The Cell Cycle:

Why do cells divide?

Reasons for Cell Division

1. Growth and Development (why you aren’t the same height as you were 10 years ago)

2. Repair/Replace damaged cell or cell parts (think of a nasty scrape and how it heals)

3. Limiting Cell Size (cell limited by its protein needs which is regulated by DNA)

4. Reproduction (related to meiosis…more on this next week)

BASIC GENETICSBASIC GENETICS

• Each cell in the human body contains two sets of 23 chromosomes

• Mitosis identically replicates this information

• Each cell therefore has the same genetic material

When cell is not dividing… ● DNA molecules in extended, uncondensed form = chromatin

● Cell can only use DNA to produce molecules when in extended state.

When cell is preparing for division…

● DNA molecules condense to form chromosomes prior to division.

– each chromosome is a single molecule of DNA

– easier to sort and organize DNA into daughter cells

• Unreplicated eukaryotic chromosome

arm armcentromere

Prior to cell division:• chromosomes (DNA) are replicated (duplicated)

duplicatedchromosome

• duplicated chromosome – attached at their centromeres – as long as attached, known as Sister Chromatids

daughterchromosomes

sisterchromatids

The Cell Cycle:

Interphase(comprises 95 % of the cell cycle): G1, S, G2

(M) Phase: Mitosis (prophase, metaphase, anaphase, telophase)

So if the whole cell cycle takes 8 hours for this particular cell…then Mitosis would only take 24 minutes.

G1:This is where organelle duplication occurs. It also is the checkpoint for the following: CELL SIZE, NUTRIENTS, GROWTH FACTORS and DNA DAMAGE

• Is called the “restriction point” in animal cells (why do you think this is?)

• Is much shorter in embryonic and cancer cells

• In cancer cells can even be nonexistent. (Why do you think this is?)

G0: Cells that do not require a lot of replication such as neural tissue remain in this stage for a long time.

(Any other types of tissues that you think would remain in this stage? How about types of tissues that replicate fast?)

S Phase:• The cell is committed to cell division once this process starts

• DNA and centrosome (remember this structure?) replication occur during this phase

• Replication of DNA leads to 2 identical sets of daughter chromosomes

G2:• Cell growth continues

• Enzymes and proteins are synthesized for cell division

• DNA damage is checked for. Proper completion of DNA synthesis is required before cell can initiate mitosis (so what happens if DNA is damaged?)

Embryonic Cells:• Cell growth is not part of the cell cycle

• All energy is put into DNA synthesis (requires ATP!)

G1 does not occur and G2 is very short (If interphase is 95% of the cell cycle then what does this mean about how long it takes for the cell division to occur?)

• Each round of division subdivides the original cytoplasm into smaller and smaller cells until adult cell size is reached (what do I mean by “adult” cell size?)

The Division of Somatic cells (non reproductive cells) in eukaryotic organisms.

- A single cell divides into two identical daughter cells

- Daughter cells have same # of chromosomes as does parent cell.

(M) Phase = Mitosis

Mitotic Cell Division

2 major processes:

• Mitosis – nuclear division => preserves diploid number of chromosomes

• Cytokinesis – cytoplasmic division

=> cell divides into two daughter cells

Mitosis 4 sub-phases:

1st – Prophase

2nd – Metaphase

3rd – Anaphase

4th – Telophase and Cytokinesis

Secret to remembering phases in order…(IPMAT)

DEMOs: http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter11/animations.html#

http://www.cellsalive.com/mitosis.htm

Prophase

i) chromosomes condense

• 3 major events

ii) spindle fibers form

iii) chromosomes are captured by spindle

chromatin

nucleolus

nucleus

centrioles

condensingchromosomes

kinetocore Prophase

Prophase

This is what prophase actually looks like under a microscope

Metaphase

• Sister chromatids align along equator of the cell, with one kinetochore facing each pole

centrioles

spindle fibers

Sister Chromatids

Anaphase

• sister chromatids separate

• spindle fibers attached to kinetochores shorten and pull chromatids towards the poles.

• free spindle fibers lengthen and push poles of cell apart

Telophase

• spindle fibers disintegrate

• nuclear envelopes form around both groups of chromosomes

• chromosomes revert to their extended state

• nucleoli reappear

• cytokinesis occurs, enclosing each daughter nucleus into a separate cell

cytokinesis

Cytokinesis: the process by which the cytoplasm of the cell divides

WHICH PHASE IS WHICH??

How is Cell Division Controlled?

•External regulation:

•Contact inhibition

•Internal regulation:

•Proteins

•Stop and Go signals

EXTERNAL REGULATION Contact Inhibition:

• Normal cells will reproduce until they come into contact with other cells.

• When cells come into contact with other cells, they respond by not growing – the proteins on the cell membrane surface signals each other

INTERNAL REGULATIONProteins:Specific proteins called cyclins have been found to regulate the cell cycle in

eukaryotic cells– The amount of this protein in the cell rises and falls in time with the

cell cycle.

A sample of cytoplasmis removed from a cellin mitosis.

The sample is injectedinto a second cell inG2 of interphase.

As result, the secondcell enters mitosis.

Cyclins were discovered during a similar experiment to this one

INTERNAL REGULATION cont..

Stop and Go signals:

If the centromere is not properly attached to spindle microtubules a molecular signal is sent that delays anaphase until they are correctly attached