when would a cell divide? growth repair or replacement cancer different cells divide at different...

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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