in eukaryotes, heritable information is passed to the next generation via processes that include the...
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In eukaryotes, heritable information is passed to the
next generation via processes that include the cell cycle and
mitosis or meiosis plus fertilization
Cell Cycle and Mitosis 8.1 to 8.11
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Genetic information is stored and transmitted through DNA
All the DNA in a cell constitutes the cell’s genome
A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)
DNA molecules in a cell are packaged into chromosomes
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Fig. 12-3
20 µm
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Genetic Information is stored and transmitted through DNA
Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
Somatic cells (nonreproductive cells) have two sets of chromosomes - DIPLOID
Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells - HAPLOID
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division
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Fig. 12-40.5 µm Chromosomes
Chromosomeduplication(including DNAsynthesis)
Chromo-some arm
Centromere
Sisterchromatids
DNA molecules
Separation ofsister chromatids
Centromere
Sister chromatids
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The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell
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Interphase
Majority of the cell cycle
Time when a cell’s metabolic activity is very high and the cell performs various functions
3 stages◦G1 – cell growth◦S – DNA replication (Synthesis of DNA)◦G2 – prepare for mitosis
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M Phase (mitotic phase)
About 10% of the cell cycle
2 stages◦Mitosis – nuclear division◦Cytokinesis – cytoplasm division
Results in two genetically identical cells
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The cell cycle is directed by internal controls or checkpoints.
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Cell Cycle Checkpoints
• For many cells, the G1 checkpoint seems to be the most important one
• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase
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Cell Cycle Checkpoints
Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)
The activity of cyclins and Cdks fluctuates during the cell cycle
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
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Fig. 12-17
M G1S G2
M G1S G2
M G1
MPF activity
Cyclinconcentration
Time(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle
Degradedcyclin
Cdk
G 1S
G 2
M
CdkG2
checkpointCyclin isdegraded
CyclinMPF
(b) Molecular mechanisms that help regulate the cell cycle
Cyclin
accu
mula
tion
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Internal and External signals provide stop-and-go signs at the checkpoints
An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide
For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture
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Fig. 12-18
Petriplate
Scalpels
Cultured fibroblasts
Without PDGFcells fail to divide
With PDGFcells prolifer-ate
10 µm
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Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
(a) Normal mammalian cells (b) Cancer cells25 µm25 µm
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Mitosis passes a complete genome from the parent cell to the daughter cell
Mitosis is conventionally divided into five phases:◦Prophase◦Prometaphase◦Metaphase◦Anaphase◦Telophase
Cytokinesis is well underway by late telophase
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Fig. 12-6b
PrometaphaseProphaseG2 of Interphase
Nonkinetochoremicrotubules
Fragmentsof nuclearenvelope
Aster CentromereEarly mitoticspindle
Chromatin(duplicated)
Centrosomes(with centriolepairs)
Nucleolus Nuclearenvelope
Plasmamembrane
Chromosome, consistingof two sister chromatids
Kinetochore Kinetochoremicrotubule
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Fig. 12-7
Microtubules Chromosomes
Sisterchromatids
Aster
Metaphaseplate
Centrosome
Kineto-chores
Kinetochoremicrotubules
Overlappingnonkinetochoremicrotubules
Centrosome 1 µm
0.5 µm
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Fig. 12-6d
Metaphase Anaphase Telophase and Cytokinesis
Cleavagefurrow
Nucleolusforming
Metaphaseplate
Centrosome atone spindle pole
SpindleDaughterchromosomes
Nuclearenvelopeforming
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Fig. 12-8b
Kinetochore
MicrotubuleTubulinSubunits
Chromosome
Chromosomemovement
Motorprotein
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Fig. 12-9
Cleavage furrow100 µm
Contractile ring ofmicrofilaments
Daughter cells
(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM)
Vesiclesformingcell plate
Wall ofparent cell
Cell plate
Daughter cells
New cell wall
1 µm
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Summary of Mitosis
What must occur before mitosis?◦DNA replication◦Pass G2 checkpoint
What is “checked” at the ‘M’ checkpoint?◦Chromosome attachment to microtubule
What follows mitosis?◦Cytokinesis
What are the products of the mitosis?◦2 genetically identical cells
Why do we need mitosis?◦Growth, repair, asexual reproduction