The Great DivideThe Great Divide

05/04

DO NOW…DO NOW…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?

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

the volume of the cell increases, and a cell can no longer absorb nutrients and get rid of wastes fast enough.

So what’s a cell to do?

Solution: divide in 2!

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

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 (1st gap) = small cell is absorbing nutrients, growing & making proteins

S (synthesis) = cell is continuing

to grow & duplicates its DNA (i.e. chromosomes) in preparation for making duplicate cells during mitosis

G2 (2nd gap) = cell keeps grow- ing & making proteins; it

grows too big…solution = divide in 2

The Mitotic PhaseThe Mitotic Phase Equal distribution of chromosomes (DNA)

into 2 identical daughter cells

Divided into 4 stages of Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

ProphaseProphase

DNA condenses (gets shorter & thicker) so they are now visible

Appear as sister chromatids

Nuclear membrane dissolves

The centrioles move to opposite poles & spindle fibers form between them http://www.biostudio.com/demo_freeman_dna_coiling.htm

MetaphaseMetaphase

Chromosomes line-up along the center and attach to the spindle fibers

AnaphaseAnaphase

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 cytoplasm

distributed equally between the 2 new cells

In animals, a cleavage furrow forms from outside in

In plants, a cell plate forms from inside out

PlantAnimal

What Mitosis Actually Looks What Mitosis Actually Looks LikeLike

InterphaseProphase Metaphase

Anaphase Telophasehttp://www.sci.sdsu.edu/multimedia/mitosis/mitosis_gif2.html

http://science.nhmccd.edu/biol/bio1int.htm

What Happens After What Happens After Mitosis?Mitosis? The cell

returns to interphase

Chromosomes uncoil back into thin strands of DNA

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

Identify the phase in the following 18 cells: