Binary Fission
Asexual cell division for unicellular organisms
Purpose: reproduction; division of one cell reproduces the entire organism
Occurs in bacteria, archaea, and protists
bacterial chromosome replication —> segregation (proteins bind to chromosomes and separate them) —> other proteins (tubulin homologs) divide the cytoplasm —> PG (peptidoglycan?) is synthesized
Mitosis
a type of cell division where the daughter cells are identical to the parent cell (clones—same genetic content)
purpose: reproduction, growth and development, and tissue renewal within multicellular organisms; essential for the development of the zygote into the adult organism
number of chromosomes is conserved in division
Haploid cells
contain one copy of each chromosome
individuals have just one copy of each allele
Diploid cells
contain two copies of each chromosome
individuals have two copies of each allele
Meiosis
a type of cell division where the daughter cells have half as much genetic material as a parent cell
not genetically identical to parents—recombinated DNA; used in gametes of diploid organisms
2n —> n (chromosome number is halved)
Cellular replication (basic)
copy the DNA
separate the copies
divide the cytoplasm to create two complete cells
main purpose is to transmit the mother cell's genetic information (usually DNA) to the daughter cells
Chromosome
carrier of genes
a single, long double-helix of DNA wrapped around proteins called histones
two attached sister chromatids are still considered a single chromosome
unreplicated: consists of a single, long DNA double helix wrapped around proteins
replicated: consists of two copies of the same DNA double helix
condensed replicated: consists of DNA condensed around its associated proteins, resulting in a compact chromosome
Gene
a section of a chromosome that codes for a particular protein or nucleic acid, which affect traits
Chromatid
one double-stranded DNA copy of a replicated chromosome (+ its associated proteins)
Sister chromatids
chromatids attached at the centromere
Cohesins
proteins that attach sister chromatids along their entire lengths
once mitosis begins, these connections are removed except for at the centromere
Interphase
nondividing phase of the cell cycle
phase in which cells spend most of their time
chromosomes are uncoiled or loosely coiled (chromatin)
cells are growing and preparing for division or are fulfilling their specialized functions
S (synthesis) phase
stage within interphase in which DNA replication occurs
chromosome replication occurs ONLY during interphase
Gap/Growth phases (G1 and G2)
G1 comes before S phase and G2 comes after
responsible for protein synthesis and organelle duplication
existence confirmed by pulse-chase experiments
M (mitotic) phase
chromosomes are condensed into compact structures
division of replicated chromosomes to daughter cells; one copy of each chromatid goes to each daughter cell
Cytokinesis
division of the cytoplasm
separates the mother cell into two daughter cells
Cell Cycle (IPPMAT)
Interphase
mitosis: Prophase, Prometaphase, Metaphase, Anaphase, Telophase (+ cytokinesis)
Prophase
sister chromatids condense, and the mitotic spindle begins to form
the nuclear envelope begins to dissociate into vesicles
nucleolus is no longer visible
Prometaphase
the nuclear envelope has completely dissociated into vesicles and the mitotic spindle is completely formed
early in prometaphase, kinesin and dynein motors attached to the kinetochores "walk" the chromosomes up/down the microtubules until the chromosomes reach the plus ends, at which point the kinetochore proteins secure their attachment to the spindle
Mitotic spindle (mitotic spindle apparatus)
ensures that each daughter cell will obtain the correct number and types of chromosomes
responsible for organizing and sorting the chromosomes during mitosis
composed of microtubules
Microtubule organizing centers
centrosomes (in animals and certain plants and fungi) that duplicate at the beginning of the M phase
each defines a pole
animal cells have centrioles, while other eukaryotes do not
Centrioles
cylindrical structures consisting of microtubule triplets
located inside animal centrosomes
Astral microtubules
microtubules that position the spindle in the cell
extend from the MTOCs
interact with proteins on the plasma membrane
Polar microtubules
microtubules that separate the two poles and push away during anaphase
extend from each spindle pole and overlap with one another
Kinetochore microtubules
microtubules attached to the kinetochore bound to centromeres
play a central role in anaphase; remain stationary and shorten as subunits are lost from the + ends
