G1 (Restriction) Checkpoint
* End of G1, just before onset of the S phase (DNA replication) * Yeast “start”; other eukaryotes “restriction point” * The options for the cell at this point:
* divide, delay division, or exit the cell cycle
* Cells can exit the cell cycle at this point into an arrested stage (G0) * When this checkpoint is passed, cdk4 and cyclin D interact. This interaction results in phosphorylation of the retinoblastoma protein, which in turn allows activation of the transcription factor E2F. Active E2F promotes expression of the cyclin E gene. Cyclin E (protein) and cdk 2 interact to promote the G1 to S phase transition. DNA Replication Checkpoint (end of G2)
* Cell will not proceed with mitosis if DNA replication is not complete * The way the cell senses this is not understood completely * This checkpoint involves signals that block the activation of M phase cyclin-cdk complex (MPF) by inhibiting the activity of cdc25 protein phosphatase. * Cells with mutations in this checkpoint pathway or cultured mammalian cells treated with caffeine will proceed through mitosis with unreplicated DNA. Spindle-attachment checkpoint
* Before anaphase (separation of chromosomes) there is a checkpoint to ensure the chromatids are correctly attached to the mitotic spindle * The kinetochore (where the chromatids attach to the spindle) is the structure that is monitored * Unattached kinetochores negatively regulate the activity of cdc20-anaphase promoting complex (APC), and anaphase is delayed Mitosis
* Degradation of the M phase cyclin/cdk complex (aka MPF) is required to proceed with the final activities of mitosis (spindle disassembly and formation of the nuclear envelopes). * This degradation is accomplished by ubiquitinylation of the complex. * The cdc20-APC complex is responsible for signaling the degradation and exit from mitosis. * The B-type cyclins remain active throughout M-phase, but their activity immediately ceases once cell division is complete and the two daughter cells once again enter G1. 2. Differentiate oncogene and tumor-suppressor gene.
Cells contain many normal genes that are involved in regulating cell proliferation. Some of these genes can be mutated to forms that promote uncontrolled cell proliferation. The normal forms of these genes are called proto-oncogenes, while the mutated, cancer-causing forms are called oncogenes. In contrast to tumor suppressor genes, which put the brakes on cell proliferation, oncogenes actively promote proliferation (analogous to the gas petal of the cell cycle). Mutations that convert proto-oncogenes to oncogenes typically increase the activity of the encoded protein or increase the expression of the normal gene. Such mutations are dominant or gain-of-function mutations. Therefore, only one copy of the gene needs to be mutated in order to promote cancer. Oncogenes were first identified in oncogenic retroviruses that had picked up a cellular oncogene (c-onc) and incorporated it into the viral genome to produce a viral oncogene (v-onc). J. Michael Bishop and Harold Varmus of UCSF were awarded the 1989 Nobel Prize in Medicine for the discovery of the cellular origin of the viral oncogenes.
On the other hand, tumor suppressor genes can be defined as genes which encode proteins that normally inhibit the formation of tumors. Their normal function is to inhibit cell proliferation, or act as the “brakes” for the cell cycle. Mutations in tumor suppressor genes contribute to the development of cancer by inactivating that inhibitory function. Mutations of this type are termed loss-of-function mutations. As long as the cell contains one functional copy of a given tumor suppressor gene (expressing enough protein to control cell proliferation), that...