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When do cells divide? Reproduction Replacement of damaged cells Growth of new cells In replacement and growth cell divisions, how
should daughter cells compare to parent cell? The daughter cells should be identical copies
of the parent cell
How can identical daughter cells form? The genome must be copied and then
divided such that each daughter cell gets one of the copies.
Genome = all the genes in an organism
Important terms in eukaryotic cell division Chromosome - threadlike structures that are
composed of DNA and protein Replication - process whereby DNA is
identically copied (before cell division) Mitosis - division of the nucleus Cytokinesis - division of the cytoplasm Chromatin - DNA and protein complex that is
thin and fibrous; it will condense into distinct chromosomes during cell division
Important terms in eukaryotic cell division Chromatid - after replication the chromosome
consists of 2 sister chromatids joined at the centromere
Centromere - specialized region of the chromosome, where chromatids are joined. Each chromosome has one centromere
Interphase 90% of cell cycle is spent in this phase G1 = first growth phase S = synthesis phase, DNA synthesis
(replication) occurs here G2 = second growth phase
G2 phase of Interphase in animal cells Nuclear envelope is visible One or more nucleoli are present Centrioles are replicated and the 2 pairs are
near nucleus Aster forms around each pair of centrioles Chromosomes are loosely packed into
chromatin fiber, not distinguishable
Prophase: In the Nucleus: Nucleoli disappear Chromosome fibers condense into discrete
chromosomes Each chromosome consists of 2 sister
chromatids joined at the centromere
In the Cytoplasm: Mitotic spindle begins to form Spindle consists of microtubules arranged
between the centrosomes Centrosomes move apart due to lengthening
of microtubules
Prometaphase: Nuclear envelope breaks apart Each chromatid has specialized structure
called kinetochore located at the centromere region
Kinetochore microtubules interact with chromosomes at the kinetochore region
The kinetochore microtubles cause the chromosomes to move
Nonkinetochore microtubules radiate from each pole
Metaphase Chromosomes move to the metaphase plate
and line up there The centromeres of the chromosomes are all
aligned on the metaphase plate Each sister chromatid of one chromosome,
has a kinetochore microtubule attached to it from opposite poles
Kinetochore microtubules + nonkinetochore microtubules = spindle fiber
Anaphase Kinetochore microtubules shorten and non-
kinetochore microtubules lengthen Centromeres divide and each chromosome
has no sister chromatid component The shape of the cell elongates into an ellipse Chromosomes are pulled to the opposite poles
Telophase Nonkinetochore microtubules continue to
elongate the cell New daughter nuclei form at the two poles New nuclear envelopes are formed around the
chromosomes Nucleoli reappear Chromosomes uncoil into chromatin fiber Last phase of mitosis
Cytokinesis Begins before telophase has completed Evidenced by cleavage furrow in animal cells
and cell plate in plant cells
Diatoms; nuclear envelope stays, microtubules inside nucleus
Most other eukaryotes; spindle forms outside of nucleus, and nuclear envelope breaks apart
Checkpoints in the cell cycle: If it passes the G1 checkpoint cell divides -if not, enters G0 phase and does not divide
Cyclin protein levels fluctuate according to cell cycle stage. When cyclin is high the Cdk attaches and phosphorylation leads to breakdown of nuclear envelope. Later MPF initiates cyclin breakdown