Cell Division: Mitosis

Chapter 8

Prokaryotic Cell Division

• Binary fission: dividing in half• Steps

– Chromosomes duplicate and move to ends of cell

– Cell elongates– Plasma membrane pinches off into 2

‘daughter cells’

Chromosomes

• 1 chromosome is 1 DNA molecule w/ 100’s to 1000’s of genes

• Composed of chromatin– DNA and protein

• Number varies within species– 46 in humans– 78 in dogs– 38 in cats– 254 in shrimp– 1200 Adders-tongue (fern)

• Chromosomes replicated to form 2 sister chromatids– Joined by centromere

• Division splits chromosome apart– Each becomes a chromosome– Count centromeres to determine

number

Eukaryotic Cell Division

The Eukaryotic Cell Cycle (Overview)

Mitotic Phase

• Interphase – About 90% of time– Chromosomes not visible yet– 3 Subphases

• G1 phase = cell growth and maturation• S phase = synthesis, chromosomes

replicated• G2 phase = growth and prep for division• G0 phase = not dividing or quienescent

• Mitotic (M Phase)– Actual cell division– Mitosis – nuclear division

• Prophase• Metaphase• Anaphase• Telophase

– Cytokinesis – cytoplasmic division• Repeat

Prophase Events

• Sister chromatids condense

• Nuclear envelope begins to disappear

• Centrosomes (centrioles in animal cells) move to opposite ends of cell

• Mitotic spindles form

Metaphase Events

• Centrosomes at opposite ends of cells

• Sister chromatids line up with centromere on metaphase plate

• Microtubules attached to each chromatid at the centromere

Anaphase Events

• Sister chromatids separate

• Single chromosomes move toward opposite ends of the cell– Microtubule ‘tug of war’

• Cell elongates

Telophase Events

• Daughter nuclei form

• Nuclear envelope reforms

• Chromosomes begin to uncoil

• Mitosis is complete

Cytokinesis

• Division of cytoplasm– Begins at the end of telophase (late

anaphase too)

• Animal cells– Cleavage furrow forms

• Microfilaments interact to pinch plasma membrane in 2

• Plant cells– Cell plate forms

• Vesicles collect in the middle until fuse with plasma membrane

Apoptosis

• Programmed cell death• Necessary for normal development

– Maintains balance of cell growth– Organism shape/form development– Sunburn peeling

• Tightly regulated to maintain balance

Cell Division Control• Cyclical control system regulates with ‘checkpoints’

– 3 major ones: G1, G2, and M– Ensures previous processes complete before allowing continuation of

division• Possible number of divisions varies

– Ends of chromosomes dictate– Skin cells and those of GI tract divide regularly– Liver cells rarely divide

• Need growth factors, proteins secreted by the body to stimulate growth– Cells respond to specific ones

• Density-dependent inhibition– Cells stop dividing if become crowded– Remove cells and dividing restarts

• Anchorage dependence– Must have contact w/ a solid surface

Cancer• Cancer is a disease of the cell cycle

– Only 1 cell needs to be transformed into a cancer cell– Immune system normally detects then destroys– Undetected can proliferate = tumor

• Benign stay in original location– Can be removed by surgery so don’t disrupt organs

• Malignant can spread into other areas– Metastasis: cancer cells spread via the circulatory system from original site

• Don’t follow normal signaling– No density-dependent inhibition (overlap)– No check points– Make own growth factors to increase proliferation– Unlimited number of divisions

• Often named for site of origin– 4 categories

Cancerous Cell Growth

Categories of Cancer

• Carcinomas– External or internal coverings of the body– E.g. skin or intestinal linings

• Leukemias and lymphomas– Blood forming tissues– E.g. bone marrow or lymph bodies

• Sarcomas– Tissues that support the body– Eg. Bones or muscle

Cancer Treatments

• Radiation for localized cells– Damages DNA of cancer cells more than normal

ones– Lost ability to repair

• Chemotherapy for metastatic tumors– Drugs interfere with cell division (normal and

abnormal) and cause side effects• Nausea from intestinal cells• Hair loss from hair follicle cells• Infections from immune system cells