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