Growth factors bind to surface receptors on the cells: transmembrane proteins relay information to the cell by signal transduction. Type types of growth factors: growth factors- stimulate cell division, growth-inhibiting factors- inhibit cell division, healthy cells divide when growth factor >growth- inhibiting factor balance favors cell division. Cancer may be caused by mutations in growth and growth inhibiting factor genes. Cancer (far right side): changes occurring in proteins important in maintain the cell. Just like a cell needs to grow-it also regulates number of cells through apoptosis. Display serine/threonine kinase activity only after binding with a cyclin (smaller than cyclin) Induces a conformational change enabling kinase activity and phosphorylation of substrates. Cdks are present in constant amounts all along the cell cycle. Cdk1-cyclin a/b trigger mitosis, kinase activity drops at the end m phase. Cdk activity is gone marking the end of mitosis and allows replication origins: replication origins are loaded with a protein called pre-replicative complex.