proteins from the kinetochore attach to a ring that surrounds the kinetochore microtubule; as the + end disassembles, the ring moves along the microtubule
Metaphase
sister chromatids align along the metaphase plate
polar microtubules overlap in the middle of the cell, forming a pole-to-pole connection
Anaphase
cohesins that hold together sister chromotids at the centromeres split
individual chromosomes move toward the poles as kinetochore microtubules shorten
creates two identical sets of daughter chromosomes at each pole
both the shrinking of kinetochore microtubules and the movement of the poles away from each other due to the pushing of the polar microtubules' motor proteins pull the chromosomes apart
Telophase
chromosomes decondense and the nuclear envelope reforms
Cytokinesis in plants
vesicles containing cellulose from the Golgi apparatus bring membrane and cell wall components to the middle of the cell, which fuse to form a cell plate
Cytokinesis in animals (and other eukaryotes)
a ring of actin and myosin filaments contracts inside the cell membrane, pinching inward to form a cleavage furrow
the ring shrinks and tightens until division is complete
Cell cycle length variation
variation most commonly occurs in the G1 phase
rapidly dividing cells, such as epithelial skin cells, essentially eliminate the G1 phase while non-dividing cells get permanently stuck in the G1 phase
variation may also vary in response to different conditions, indicating that the cell cycle is regulated
G0 Phase
resting phase of the cell cycle in which the cell continues to function but does not divide
Mitosis promoting factor (MPF)
is present in the cytoplasm of M-phase cells and induces mitosis in all eukaryotes
consists of two units: cyclin and cyclin-dependent kinase (Cdk)
concentration increases during interphase and peaks in M phase before decreasing again
active when cyclin concentrations are high
Cyclin
regulatory protein
component of MPF
CYCLES during the cell cycle
high concentration before/during the M phase; during anaphase, degradation proteins activate and decrease concentrations of cyclin
Cyclin-dependent kinase
catalyzes the phosphorylation of other proteins to start the M phase
regulated by cyclin; active only when bound to the cyclin subunit
2 phosphorylation sites (1 activation site, 1 inhibition site); activation site but not inhibition site must be phosphorylated for Cdk to be active
MPF regulation
the enzyme complex that is activated during anaphase attaches proteins to the cyclin subunit, marking it for destruction and leading to the deactivation of MPF
Cell cycle checkpoints
regulatory molecules at each checkpoint allow a cell to "decide" whether to proceed with division
if these regulatory molecules are defective, the checkpoint may fail
G1 checkpoint
occurs late in the G1 phase
pass if:
cell size is adequate
nutrients are sufficient
social signals are present
DNA is undamaged (if damaged, p53 activates and either pauses the cell cycle so damage can be repaired, or it triggers apoptosis)
G2 checkpoint
occurs between G2 and M
pass if:
chromosomes have replicated successfully
DNA is undamaged
activated MPF is present (only possible if first two criteria are met)
M-phase checkpoint 1
regulates transition from metaphase to anaphase
pass if chromosomes have attached properly to the spindle apparatus (occurs during metaphase)
M-phase checkpoint 2
regulates transition from anaphase to telophase
pass if chromosomes have properly segregated and MPF is COMPLETELY absent
if chromosomes do not fully separate during anaphase, remaining MPF activity will prevent the cell from entering telophase and undergoing cytokinesis
Cancer
a complex family of diseases caused by cells that grow in an uncontrolled fashion, invade nearby tissues, and spread to other sites in the body
200 types of cancers, all arising from cells in which cell-cycle checkpoints have failed (many are thought to arise from cells with defects in the G1 checkpoint)
arise from 2 types of defects: defects that activate the proteins required for cell growth when they should not be active, and defects that prevent tumor suppressor genes from shutting down the cell cycle
Malignant tumors
cancerous and invasive tumors
metastasize
can spread throughout the body via the blood or lymph and initiate secondary tumors
Benign tumors
noncancerous, noninvasive tumors
Social signals
cells respond to signals from other cells
social control is based on growth factors, which allow cells to pass the G1 checkpoint
Growth factors
small proteins that stimulate division
found in serum (the liquid portion of blood)
cancer cells divide without growth factors