The Great DivideThe Great Divide

“MITOSIS IN A NUTSHELL”

To Be To Be Answered…Answered…THINK:

How many cells are you composed of?

When an organism grows bigger do you get more cells or just bigger cells or both?

When do your cells divide the fastest? Slowest?

Do cells ever stop dividing?

Are all cells capable of division and replacement?

WHY DO CELLS DIVIDE?•To keep a workable ratio

of surface area to volumeV = l x w x h SA = l x w x 6

1 cm 2cm 3 cm V = 1 cm3 V = 8 cm3 V = 27 cm3

SA = 6 cm2 SA = 24 cm2 SA = 54 cm2

Ratio = 6/1 Ratio = 24/8 Ratio = 96/64 6:1 3:1 2:1-As cell gets larger, ratio decreases

Why Would a Cell Why Would a Cell Divide?Divide? As cells absorb nutrients and get larger,

the volume of the cell increases faster than the surface area

This means that a cell can no longer absorb nutrients and get rid of wastes fast enough to support its demands (volume) = STARVE TO DEATH

So what’s a cell to do?

Solution: divide in 2!

Surface area for exchange not great enough to support cell’s needs

When Would a Cell When Would a Cell Divide?Divide? Growth

Repair or Replacement

Cancer

Different cells divide at different rates:

Most mammalian cells = 12-24 hours

Some bacterial cells = 20-30 minutes

Getting Older…Getting Older… All cells are only allowed to complete a

certain number of divisions

Then they die (programmed cell death)

How does cell division change over a lifetime?

Childhood = cell division > cell death

Adulthood = cell division = cell death

The Later Years = cell division < cell death

The Cell CycleThe Cell Cycle

INTERPHASE

Stages of the Cell Stages of the Cell CycleCycle 2 stages = interphase (growth &

replication of DNA) & mitotic phase (division of cell into 2 daughter cells)

Cell spends about 90% of the time in interphase

InterphaseInterphase Divided into 3 phases:

G1 = Growth Phase – Cells grow and develop, undergo protein synthesis and rapid growth

S = Synthesis Phase - DNA replication occurs (i.e. chromosomes copied) in preparation for making new cells during mitosis

G2 =Second Growth Phase = centrioles replicate, cell prepares for cell division, makes new cell parts

Becomes too large = divide in 2

The Mitotic PhaseThe Mitotic Phase Equal distribution of 2 sets of chromosomes

(DNA) into 2 identical daughter cells

Divided into 4 stages of Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Cell Cycle TidbitsCell Cycle TidbitsHow long is one cell cycle?

Depends on the cell- skin cells = ~24 hours, nerve cells = never after maturity, cancer cells = very short

Remember: every cell only has a certain # of divisions it can undergo, then it dies = (programmed cell death)

Mitosis: A Closer LookMitosis: A Closer Look

Prior to entering the mitotic phase, the cell has just come out of Interphase

Replicated DNA during S (synthesis)

2 complete set of chromosomes that must be distributed equally between 2 cells = mitosis

Interphase

ProphaseProphase Chromatin

condenses visible chromosomes

Appear as sister chromatids held together by centromere

Nuclear membrane dissolves

The centrioles migrate to opposite poles & spindle fibers form between them

Duplicated Chromosome

Mitotic Appearance (Prophase)

CentrioleCentriole CentromereCentromere SpindleSpindle Chromosome Chromosome

(chromatids) (chromatids) AsterAster

MetaphaseMetaphase

Chromosomes line-up on the metaphase plate

Centromeres are attached to spindle fiber

AnaphaseAnaphaseCentromeres divide

Spindle fibers contract

Result = sister chromatids are pulled away from one another towards the poles

TelophaseTelophase

The chromosomes reach the poles

Nuclear membranes form around the 2 new nuclei

CytokinesisCytokinesis The cytoplam distrubted equally between the 2 new cells

In animals, the cell membrane pinches together

In plants, a cell plate forms from inside out

PlantAnimal

Cytokinesis in plants

A cell plate made up of cell-wall components gradually forms in the middle of the cell.

Cytokinesis in animals

A cleavage (or division) of the animal cell. The membrane pinches together forming 2 cells

Cleavage Furrow

ANIMAL VS. PLANT MITOSIS

ANIMAL CELLANIMAL CELLCentriole and Centriole and

aster presentaster presentDaughter cells Daughter cells

separated by separated by cleavage cleavage furrowfurrow

PLANT CELLPLANT CELLNo visible No visible

centriole or centriole or asteraster

Daughter cells Daughter cells separated by separated by cell platecell plate

What Mitosis Actually Looks What Mitosis Actually Looks LikeLike

InterphaseProphase Metaphase

Anaphase Telophase

Figure 9.8 – Part 2

Figure 9.8 – Part 2

Mitosis consists of several different phases.

Summary of MitosisSummary of Mitosis

What Happens After What Happens After Mitosis?Mitosis?

The cell returns to interphase

Chromosomes uncoil back into chromatin

The cycle repeats itself over & over…

At What Stage Are Our Cells At In The Cell At What Stage Are Our Cells At In The Cell Cycle?Cycle?

Different cells can be in different stages

Interphase

Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Can You Identify the Stages of Can You Identify the Stages of Mitosis?Mitosis?

Put the following mitosis stages in the correct sequence

MitosisCan you name the stages?

A

B

C

D

E

The GuaranteeThe Guarantee

The product of mitosis is 2 cells

The daughter cells are identical to each other & to the mother cell

Mother cell

Identical daughter

cellsWhy is this so important?

The Daughter CellsThe Daughter Cells

In humans, the 2 daughter cells will have 46 chromosomes (23 pairs)

1 chromosome originally from mom & 1 from dad

Each chromosome is said to have the same gene sequence

Mother cell

Identical daughter

cells