Cell Cycle● Interphase: time between divisions – G1, S, and G2● Mitosis: division of the chromosomes● Cytokinesis: division of the cytoplasm● Checkpoints in the cell cycle regulate cell division 6
Interphase● G1 – Begins immediately after division – New organelles formed – End of G1, cell has doubled in size● S phase – Duplicate copy of each chromosome● G2 phase – Cell prepares to begin mitosis 7
G2/M checkpoint 4 Cell division3 DNA repair Mitosis 1 G2 Cell grows, doubles in G1 size S Chromosome 2 duplication 8 G1/S checkpoint Stepped Art p. 181
Stages of Mitosis: Metaphase● Chromosomes, with spindle fibers attached, move to middle of cell 14
Stages of Mitosis: Anaphase● Centromeres divide.● Converts each sister chromatid to a chromosome.● Chromosomes migrate to opposite ends of cell.● Complete set of 46 chromosomes at each end of cell. 15
Hunt Hartwell Nurse (2001 AD) This years Nobel Laureates have discovered the key regulators of the cell cycle- cyclin dependent kinase(CDK) and cyclin. Together these two components form an enzyme, in which CDK is comparable to a molecular engine that drives the cell through the cell cycle byaltering the structure and function of other proteins in thecell. Cyclin is the main switch that turns the CDK engine on and off. This cell-cycle engine operates in the same way in such widely disparate organisms as yeast cells, 22 plants, animals, and humans.
Cell-cycle checkpoints● Restriction point – Regulate initiation of DNA replication● G2-M checkpoint – Checks DNA damage● Spindle checkpoint – Checks chromosome alignment 24
DNA Damage Check Point● DNA Damage can be checked at various sites. (Before S, During S, After S )● Detected by DNA bound proteins ATM & ATR.● Transcription factor p53 is activated after damage is detected. (guardian of genome)● It activates p21 ( a CKI) that blocks CDK2/4. 25
Replication Checkpoint● Before entering the M phase of the cycle.● If some errors in the replication of the genome is identified by chk1 and CDC25C.● The activation of CDK1 is prevented thus causing prevention of M phase of cycle. 26
Spindle Integrity checkpointmicrotubules in green, chromosomes in blue, and kinetochores in pink 27
Spindle Integrity Checkpoint● The most dangerous time for a cell. -Devita● Aligning duplicated chromosomes by attaching them to bipolar attachments to the spindle.● The sensors of this check point are present in the kinetochores.● APC/C Cofactor ~ CDC20 is inhibited unless there is equal tension on both the sides.● CDC20 when active inhibits securin which then enables the seperation process. 28
Cell Cycle and Cancer● Cancer is partly a disease of uncontrolled proliferation.● Cell cycle and check point genes are found misregulated / mutated in cancer.● Protooncogenes● Tumor Supressor Genes 30
Protooncogene / Oncogene● A proto-oncogene is a normal gene that can become an oncogene due to mutations.● The resultant protein : oncoprotein.● The mutations are generally dominant.● Examples: SRC, RAS, WNT, MYC, cSIS 32 Pancreas colon breast burkitts GBM
Tumor Supressor Gene● ~ anti-oncogene.● Mutations in these genes cause loss of function, thus causing malignancy.● Mutations are generally recessive, thus heterozygosity is necessary for mutation to be apparent. two-hit hypothesis● Examples: pRb, p53, APC, BRCA 33
Proto-Oncogene RAS● Produces RAS protein that: – Attaches to inside of plasma membrane – Is part of a pathway that turns on cell division – Is signaled by growth factors from outside the cell● RAS mutations in many types of cancer, including colon, lung, pancreatic, and stomach cancer● Mutant RAS stuck “on” and produces 35 uncontrolled cell division.
BRCA1 ● Described in 1970s after analyzing pedigrees of 1,500 families. ● In 1990, identified in chromosome 17 ● Dominantly inherited, carry one mutant copy gene, develop breast cancer if other copy mutates82%: breast cancer44%: ovarian cancer 36Another gene BRCA2 is in Chromosome 13, rare and causes susceptibility to breast cancer.