Cell Cycle

Cell Cycle•describes the life cycle of a cell•differs fundamentally between prokaryotes & eukaryotes

•shares four features among all cell types–reproductive signal–DNA replication–genome segregation–cytokinesis

•produces a new individual or new parts

Prokaryotic Cell Cycle•one circular chromosome–packed on a protein frame in the nucleoid

–anchored to the plasma membrane•chromosome replication yields two anchored circles

•cell growth separates anchored circles

•annular pinching separates daughter cells

Binary Fission

binary fission in a

bacteriumFigure 9.2

chromosome numbers vary widely

Table 9.1

Eukaryotic Cell Cycle•multiple linear chromosomes–wrapped on a protein frame–contained within the nucleus

•replication yields two complete sets of chromosomes

•mitosis segregates one set of chromosomes to each end of the cell

•cytokinesis divides the cytoplasm to form two new cells

Eukaryotic Cell Cycle

•mitosis, cytokinesis (M phase) and interphase

•interphase represents most of the cell cycle

eukaryotic cell cycleFigure 9.3

growth &

biosynthesis

growth &

biosynthesis

Cell Cycle Decision Making•G1 => S transition & G2 => M transitions require signals

•CDKs signal cell cycle transitions–Cyclin-Dependent Kinases•kinases phosphorylate target proteins•phosphorylation activates (some) proteins

–Cyclins activate CDKs by allosteric binding

•Cyclins cycle

eukaryotic cell cycleregulationFigure 9.4

“Go!”“Don’t go!”

“Radiation

damage!Don’t go!”“Go!”

“Okay, go!”

“Go!”

Replicate!

external signals stimulate cell division

•growth factors stimulate cell division–platelet-derived growth factor•wound healing

–interleukins•immune system function

–erythropoietin•red blood cell production

•each provides information to the cell regarding the needs of the body

Eukaryotic Chromosomes•chromsomes consist of chromatin–double stranded DNA–protein “spools”

•chromatin is ~1/2 DNA & 1/2 protein•chromatin is highly organized–DNA wraps around histone proteins to form nucleosomes

–during cell division, nucleosomes coil tightly to form highly-condensed chromatin

organization of chromatinFigure 9.5

organization of

chromatinFigure 9.6

Replication•during S phase–partially wound DNA is replicated to form two identical copies of each chromosome

–two sister chromatids remain attached at the centromere

•each chromosome enters M phase as two linked sister chromatids

•mitosis separates the chromatids and distributes one to each daughter cell

mitosis: chromatin & cytoskeletonFigure 9.8

events of mitotic phases

•Prophase–chromosomes condense–nucleolus disorganizes–spindle apparatus begins to organize

–kinetochores forms

mitosis: cartoon versionFigure 9.8

Mitotic spindle apparatusFigure 9.7

events of mitotic phases

•Prometaphase–nuclear envelope is fragmented

–spindle fibers bind kinetochores

–chromosomes begin to migrate to equatorial plate

events of mitotic phases•Metaphase–chromosomes are aligned at equatorial plate

•Anaphase–sister chromatids separate–daughter chromosomes migrate to poles

•Telophase–prophase is reversed

Animal Cytokinesis Plant

Figure 9.10

Cytokinesis: Division of Cytoplasm

•Animals–annular pinching by actin & myosin ring

•Plants–deposition of cell plate by Golgi vesicles

•Organelles are distributed to daughter cells ~randomly

Modes of Reproduction•asexual reproduction–production of genetic clones through mitotic cell divisions

–common among plants (vegetative propagation) and unicellular eukaryotes

–eliminates costs & risks associated with sexual reproduction

–offspring lack genetic variability

Modes of Reproduction•sexual reproduction–offspring exhibit genetic variability•each bears a unique combination of parental genetic contributions

–requires•meiosis - reduction of chromosome number from 2n (diploid) to 1n (haploid)•fertilization - combination of 1n parental contributions to produce 2n offspring

Fungal/animal life cyclesFigure 9.12

Modes of Reproduction•meiosis produces–gametes - animals, some protists–spores - fungi, plants, some protists•produce 1n adults•produce gametes

•fertilization (gamete fusion) produces–zygotes•produce 2n adults and/or•undergo meiosis

Modes of Reproduction•meiosis–two divisions–reduces 2n parent cell to 1n products–always produces 4 haploid products•begins with 4 homologous chromatids

–recombination produces novel chromatids

–phases resemble mitotic phases, except•meiosis I - homologs pair at prophase•meiosis I - homologs separate at anaphase

Meiosis I: cartoon versionFigure 9.14

Modes of Reproduction

•meiosis I–prophase I•chromosomes condense, spindle forms•nuclear envelope fragments•homologs pair (synapsis)•crossing over

crossing- over

occurs during prophase IFigure 9.15 Figure 9.16

Modes of Reproduction•meiosis I–metaphase I•homolog pairs align at equatorial plate

–anaphase I•homologs separate to poles

–telophase I•may or may not happen

Modes of Reproduction•interkinesis–period between telophase I and prophase II

–no replication occurs

Modes of Reproduction•meiosis II–prophase II•reverses telophase I

–metaphase II•chromosomes align at equatorial plate

–anaphase II•sister chromatids separate to poles

–telophase II•reverses prophase

mitosis => identical daughters

meiosis => variable

daughters Figure 9.17

nondisjunction at

anaphase I

of meiosisFigure 9.18

Errors of Reproduction

•meiotic errors may result in chromosomal disorders–aneuploidy - incorrect genetic complement•nondisjuction•translocation

Errors of Reproduction

•meiotic errors may result in chromosomal disorders–polyploidy•3, 4, or more sets of chromosomes•can perform mitosis (reproduce asexually)•1, 3,5, etc. cannot perform meiosis

Cell Death•death by necrosis–damage, poison, starvation

•death by apoptosis (programmed cell death)–discards un-needed or old cells

–signals are common in many organisms

–many cancers result from failed apoptosis

apoptosis: programmed cell death

Figure 9.18

cell death mechanismsTable 9.2