NOTES: Cell Division (CH 3)

Cell Division

• In simple terms: 1 cell divides into 2 new cells

3 main stages of the cell cycle1. Interphase• longest stage (90%); preparation for cell division

2. Mitosis (10%)• nucleus divides into 2 nuclei, each with • nucleus divides into 2 nuclei, each with the same # and kind of chromosomes (DNA) as the parent cell

3. Cytokinesis

• cytoplasm divides forming 2 distinct cells

• MITOSIS = the division of the nucleus

• Why is it important that the nucleus divides?– it stores the DNA (information of life)

– all new cells need this information

• Chromosomes =tightly coiled

chromatin (DNA); consist of 2 identical chromatids (sister chromatids); which chromatids); which are connected in the center by a CENTROMERE

**a human cell entering mitosis contains **a human cell entering mitosis contains **a human cell entering mitosis contains **a human cell entering mitosis contains

46 chromosomes46 chromosomes46 chromosomes46 chromosomes

centromere

CHROMATIN

CHROMOSOME

DNA

Mitosis is one, continuous event, but it can be described as happening in 4 phases:

1. Prophase

2. Metaphase2. Metaphase

3. Anaphase

4. Telophase

**Remember, the cell is coming out of Interphase…

PROPHASE (early)• chromatin condenses & chromosomes become visible

• Centrosomes / • Centrosomes / centrioles separate and move to the opposite sides of the nucleus;

PROPHASE (late)

• nuclear envelope breaks down and the nucleolus disappears

• spindle fibers connect to chromosomes at their centromeres;

METAPHASE

**chromosomes line up in the center of the cell (metaphase plate);

**spindle fibers connect from the poles (end) of the spindle to the centromere of each chromosome

ANAPHASE

• centromeres split, causing the sister chromatids to separate

• chromosomes are pulled apart to opposite ends of the cell (the spindle fibers shorten and “reel them in” to the poles)

TELOPHASE• chromosomes uncoil into chromatin;

• new nuclear envelope forms around the chromatin;forms around the chromatin;

• spindle breaks apart;

• nucleolus reappears in each new nucleus

Finally…

CYTOKINESIS

• cell membrane pinches in & divides cytoplasm/cell cytoplasm/cell

Then the cell returns to Interphase… and the process continues

Regulation of the Cell Cycle

CANCER

• cancer cells do not respond to body’s control mechanisms

• cancer cells divide excessively, invade other tissues, and can kill the organism if left unchecked.

HOW?

• cancer cells may make their own growth factors;

• cancer cells may have an abnormal growth factor signaling system;factor signaling system;

• cancer cells divide indefinitely (as opposed to normal cells, which typically divide about 20-50 times before they stop).

• Normally, the immune system recognizes and

destroys transformed or mutated cells which

are growing abnormally

• if abnormal cells evade the immune system,

they may form a TUMOR.

Bronchus

tumor

• if the cells remain at the original site, the mass is called a BENIGN TUMOR and can be completely removed by surgery.

• if the tumor cells have invaded other tissues / organs, it is a MALIGNANT TUMOR.

Properties of malignant tumors:

• excessive cell proliferation

• may have unusual numbers of chromosomeschromosomes

• may have abnormal metabolism

• abnormal cell surface properties

• if cancer cells separate from the original tumor and spread into other tissues, entering the blood and lymph vessels, they may vessels, they may invade other parts of the body and develop into new tumors…this is called… METASTASIS.

Treatments

• surgery (for benign tumors)

• radiation

• chemotherapy

**Factors which can cause an “alteration

of genes” (a.k.a. MUTAGENS) include:

1. Chemicals

2. Radiation2. Radiation

• Examples of Chemical Mutagens:– cigarette smoke

– DDT

– chewing tobacco

– pollution

– Heavy metals – Heavy metals

(e.g. chromium-6)

• Examples of Radiation Mutagens:– sun (UV rays)

– nuclear waste

– x-rays

skin cancer caused by

too much sun

spots: sun damage