Cell Cycle and Cell Division

Chapter 5

Belk and Borden

Biology: Science for Life

Cell Division

• - the process a cell undergoes in order to make copies of itself.

• Why do cells divide?

What is Cancer?

• “mitosis gone mad”• - a disease that begins

when a single cell escapes from the regulation of its own division

• Tumor• Benign vs Malignant

The Cell Cycle

• -all the events that occur when a cell divides

• -time required can vary from minutes to days, depending on the cell– Salmonella = 29 min

– Red B. C. = 120 days

DNA & Chromosomes

• DNA as chromatin strands and chromosomes

• Sister Chromatids• Centromere• Why do chromosomes

form?

Interphase (G1, S, G2)(about 90% of the cell cycle time)

• Cell grows during all 3 subphases by making proteins and organelles

• G1 = first growth phase

Interphase (G1, S, G2) (about 90% of the cell cycle time)

• S phase = DNA is copied (synthesis) so each daughter cell has a complete set of chromosomes at the end of the cell cycle

Interphase (G1, S, G2) (about 90% of the cell cycle time)

• G2 phase = second growth phase

• All through interphase, the cell is fulfilling its function

• At maximum volume to surface ratio, cell either remains in interphase or enters mitosis

Mitosis: Prophase

• Chromatin condenses into chromosomes

• Each duplicated chromosome appears as 2 identical sister chromatids

• Mitotic spindle begins to form in cytoplasm

Mitosis: Prometaphase

• Nuclear membrane disintegrates

• Spindle fibers interact with chromosomes

• The “dance of the chromosomes” begins

Mitosis: Metaphase

• Chromosomes are aligned at middle of cell

• Identical chromatids of each chromosome are attached to spindle fibers radiating from opposite poles of the cell

Mitosis: Anaphase

• Paired centromeres of each chromosome separate, freeing sister chromatids from eachother

• Each chromatid now a chromosome

• Sister chromatids are pulled to opposite poles of the cell

Mitosis: Telophase

• 2 new nuclei form at two poles of the cell where chromosomes have gathered

• Chromosomes uncoil and revert to chromatin form

• Mitosis (division of one nucleus into two identical nuclei) is complete

Cytokinesis

Allium root tip (onion) a mitosis classic!

Classify Cells relative to Cell Cycle Phase link

Control of the Cell Cycle

• Cells require nutrients and growth factors to divide (example PDGF & fibroblasts)

• Density dependent inhibition also regulates cell division

• Adhesion of cells is also involved. Cells normally stop dividing if they lose their anchorage.

• The role of regulatory proteins at the G1 “restriction point” is also critical.

Control of the Cell Cycle

• Regulatory proteins pace and control cell cycle events at “checkpoints”

• G1 checkpoint—monitors nutritional status, growth factors, & cell density; “restriction point”

• G2 checkpoint—monitors DNA replication and cell size

• Metaphase checkpoint—monitors chromosome attachment to spindle fibers

Mutations

• Changes (errors) in the genes that code for cell cycle regulatory proteins and tumor suppressor proteins are involved in cancer

• Proto-oncogenes code for growth factors• Mutated proto-oncogenes (oncogenes) can

overstimulate cell division and override the G1 checkpoint

• Mutated tumor supressor genes fail to produce the proteins that prevent tumors

Healthful Decisions to reduce Cancer Risk

• Avoid carcinogenic addictions!

• Eat whole foods (fresh fruit and grains)

• Exercise regularly to boost immune system

• Don’t drink alcohol in excess

• Don’t get sunburned

• Undergo regular screening and self-examination