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Somatic Cell Cycle

-3 periods: interphase, mitosis, and cytokinesis



-G1 phase and G2 phase characterized by cell growth, nutrient accumulation, and organelle duplication.
-S phase is when DNA replication occurs
-the cell is preoccupied with nutrient accumulation, protein and membrane synthesis, organelle duplication as well as a physical increase in size.



-M phase: prophase, anaphase, metaphase, telophase
-2 daughter cells identical to parent cell
-cellular goal is to produce two daughter cells that are not only identical to each other but also identical to mother cell in terms of DNA and cytoplasmic contents


Cytokinesis (What happens after)

-final period when daughter cells separate completely
-after each daughter cell faced with decision to re-enter the cell cycle or exit
-if it exits (G0) phase, will go into a quiescent state or will terminally differentiate.
-if it goes into quiescent state it can re-enter cell cycle, cannot if differentiates


S phase

-each chromosome is decondensed thereby allowing replication enzymes access to genomic DNA
-replication will initiate at hundreds or thousands of discrete locations called “replication origins”
-a single origin is used in bacteria, three are used in archaea while hundreds or thousands are used in higher eukaryotes



-replicated DNA will begin to re-condense into more discrete structures
-involves complex interactions with protein complexes called histones
-each duplicated chromosome will contain two identical sister chromatids that are joined together at the centromere
-The nuclear envelope will break down as the cell prepares to segregate chromosomes
-in the cytoplasm the mitotic spindle (centrosome and microtubule networks) will begin to
-two centrosomes will migrate to opposite poles of the cell and will be linked to the plasma membrane by astral microtubules
-spindle microtubules will link the centrosomes to the kinetochores of each chromosome



-the condensed chromosomes will line up along one axis of the cell halfway between the two spindle poles
-a cell will not continue on to anaphase until all cells are attached properly to spindle microtubules and lined up at the metaphase plate
-mechanisms that monitor this event are collectively called the “spindle assembly checkpoint” or “mitotic checkpoint"
-Failure of the spindle assembly checkpoint to be engaged by the cell can result in aneuploidy and has been implicated in tumor formation and the onset of cancer



-After successful engagement of the spindle assembly checkpoint the sister chromatids from each chromosome are pulled apart by the mitotic spindle apparatus
-chromatids move to opposite ends of cell
-spindle pole with have acquired a set of chromosomes that is eqivalent to what parental cell had prior to S phase
-cells transition into anaphase a relatively large set of mitotic proteins that participated in spindle assembly, sister chromatid attachment and chromosome movement, must be degraded
-Anaphase Promoting Complex (APC) is a multi-protein molecular machine that targets these factors for degradation. Similar types of machinery degrade suites of proteins at the M/G1 transition as well
-must get rid of all dividing proteins so cell doesn't try dividing again



-each set of chromosomes has completed its migration to the spindle pole
-nuclear envelope begins to re-form around each set of chromosomes and the nucleolus reappears
-cytoplasm and membrane of daughter cells remain attached by midbody
-centrioles/centrosomes duplicate so each daughter cell has a pair


Cytokinesis (Process)

-characterized by the final separation of the two daughter cell membranes and cytoplasm
-accomplished by contraction of a bundle of aligned actin filaments and myosins
-contractile ring forms in a plane that is perpendicular o that of spindle assembly


Embryonic Oscillator

-cell oscillates between S and M phases without intervening growth phases
-allows for rapid cell division without overall increase in the organisms size



-cells oscillate beteen the S and G1 phases
-results in polyploid cell
-i.e. leaves of most plants as are cells that line the gut of most organisms
-cells that must secrete large volumes of enzymes such as the salivary glands and liver cells
-do this to have many more copies of certain genes
--cell constantly duplicating DNA then growing a bit



-diploid stem cells in the gonad undergo two meiotic divisions to yield 4 haploid gametes
-During the earliest stages of the first meiotic division all chromosomes are duplicated, so for a brief period of time a human cell in meiosis I will have 46 duplicated chromosomes and 92 chromatids (homologues separated)
-During the second meiotic division DNA is not replicated (sister chromatids separated)


Meiosis vs. Mitosis

-meiosis produces 1N
-mitosis produces 2N
-recombination only happens in meiosis


Errors in Meiosis and Mitosis

-can occur during spermatogenisis and oogenesis more common in oogenesis because they're halted in meiosis I for several decades


Turner Syndrome

-affects females
-X and no other sex chromosome


Klinefelter Syndrom

-affects males


Down Syndrome

-affects males and females
-3 copies of chromosomes 21