What do all cells require to survive?

• A complete set of genetic instructions

– to produce required molecules

– to direct life processes• Genetic instructions are coded in the DNA of cells

Why do cells divide?

• Growth

• Repair injuries

• Natural replacement

The Cell Cycle

• Defined as the activities of a cell from one cell division to the next, during which…

– the cell grows, adding more cytoplasmic constituents

– the DNA is replicated

– the cell divides into two identical daughter cells

During non-division phase of cell cycle

• DNA molecules in extended, uncondensed form = chromatin

– a cell can only use DNA to produce molecules when in extended state

“chromatin” means colored thread

During division phase of cell cycle

• DNA molecules condense to form chromosomes prior to division

– easier to sort and organize DNA into daughter cells

Structure of a chromosome

• unreplicated chromosome

arm armcentromere

“chromosome” means colored body

Prior to cell division:• chromosomes (DNA) are replicated (duplicated)

duplicatedchromosome

• duplicated chromosome – attached at their centromeres – as long as attached, known as sister chromatids

Interphase

“I am a cell”•The cell spends a majority of it’s time in this phase. During interphase…

• the cell grows in size• it makes copies of its organelles•It makes copies of its DNA

Mitosis

4 phases:

1st – Prophase

2nd – Metaphase

3rd – Anaphase

4th – Telophase

Prophase“P is for pairs”

chromosomes condensed and in pairs: 2 sister chromatids attached at the centromere

spindle fibers form

chromosomes are captured by the spindle fibers

one pair of centrioles migrates to one pole of cell, the other pair migrates to opposite pole of cell

nuclear envelope disintegrates and nucleolus disappears

chromatin

nucleolus

nucleus

centrioles

condensingchromosomes

Metaphase“M is for middle”

• chromosomes align along the equator (middle) of the cell

centrioles

spindle fibers

chromosomes

Anaphase“A is for apart”

• sister chromatids separate• spindle fibers attached at centromeres shorten and pull chromatids poleward

Telophase“T is for Two nuclei”

• spindle fibers disintegrate• nuclear envelopes form around both groups of chromosomes making TWO nuclei

• chromosomes revert to chromatin• nucleoli reappear

chromosomesdecondensing

nuclear envelopereforming

nucleolus reappears

pinching of cellmembrane at equator

cytokinesis

End of cycle – Cytokinesis – membranes finish pinching apart and cells become separate.

• Differences in Plant cells: – stiff cell wall makes pinching impossible – Golgi complex buds off vesicles filled with carbohydrate – vesicles line up at equator and fuse, producing a structure called the cell plate – cell plate becomes new cell wall between the two cells