cell cycle - 5 phases
- G1 – primary growth phase of cell
- includes major part of a cell’s life for most organisms
- S - phase where genome is replicated
- G2 - 2nd growth phase; preparations made for separation of genomes
- organelles replicate, chromosomes condense, microtubules assemble
- interphase - collective name for G1, S, G2 phases
- M (mitosis) - phase where microtubules pull sister chromatids apart
- divided into prophase, metaphase, anaphase, telophase
- C (cytokinesis) - cytoplasm divides, forms 2 daughter cells
- actin acts as drawstring to pinch animal cells in 2
- plate forms between dividing cells w/ cell walls
- embryonic cells have shortest cell cycles
- G0 phase - resting state before DNA replication
- most cells in body are in this state at any given time
- neurons/muscle cells never leave this phase after maturing
interphase - prepares for mitosis
- major portion of growth during G1 phase
- chromosome creates 2 sister chromatids attached at centromere during S phase
- kinetochore - protein disk bound to specific DNA sequence at centromere
- proteins made, organelles produced during G1/G2 phases
- DNA only replicates during S phase
- condensation - DNA coils together w/ help of motor proteins
- centrioles - microtubule-organizing centers that form during G2
- tubulin - protein that makes up microtubules
mitosis
- prophase - forming mitotic apparatus
- begins when condensed chromosomes become visible to light microscope
- ribosomal RNA synthesis stops when area of chromosome that codes for rRNA condenses
- centrioles move towards poles as spindle fibers form between them; spindle apparatus made of microtubules form
- nuclear envelope breaks down, gets absorbed by endoplasmic reticulum during spindle formation
- aster - radial arrangement of microtubules on centrioles towards membrane; braces centrioles against membrane; no asters in plant cells
- microtubules must link sister chromatids to opposite sides or they won’t separate later
- metaphase - centromere alignment
- chromosomes align in center of the cell in circular array
- metaphase plate - imaginary plane perpendicular to axis of chromosome circle
- anaphase - shortest phase
- centromeres split in 2, freeing sister chromatids
- separase - enzyme that cleaves the cohesin protein holding the chromatids together
- anaphase-promoting complex (APC) - makes centromeres divide at the same time
- poles move apart, centromeres move towards poles
- microtubules shortens as tubulin subunits are removed (microtubules don’t contract)
- telophase - nuclei reforms
- spindle disassembles
- microtubules broken down into tubulin that can be used for cytoskeleton of daughter cells
- nucleus forms around sister chromatids
cytokinesis - phase where cell actually divides
- relocation of organelles takes place in S/G2 phase
- cleaves cell into equal halves
- animal cytokinesis - uses constricting actin filament belt
- actin filaments slide by each other, forms cleavage furrow, eventually slices into cell’s center
- plant cytokinesis - creates cell plates between daughter cells
- middle lamella - space between daughter cells, filled w/ pectins
- fungi/protist cytokinesis - nucleus doesn’t dissolve during mitosis
- nucleus divides after mitosis completes
- nothing determines how organelles get distributed
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