when would a cell divide? growth repair or replacement cancer different cells divide at different...
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When Would a Cell Divide? Growth
Repair or Replacement
Cancer
Different cells divide at different rates:
Most mammalian cells = 12-24 hours
Some bacterial cells = 20-30 minutes
Prophas
eM
etap
hase
Anaphas
e
• During interphase a cell performs all of its regular functions and gets ready to divide
• Metabolic activity is very high
Most of the life of a cell is spent in InterphaseCell does most of its’ growth during interphase
Figure 8.5
• Untwisting and replication of DNA: S phase
Figure 10.4B
Chromosomes condense at the start of mitosis.
• DNA wraps around proteins (histones) that condense it.
DNA doublehelix
DNA andhistones
Chromatin SupercoiledDNA
Structure of Chromosomes
– Homologous chromosomes are identical pairs of chromosomes.
– One inherited from mother and one from father– made up of sister chromatids joined at the
centromere.
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• This phase spans the time from the completion of DNA synthesis to the onset of cell division
• Following DNA replication, the cell spends about 2-5 hours making proteins prior to entering the M phase
G2 Phase
Figure 8.5
INTERPHASE PROPHASE
Centrosomes(with centriole pairs)
Chromatin
Nucleolus Nuclearenvelope
Plasmamembrane
Early mitoticspindle
Centrosome
CentrosomeChromosome,consisting of twosister chromatids
Fragmentsof nuclearenvelope
Kinetochore
Spindlemicrotubules
Figure 8.6
METAPHASE TELOPHASE AND CYTOKINESIS
Metaphaseplate
Spindle Daughterchromosomes
Cleavagefurrow
Nucleolusforming
Nuclearenvelopeforming
ANAPHASE
Figure 8.6 (continued)
• In animals, cytokinesis occurs by cleavage– This process pinches the
cell apart– The first sign of cleavage
is the appearance of a cleavage furrow
Cytokinesis differs for plant and animal cells
Figure 8.7A
Cleavagefurrow
Cleavagefurrow
Contracting ring ofmicrofilaments
Daughter cells
– As the daughter chormosomes move to opposite poles
– The cytoplasm constricts along the plane of the metaphase plate
The process of cytokinesis divides the cell into two genetically identical cells
Cytokinesis differs for plant and animal cells
Figure 8.7A
Cleavagefurrow
Cleavagefurrow
Contracting ring ofmicrofilaments
Daughter cells
Plant Cell Telophase/Cytokinesis
• When the cell divides, the sister chromatids separate
– Two daughter cells are produced
– Each has a complete and identical set of chromosomes
Centromere Sister chromatids
Figure 8.4C
Chromosomeduplication
Chromosomedistribution
todaughter
cells
When Would a Cell Divide? Growth
Repair or Replacement
Cancer
Different cells divide at different rates:
Most mammalian cells = 12-24 hours
Some bacterial cells = 20-30 minutes
Explain how mitosis ensures that daughter nuclei are genetically identical.
Cells divide at different rates.
• The rate of cell division varies with the need for those types of cells.
• Some cells are unlikely to divide (G0).
Cell size is limited.
• Volume increases faster than surface area.
• Surface area must allow for adequate exchange of materials.
– Cell growth is coordinated with division.
– Cells that must be large have unique shapes.
It’s too late to apoptise!
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