVISION
	Intro
	Cell division functions in reproduction, growth, and repair
		1. Explain how cell division functions in reproduction, growth, and repair.
			a. Single-celled and other organisms reproduce asexually by cell division
				i. Asexual spores
				ii. Binary fission in bacteria
				iii. Growth, breakage, and establishment of vines
			b. Growth may increase the sizes or numbers of cells
			c. Wounding releases growth factors that stimulate mitosis
	Cell division distributes "identical" sets of chromosomes to daughter cells
		2. Describe the structural organization of the genome.
			a. Organisms may have hundreds to tens of thousands of genes
			b. Genes, controlling sequences, spacers, and junk are strung on chromosomes
			c. Chromosome numbers
				i. Bacteria may have one or a few circular or linear chromosomes
				ii. Eukaryotes may have one to >1000 linear chromosomes
				ii. Members of species tend to have characteristic chromosome numbers
		3. Describe the major events of cell division that enable the genome of one cell to be 
		passed to two daughter cells.
			a. Immediately after mitosis each chromosome is a single DNA
			b. DNA replication makes two attached copies
			c. As cells enter mitosis the doubled, X-shaped chromosomes condense
			d. At anaphase the two copies, the chromatids separate
		4. Describe how the chromosome number changes throughout the human life cycle.
			a. n = haploid number = 23 for humans
			b. Fertilization: sperm (n) + egg (n)  zygote (2n)
			c. Mitosis: zygote (2n)  body cells (2n) + germ cells (2n)
			d. Meiosis: germ cells (2n)  sperm (n) + egg (n)

THE "MITOTIC" CELL CYCLE
	The mitotic phase alternates with interphase in the cell cycle: an overview
		5. List the phases of the cell cycle and describe the sequence of events that occurs 
		during each phase.
			a. Interphase = 90% of the cell cycle
			b. Growth in size and number of organelles is continuous
			c. Phases
				i. G1 immediately follows mitosis
				ii. DNA replication occurs during the S or synthetic phase
				iii. G2 is between S and mitosis
	The mitotic spindle distributes chromosomes to daughter cells: a closer look
		6. List the phases of mitosis and describe the events characteristic of each phase.
			a. Prophase
				i. Chromosomes condense
				ii. Cytoskeleton reorganizes into two (centrioles separate in animals)
				iii. Nucleolus disappears
				iv. Nuclear membrane disassembled
			b. Prometaphase
				i. Chromosome condensation completed
				ii. Chromosomes attach to spindle at their kinetochores
				iii. Chromosomes move toward cell equator
			c. Metaphase
				i. Chromosomes aligned on equator
				ii. Spindle complete; chromatids attached to opposite fibers
			d. Anaphase
				i. Centromeres split
				ii. Sister chromatids move to opposite poles
				iii. Poles move farther apart
			e. Telophase
				i. Sister chromatids reach opposite poles
				ii. Chromosomes begin to decondense
				iii. Nuclear membrane form
		7. Recognize the phases of mitosis from diagrams and micrographs.
			a. Prophase: sharp boundary around single pile of condensing material
			b. Prometaphase: no distinct boundary around single pile of condensed material
			c. Metaphase: chromosomes in a single line across cell
			d. Anaphase: two groups of chromosomes, arms tailing
			e. Telophase: two tight groups at opposite ends of the cell
		8. Draw or describe the spindle apparatus, including centrosomes, kinetochore 
		microtubules, non-kinetochore microtubules, asters, and centrioles (in animal cells).
			a. Centrosomes lie at the centers of the asters
			b. Kinetochore microtubules extend from the centromeres to the centrosomes
			c. Non-kinetochore microtubules extend from centrosomes and overlap
			d. Asters are star-shaped bodies made up of microtubules radiating from a 
			centrosome
			e. Centrioles lie at the center of the centrosomes
		9. Describe what characteristic changes occur in the spindle apparatus during each phase 
		of mitosis.
			a. Interphase: centrioles duplicate and begin to separate
			b. Prophase: spindle begins to form; more tubules involved, seen as asters
			c. Prometaphase: some microtubules attach to kinetochores
			d. Metaphase: symmetry
			e. Anaphase: kinetochore microtubules shorten
			f. Telophase: spindle reorganizes for multi-functionality in interphase
		10. Explain the current models for poleward chromosomal movement and elongation of 
		the cell's polar axis.
			a. Dynein in kinetochores pulls chromatids along spindle fibers toward poles
			b. Spindle fibers disassemble after kinetochore goes by
			c. Overlapping non-kinetochore microtubules have attached motor proteins
				i. Motor proteins push on overlapping segments
				ii. Force pushes poles farther apart
	Cytokinesis divides the cytoplasm: a closer look
		11. Compare cytokinesis in animals and plants.
			a. Animals: cleavage furrow pinches the cell in two
			b. Plants: cell plate forms at equator
	Mitosis in eukaryotes may have evolved from binary fission in bacteria
		12. Describe the process of binary fission in bacteria and how this process may have 
		evolved in eukaryotic mitosis.
			a. Binary fission
				i. Copies of chromosomes attached to membrane/cell wall
				ii. Wall grows between attachment points
				iii. Copies separate
				iv. Dividing cell wall forms in the middle
			b. Evolution
				i. Bacterial chromosome/membrane attachment proteins?
				ii. Homologous genes in eukaryotes?
				iii. Kinetochores incorporate membrane attachment sites? 

REGULATION OF THE CELL CYCLE
	Intro
	A molecular control system drives the cell cycle
		13. Describe the roles of checkpoints, cyclin, Cdk, and MPF in the cell cycle control 
		system.
			a. Checkpoints 
				i. In G1, G2, and mitosis
				ii. Block the cell cycle unless conditions are met
				ii. = completion of cellular processes and environmental cues
			b. Cyclin
				i. Cyclin rises steadily through the cell cycle then falls at mitosis
				ii. Binds to and activates kinases
			c. Cdk
				i. Checkpoint go-aheads are given by cyclin-dependent kinases
				ii. Cyclin rises, binds to Cdk, allows go-ahead
			d. MPF
				i. First cyclin-Cdk complex identified
				ii. Initiates mitosis by phosphorylating rpoteins
	Internal and external cues help regulate the cell cycle
		14. Describe the internal and external factors that influence the cell cycle control system.
			a. Internal signals
				i. Unattached kinetochores send a signal
				ii. Once all are attached the signal stops
				iii. Proteolytic enzymes activate to split centromeres, degrade cyclin
			b. External signals
				i. Lack of an essential nutrient or growth factor
				ii. Density-dependent inhibition; stop growing when dish is covered
				iii. Anchorage dependence
	Cancer cells have escaped from cell-cycle controls
		15. Explain how the abnormal cell division of cancerous cells differs from normal cell 
		division.
			a. In vitro
				i. No density-dependent inhibition
				ii. Random checkpoints in the cell cycle
				iii. Immortal instead of death after 20-50 divisions
			b. In vivo transformation
				i. Loss of density-dependent inhibition
				ii. Loss of cell type phenotype
				iii. Loss of anchorage dependence  metastasis