Why do Cells Why do Cells Divide? Divide?

Cell Size LimitsCell Size Limits

1. Replacement of Cells1. Replacement of Cells

Humans need to replace old, worn Humans need to replace old, worn out cells from normal wear and tearout cells from normal wear and tear

2. Rate of Diffusion 2. Rate of Diffusion

Remember, the cell membrane allows Remember, the cell membrane allows nutrients (ex. Glucose, oxygen, etc.) in nutrients (ex. Glucose, oxygen, etc.) in and wastes out (diffusion)and wastes out (diffusion)

Diffusion is Diffusion is fastfast over over shortshort distances but distances but slowslow over over longlong distances distances

So, b/c of slow rate of diffusion, cells can’t So, b/c of slow rate of diffusion, cells can’t be giants b/c they would starve to death be giants b/c they would starve to death or be poisoned from the build up of or be poisoned from the build up of wastes!!wastes!!

Diffusion AnimationDiffusion Animation Remember diffusion works best over Remember diffusion works best over

short distances!!short distances!! diffusion animationdiffusion animation

3. Need enough DNA to 3. Need enough DNA to support protein needs of cellsupport protein needs of cell Since DNA codes for making protein Since DNA codes for making protein

there needs to be enough to support the there needs to be enough to support the protein needs of the cellprotein needs of the cell

ex. Think how much protein is in the ex. Think how much protein is in the cell membrane and if the cell cell membrane and if the cell size increases than you would size increases than you would need more protein to build itneed more protein to build itYOU ONLY HAVE SO MUCH DNA YOU ONLY HAVE SO MUCH DNA

AVAILABLE!!AVAILABLE!!

4. Surface Area to Volume 4. Surface Area to Volume RatioRatio

Need a proper SA:Vol ratio to Need a proper SA:Vol ratio to support the needs of the cellsupport the needs of the cell

You need the cell to remain small!!You need the cell to remain small!! For example, if the cell size doubles For example, if the cell size doubles

it would need 8x more nutrients to it would need 8x more nutrients to survive and it would create 8x the survive and it would create 8x the waste to excrete!!waste to excrete!!

The vol. increases faster than the The vol. increases faster than the surface area of the cell membrane!!surface area of the cell membrane!!

Surface area to volumeSurface area to volume

surface area to volumesurface area to volume

Cell Division Cell Division RequirementsRequirements

I) I) Cell DivisionCell Division A. Mitosis- nuclear division *A. Mitosis- nuclear division * B. Cytokinesis- Division of the B. Cytokinesis- Division of the

cytoplasmcytoplasm

Reminder from cell theoryReminder from cell theory All cells come from pre-existing All cells come from pre-existing

cells!!cells!! -Cell Division follows this b/c it’s the -Cell Division follows this b/c it’s the

process where new cells are made process where new cells are made from 1 cellfrom 1 cell

Cell Division makes 2 new daughter Cell Division makes 2 new daughter cells that are identical to the parent cells that are identical to the parent cellcell

Cell Cycle- The life of a cellCell Cycle- The life of a cell

Cell Cycle- InterphaseCell Cycle- Interphase InterphaseInterphase (Nucleus enclosed in (Nucleus enclosed in

nuclear membrane, nucleolus present, nuclear membrane, nucleolus present, chromosomes appear as chromatin (in chromosomes appear as chromatin (in other words, chromosomes are not yet other words, chromosomes are not yet visible))visible))

a) G1 phase- intense cellular a) G1 phase- intense cellular activity and growthactivity and growth b) S phase- DNA synthesis- b) S phase- DNA synthesis- chromosomes replicate chromosomes replicate (copy)(copy)

Interphase continuedInterphase continued

c) G2 phase- spindle fibers are c) G2 phase- spindle fibers are

assembled/ centrioles assembled/ centrioles

replicatereplicate

- cell prepares for cell division - cell prepares for cell division

Interphase DiagramInterphase Diagram

MITOSIS PHASESMITOSIS PHASES

Prophase (1Prophase (1stst stage) stage) ProphaseProphase-- 1st stage of actual cell division 1st stage of actual cell division a.) Chromosomes condense and shorten, a.) Chromosomes condense and shorten, become distinguishablebecome distinguishable

b.) Chromatids attached near center by b.) Chromatids attached near center by centromerecentromere c.) Centrioles separate, migrate to the polesc.) Centrioles separate, migrate to the poles d.) Spindle fibers form between centrioles, d.) Spindle fibers form between centrioles, asters formasters form e.) Nuclear membrane disintegrates, e.) Nuclear membrane disintegrates, nucleolus disappears (in order to nucleolus disappears (in order to separate contents of nucleus, it must separate contents of nucleus, it must disappeardisappear))

Prophase Prophase

Early prophase Late prophase

Metaphase- 2Metaphase- 2ndnd stage stage (shortest)(shortest)

a.) chromosomes line up in pairs a.) chromosomes line up in pairs

at equator (center of cell)at equator (center of cell) b.) centromere of eachb.) centromere of each

chromosome is attached to a chromosome is attached to a

separate spindle fiberseparate spindle fiber

Metaphase diagramMetaphase diagram

Metaphase

Anaphase- 3Anaphase- 3rdrd stage stage a.) Centromere of each chromosome a.) Centromere of each chromosome separatesseparates b.) Separation of the chromatids in each b.) Separation of the chromatids in each pairpair c.) Spindle fibers appear to shorten, c.) Spindle fibers appear to shorten, pulling the chromatids apart at the pulling the chromatids apart at the centromere (now called centromere (now called chromosomes)chromosomes) d.) migration of the chromosomes ends d.) migration of the chromosomes ends with the arrival at the poles and the with the arrival at the poles and the formation of clustersformation of clusters

Anaphase diagramAnaphase diagram

Anaphase

Telophase – 4Telophase – 4thth stage stage a.) Cleavage furrow forms b/w 2 cells a.) Cleavage furrow forms b/w 2 cells

and spindle fibers disappearand spindle fibers disappear b.) Nuclear membrane forms around b.) Nuclear membrane forms around

each set of chromosomes (forms 2 each set of chromosomes (forms 2

new nuclei)new nuclei) c.) Chromosomes uncoil to form c.) Chromosomes uncoil to form

chromatin net (getting ready for chromatin net (getting ready for

interphase)interphase) d.) Nucleoli reorganizedd.) Nucleoli reorganized

Telophase DiagramTelophase Diagram

Telophase

CytokinesisCytokinesis

Cytokinesis- division of the Cytokinesis- division of the cytoplasmcytoplasm

In animal cells- cell membrane In animal cells- cell membrane pinches together, furrow forms along pinches together, furrow forms along the equator.the equator.

-In plant cells- cell plate forms in the -In plant cells- cell plate forms in the

middle of the dividing cellmiddle of the dividing cell