Regulating the Cell Cycle Where do cells come from? Where did the cell in my cheek come from? -All cells come from pre-existing cells (cell theory) What is the earliest cell then in our body? -all cells originated from your egg cell -imagine, one tiny cell grew for over 10 months into a baby -how much does a baby weigh? - 7lbs How many cells does the human body have? Trillions If the skin cell has the same DNA as the egg cell, why hasn’t your skin cell grown to be 7lbs Why aren’t our trillions of cells 7lbs each? This is gene regulation! 1/3 of genes are turned off before we are even born If these genes turn back on = cancer Oncogenes = off: growth gene Tumor suppressor genes = on If liver and skin cells have the same genes, but the liver cell has different genes ON than the skin cell

Controls on Cell Division Density-dependent Inhibition Cells will continue to grow until cells come into contact with other cells (depends on density of population) Contact inhibition: chemicals released by cells so they stop dividing as they become crowded.

When you are cut… Cells at the edges of the injury are stimulated to divide rapidly When close to healing, rate of division slows down Controls on growth are restored and everything returns to normal

Growth factors are proteins secreted by cells that stimulate other cells to divide After forming a single layer, cells have stopped dividing. Growth Factor Providing an additional supply of growth factors stimulates further cell division. The cells are still in a single layer, but smaller and more numerous Most cells require growth factors in order to begin dividing, and they stop dividing when they run out of these substances. Flooding a layer of nondividing cells with growth factor stimulates the cells to grow to a greater density than they would otherwise.

Growth factors signal the cell cycle control system Proteins, within the cell, control the cell cycle Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide G1 checkpoint Cell cycle control system is a set of proteins that both triggers and coordinates major events in the cell cycle. Control system M checkpoint G2 checkpoint Figure 8.9A

Cell cycle control system The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division Growth factor Plasma membrane Relay proteins Receptor protein G1 checkpoint Growth Factors Proteins that bind to receptors on the cell surface Stimulate the growth and division of cells Important in embryonic development and wound healing Signal transduction pathway Cell cycle control system

Internal regulators exist at checkpoints throughout the cell cycle. Allow the cell cycle to proceed only when certain processes have happened inside the cell Check to see that a cell doesn’t enter M phase until all chromosomes have been replicated CDK & Cyclin

Cancer

Uncontrolled Cell Growth Cancer is a disorder in which some of the body’s own cells lose the ability to control growth Malignant cells are cancerous cells Apoptosis Animation Apopotosis Multiple Apoptosis Explanation

Uncontrolled Cell Growth Cancer cells do NOT respond to signals that regulate cell growth They will divide uncontrollably and form tumors Cancerous cells may break loose from tumors and spread throughout the body (metastasize)

What causes cancer? A mutation to the DNA that controls the cell cycle The average cancer cell has between 6 and 8 different mutations What do these mutation affect? Genes that control the cell cycle Genes that control cell death

Cell Cycle controlled by: 2 types of genes Proto-oncogenes Encode proteins that promote cell division (gas pedal on a car: “GO!”) Tumor-suppressor genes Encode proteins that inhibit cell division (brake pedal on a car: “STOP!”) Cancerous cells have mutated proto-oncogenes AND tumor suppressor genes = “Always on GO!” Which of these do you think is mutated in cancer cells? Tumor-suppressor gene

What causes the loss of growth control? All cancers have one thing in common: The control over the cell cycle has broken down Some cancer cells no longer respond to external growth regulators Some cancer cells fail to produce internal regulators to ensure orderly growth Defect in gene p53

Gene p53 Main function is to halt the cell cycle until all chromosomes have been properly replicated Damaged/Defective p53 cause the cells to lose the information needed to respond to signals that would control their growth

Cancer Benign tumors remain clustered and can be removed. Cancer Cell Behavior Benign tumors remain clustered and can be removed. Malignant tumors metastasize, or break away, and can form more tumors. cancer cell bloodstream normal cell Several mutations to the DNA that controls the cell cycle occurs. The p 53 gene does not find the mutations and destroy the cell. Cells start to divide uncontrollably. The new cells formed are also cancerous. Large #’s of cancerous cells from clumps called tumors. Tumors out-compete normal healthy cells for nutrients. The normal cells die. Tumors break apart and spread cancer cells to other tissues and organs and out-compete the normal cells there killing them