Regulation of the Cell Cycle SBI3UP AP Lesson

Why do cells divide? Do all cells divide at the same rate? Why not?

Recall the Cell Cycle:

Two Hypotheses: Each event in cycle triggers the next Incorrect theory Cell cycle is driven by specific molecular signals Evidence came from experiments with mammalian cells grown in culture

The Experiment: 2 cells in different phases of cell cycle were fused to form a single cell with 2 nuclei If one original cell was in S phase and the other in G1, the G1 nucleus entered S phase Known due to the chemical present in cytoplasm

If one original cell was in M phase and the other in G1 the nucleus entered M phase Known due to formation of spindle and condensing of chromosome However, the chromosomes would not be duplicated

S and G1  S phase M and G1  M phase

What if the 1st theory was correct What if the 1st theory was correct? How would the results of the experiment differed?

Experiments concluded that events of are directed by a cell cycle control system Set of molecules that trigger/coordinate cell cycle events It operates on its own The control system is subject to external/internal control

Cell Cycle Checkpoints: Point in cell cycle where stop/go signals regulate cycle Signal checks that important processes have been complete E.g. cell size, DNA replication, spindle fiber attachment Checkpoints are found in G1, G2, and M phase

The G1 Checkpoint: G1 has been known as the “restriction point” in mammalian cells If cells pass G1 checkpoint they will likely divide If cells don’t pass G1 checkpoint they go to G0 phase  non- dividing E.g. nerve cells, liver cells

What if a cell ignored the checkpoint at the G1 phase What if a cell ignored the checkpoint at the G1 phase? What would be the consequences?

Molecular Basis for the Cell Cycle Clock: Rhythmic fluctuations Abundance and activity of cell cycle control molecules Two main types of regulatory molecules (proteins): Kinases Cyclins

Phosphorylating other proteins to activate/inactivate them Kinases: Phosphorylating other proteins to activate/inactivate them Give “go ahead” at G1 and G2 checkpoints Present in constant concentrations Inactive unless attached to a cyclin Thus known as cyclin-dependent kinases (Cdk) Cdk + Cyclin  MPF Phosphorylation is the addition of a phosphate group to a protein.

MPF (maturation-promoting factor/M-phase promoting factor) Functions: As kinase Initiates mitosis Contributes to chromosome condensation and spindle formation Activates other kinases Phosphorylates a variety of proteins Eg. Phosphorylates protein of the nuclear lamina  fragmentation of nuclear envelope

4. Cdk component of MPF is recycled 3. One indirect effect of MPF is the breakdown of its own cyclin 2. MPF promotes mitosis by phosphorylating various proteins/enzymes 1. Accumulated cyclin molecules combine with Cdk molecules to produce MPF by G2 checkpoint

When kinetochores of all chromosomes are attached Internal Signals: M phase checkpoint Kinetochores not yet attached to spindle microtubules send molecular signal Sister chromatids stay together, delay anaphase When kinetochores of all chromosomes are attached Anaphase-promoting complex (APC) becomes active Triggers breakdown of cyclin and inactivation of proteins holding chromatids together Ensures right number of chromosomes in daughter cells Kinetochores are the protein on chromatids where spindle fibers attach. Located at the centromere.

External Signals: 1. Chemical Cells in culture cannot divide if missing an essential nutrient Eg. Growth factor Mitogen: a growth factor protein that promotes mitosis Eg. Platelet-derived growth factor (PDGF) Required for division of fibroblasts Fibroblast: a type of connective tissue cell with PDGF receptors Binding allows cell to pass the G1 checkpoint and divide Injury  platelets release PDGF

2. Density Dependent Inhibition Crowded cells stop dividing Cultured cells form a single layer on inner surface of container If cells are removed, cells bordering space will divide to fill in Why? Physical contact (minor) Amount of required growth factors and nutrients available (major)

3. Anchorage Dependence Requires substratum Eg. Inside of culture container or extracellular matrix of a tissue Signaled through pathways using plasma membrane proteins and cytoskeleton

How are cancer cells different from normal cells?

Cancer Cells: Do not respond normally to body’s control mechanisms No density-dependent inhibition No anchorage dependence Divide excessively Invade other tissues  may kill organism Stop dividing at random points in the cycle, instead of at checkpoints

Cancer cells are immortal can divide indefinitely if given continual supply of nutrients and an adequate environment to grow Eg. HeLa cells: a cultured cell line from 1951, Henrietta Lacks’s tumour Vs. normal cells in culture only divide 20-50 times

Hypotheses for NO density-dependent inhibition in cancer cells: Do not need growth factors to grow and divide May make a required growth factor themselves Abnormal signal pathway to convey GF’s signal even in its absence Abnormal cell cycle control system

Transformation to Cancer Cells: process that converts a normal cell to a cancer cell Escape destruction from body’s immune system Forms tumour (a mass of abnormal cells within otherwise normal tissue) If remain at original site benign tumour (no serious problem, can be completely removed by surgery) If becomes invasive to impair functions of one/more organs malignant tumour (cancer)

Malignant Tumours: Excessive proliferation Unusual number of chromosomes (cause or effect?) Metabolism may be disabled No constructive function Abnormal changes on cells’ surfaces  lose/destroy attachment to neighboring cells and extracellular matrix Can spread into nearby tissues Can secrete signal molecules to cause blood vessels to grow toward the tumour

Metastasis: a few tumour cells separate from original tumour enter blood/lymph vessels travel to other parts of body proliferate and form a new tumour

Treatment: Localized Tumour high-energy radiation Damages DNA in cancer cells normal cells can repair damage, cancer cells cannot

Treatment: Metastatic Tumour chemotherapy through circulatory system Interfere with specific steps in cell cycle Eg. Taxol prevents microtubule depolymerisation freezes mitotic spindle stops actively dividing cells at metaphase Side effects due to drug’s effect on normal cells Nausea (intestinal cells) hair loss (hair follicle cells) susceptibility to infection (immune system cells)

Things to Know: Stages of the cell cycle Cell cycle checkpoints Molecular basis for cell cycle clock Internal/external signals Cancer cells: development, types of tumours, treatments