When Would a Cell Divide?  Growth  Repair or Replacement  Cancer Different cells divide at different rates:  Most mammalian cells = hours  Some bacterial cells = minutes

Prophase Metaphase Anaphase

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 Interphase Cell 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 double helix DNA and histones Chromatin Supercoiled DNA

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. Copyright © McGraw-Hill Companies Permission required for reproduction or display

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 G 2 Phase Figure 8.5

INTERPHASEPROPHASE Centrosomes (with centriole pairs) Chromatin NucleolusNuclear envelope Plasma membrane Early mitotic spindle Centrosome Chromosome, consisting of two sister chromatids Fragments of nuclear envelope Kinetochore Spindle microtubules Figure 8.6

METAPHASETELOPHASE AND CYTOKINESIS Metaphase plate SpindleDaughter chromosomes Cleavage furrow Nucleolus forming Nuclear envelope forming 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 Cleavage furrow Contracting ring of microfilaments 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 Cleavage furrow Contracting ring of microfilaments 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 Chromosome duplication Chromosome distribution to daughter cells

When Would a Cell Divide?  Growth  Repair or Replacement  Cancer Different cells divide at different rates:  Most mammalian cells = hours  Some bacterial cells = 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 (G 0 ).

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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