Regulation of the Cell Cycle

How does a cell know when to divide and when not to divide?

Coordination of cell division Multicellular organisms need to coordinate cell division across different tissues & organs critical for normal growth, development & maintenance coordinate timing of cell division coordinate rates of cell division not all cells can have the same cell cycle

Frequency of cell division Frequency of cell division varies by cell type embryo cell cycle < 20 minute skin cells divide frequently throughout life 12-24 hours cycle liver cells retain ability to divide, but keep it in reserve divide once every year or two mature nerve cells & muscle cells do not divide at all after maturity permanently in G0 G2 S G1 M metaphase prophase anaphase telophase interphase (G1, S, G2 phases) mitosis (M) cytokinesis (C) C

Cyclins

Regulation of the Cell Cycle The cell cycle is driven by a chemical control system that both triggers and coordinates key events in the cell cycle. The cell cycle control system is regulated at certain checkpoints. Signals from inside the cell (internal signals) and from outside the cell (external signals) are involved in turning the process of cell division off and on.

Check Points A critical control point where stop and go signals can regulate the cycle. The cell division mechanism in most animal cells is in the “off” position when there is no stimulus present. Specific stimuli are required to start the processes.

Internal Factors: Cyclins An internal signal involves the cell sensing the presence of chemicals, called enzymes, which are produced inside the cell Internal regulators allow the cell cycle to proceed only when certain process have happened inside the cell. Ex. Regulatory proteins that make sure the cell does not begin mitosis until all the chromosomes have been replicated.

External Factors An external signal involves the cell sensing the presence of a chemical which was produced in other specialized cells. External regulators direct cells to speed up or slow down the cell cycle. Growth Factors are an important external regulator. Extremely important during embryonic development and wound healing Ex. Hormones Cells can also respond to physical signals from their environment. ○ Cells sense when they are too closely packed and cell division is turned off. ○ Cells sense when they are not in contact with a surface and cell division is turned on. Growth factors are a broad group of proteins that act as chemical signals released from other cells to tell other cells to divide. Growth factors bind to receptors that activate specific genes to trigger cell growth. Cells will usually respond to a combination of several growth factors not just one. Once a cell touches other cells it stops dividing. The exact reason is considered a phenomenon and is unknown. One hypothesis suggests that receptors on neighboring cells bind to each other and trigger the cytoskeleton to block signals that will trigger growth.

Examples Platelets-form clots to stop bleeding and carry growth factors that helps your body repair wounds by signaling certain cells to divide.

Apoptosis Just as some cells need to divide, some cells need to die. Apoptosis is programmed cell death. Internal or external factors stimulate genes that activate self destructive enzymes. The cell will then be recognized and ingested by immune system cells.

What happens when cell division becomes uncontrolled?

Cancer Cancer cells are an example of cells that do not regard the normal signals that stop the cell division process. Cancer cells continue to divide even when they are very densely packed and/or there is no growth factor present. Normally the body’s immune system will recognize that the cell is damaged and destroy it, but if it evades destruction, it will continue to divide and each daughter cell will be a cancer cell. Cancer cells divide much more than healthy cells.

Tumors When clumps of cancer cells form they are called tumors. Two types: Benign Malignant

Benign Tumors Benign tumors usually remain localized and do not spread. Usually harmless and may be cured simply by removal.

Malignant Tumor In a malignant tumor certain cells may break away from the tumor and spread Carried by blood & lymph system to other tissues When cancerous cells spread, we say that they metastasize. Impair functions of organs throughout body Tumors are difficult to eradicate once they metastasize.

Why are tumors harmful? Tumors Cancer cells don’t perform the specialized functions needed by the body. The body has large volumes of rapidly dividing cells that require lots of blood and energy but pose no usefulness to the body. Tumors can exert pressure on vital organs when they become too large.

Where do cancer cells come from? Cancer cells will usually come from normal cells that have suffered damage to their genes (mutations) that are associated with making proteins that regulate the cell cycle. Oncogenes are one type that will accelerate cell growth. Other genes act as cell cycle brakes. Mutations in various genes can be inherited (e.g. breast cancer).

Types of Skin Cancer The Major Types Basal Cell Carcinoma (BCC) Caused by excessive exposure to the sun. This is the least dangerous type of skin cancer because it is unlikely to spread.  Squamous Cell Carcinoma (SCC)  second most common form of skin cancer. It’s usually found on areas of the body damaged by UV rays from the sun or tanning beds. SCC is a fairly slow-growing skin cancer. It can spread to the tissues, bones, and nearby lymph nodes, where it may become hard to treat. When caught early, it’s easy to treat. Melanoma Less common, but the deadliest form of skin cancer. Those who use

Know Your ABCD’S A – Asymmetry B- Border C- Color D - Diameter

Lung Cancer: Smoking Lung cancer is the leading cause of cancer death in the United States for both men and women. (Source:Cancer Facts & Figures 2014) Lung cancer is the most preventable form of cancer death in the world. (Source: Cancer Facts & Figures 2014) Women smokers are 25.7 times more likely than women who never smoked to develop lung cancer. For men smokers, it’s 25 times the risk of men who never smoked. (Source: US Surgeon General Report 2014)

Carcinogens are substances that may cause mutation. Examples Include: Tobacco Asbestos Air Pollutants Lifestyle factors (nutrition, tobacco use, physical activity, etc.) Naturally occurring exposures (ultraviolet light, radon gas, infectious agents, etc.) Medical treatments (radiation and medicines including chemotherapy, hormone drugs, drugs that suppress the immune system, etc.) Workplace exposures

What are Common Treatments for Cancer?

Radiation Localized treatment that damages a cell’s DNA and prevents it from dividing. Cancer cells whose DNA is damaged beyond repair stop dividing or die. When the damaged cells die, they are broken down and eliminated by the body’s natural processes. Does radiation therapy kill only cancer cells? No, radiation therapy can also damage normal cells, leading to side effects. Doctors take potential damage to normal cells into account when planning a course of radiation therapy. The amount of radiation that normal tissue can safely receive is known for all parts of the body.

Chemotherapy Often used in combination with radiation and surgery. Drugs that kill rapidly dividing cells.  May kill healthy cells that divide rapidly such as  cells in the bone marrow, digestive tract and hair follicles.