Cell Growth and Division

Cell division is needed to…

• Grow – most organisms grow by producing more cells

• Repair wounds and damaged cells

• Develop and change

Why do cells divide?(aka, why are cells small?)

• Two main reasons why cells can’t grow indefinitely:– A larger cell places more demands on its DNA (more

things to control and not enough DNA)– Surface-to-volume ratio decreases as size increases

• Thus, before the cell becomes too large, it divides

When do cells divide (or stop)?• When cells crowd each other, they stop dividing• When internal factors signal the cell to start or stop

dividing – e.g., p53 gene makes sure the cell doesn’t divide until

chromosomes have doubled; cyclin enzymes start/stop the cell cycle

• When external chemical or physical signals (growth regulators) stimulate or inhibit growth and division

(Cancer is an example of uncontrolled cell growth….we will discuss cancer later in the unit)

Cells divide at different rates

• The rate of cell division varies with the need for the type of cell…

What is the cell cycle?• The series of phases that cells go through as they

grow and divide.• A continuous process, but we divide it into 3 stages:

1. Interphase2. Mitosis3. Cytokinesis

http://images1.clinicaltools.com/images/gene/celldivision/cellcycle.jpg

Stage 1: Interphase• Cell life between divisions• The longest stage!• Nucleus is still present• Phases:

– Gap 0 (G0) – cells like nerves leave the cell cycle

– Gap 1 (G1) – growth– Synthesis (S) – duplicates

DNA (replication)– Gap 2 (G2 ) – doubles

organelles, prepares to divide

Stage 2: Mitosis• Division of the cell nucleus and DNA

• Phases:1. Prophase2. Metaphase3. Anaphase4. Telophase

Stage 3: Cytokinesis

• Division and splitting of the cytoplasm and cells

Mitosis + cytokinesis result in two identical daughter cells!

YouTube:Mitosis

(Review…) Interphase• Nucleus is still intact and the nucleolus is visible.• DNA is in chromatin form• Includes G1, S, and G2

Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down

prophase → pro (#1!)

Chromatin = DNA unwound (looks like spaghetti)

Chromosome = DNA condensed and organized; looks like this: 2 chromosomes

Sister chromatids = a duplicated chromosome

Centromere = the place where chromotids are connected

Some useful DNA vocabulary!

Chromatin? Chromosomes?

To recap…During prophase, chromatin condense into chromosomes… which have duplicated into sister chromatids, attached to each other at their centromeres.

Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down

prophase → pro (#1!)

Mitosis step 2: Metaphase• Chromosomes line up at the middle of the cell• A spindle fiber attaches to each sister chromatid at

the centromere

metaphase → middle

Mitosis step 3: Anaphase• Spindle fibers contract and pull apart the

chromosomes to opposite ends of the cell

anaphase → apart

Mitosis step 4: Telophase• Chromosomes return to chromatin form• Nuclear membranes and nucleolus reform• Spindle fibers disappear• Nuclear division is complete

telophase → telephone

After mitosis…Stage 3: Cytokinesis• Pinching of the cytoplasm, resulting in two

identical daughter cells.

YouTube:Mitosis Video

Mitosis Review

The cell cycle:

IPMATCI

Passed

My

Algebra

Test with a

“C”

Mitosis in plants• Plants don’t have centrioles• Cell plate forms during telophase• During cytokinesis, cell plate separates the daughter

cells and becomes the new cell wall

Blood lily mitosis

Mitosis in real cells…PMAT

Where doesn’t mitosis happen?• Nerve and blood cells are different• Sex cells (gametes) undergo a different

division process