notes 12

Molecular Biology Chapter 12 Mitosis All cells come from cells Your cells came from cell divisions in your parents, who came from cell divisions in your grandparents parents, who came from cell divisions in your grandparents etc… Common Ancestors Each generation, you double your number of ancestors 2 ancestors 1 generation back (parents) 4 ancestors 2 generations back (grandparents) Assuming a generation time of 20 years… Generations between year 1400 – present = 30 How many ancestors did you have 30 generations back? 230 = 1,073,741,824 Growth in multicellular organisms Replacing old cells, repair tissues Reproduction for single celled organisms Need to maintain the integrity of the information in the nucleus Overview of the process 1 parental cell (mother cell) ? 2 genetically identical daughter cells Critically important that each daughter cell gets the same DNA Eukaryotes average ~1 billion base pairs ~ 1 meter long Need lots of folding and compacting Chromosomes Single strand of DNA Wrapped up on proteins to reduce volume Still highly organized!!! 10,000x more compact Protein scaffold in the center In a diploid species 2 versions of each chromosome 1 from Mom, 1 from Dad Homologous chromosomes Before mitosis, they need to be copied Sister chromatids = Identical copies 2 identical copies of Mom’s chromosome, 2 identical copies of Dad’s chromosome Phases of the cell cycle 4 phases of mitosis 2 general categories M phase (mitosis) Interphase Interphase Most of the cell cycle is interphase Human stem cell: 22 hours of interphase, 2 hours of division Preparing DNA, organelles for division G1 Organelles double Mitochondria, ribosomes, chloroplasts Cell can pause here indefinitely Mature nerve and muscle cells stay here Intestinal cells move through quickly S S= synthesis DNA doubles Individual chromosomes becomes 2 sister chromosomes G2 Make the proteins needed for cell division Microtubules used to move parts around M stage (mitotic stage) Division of the nucleus Splitting the DNA for 2 new, identical nuclei Think about how this happens physically Spindle fibers Microtubules that pull chromatids apart Attached to 2 poles at opposite sides of the cell Meet at the middle, “spindle equator” Stages of mitosis Easiest to break down mitosis into pieces Prophase (prefix “pro-” = located at the front) Prometaphase Metaphase (prefix “meta-” = occurring aner, middle) Anaphase (prefix “ana-” = back, against) Telophase (prefix “telo-” = final, end) At the end of interphase DNA is copied All organelles copied Centrosomes appear This is where spindle fibers will attach Prophase Sister chromatids condense Spindle fibers form Prometaphase Nuclear membrane breaks down Kinetochore microtubules (one of the spindle fiber types) attach to the kinetochore and the centrosome Kinetochore= attachment point on chromosomes Each pair of sister chromatids will attach to 2 kinetochore microtubules The 2 kinetochore microtubules will be attached to opposite kinetochores Metaphase Chromosomes line up on the equator in a single row Metaphase plate Individual sister chromatids are attached to opposite sides of the cell Polar fibers (part of the spindle apparatus) extend from the 2 centrosomes, overlap in the middle Anaphase Kinetochore microtubules shorten Pull sister chromatids apart, towards poles Polar microtubules push against each other, move poles apart Kinetochore motors As kinetochore microtubules shorten, motor proteins carry the chromosome down the crumbling road Studying microtubules Can stain them, watch them as they work Telophase/ cytokinesis Spindles break down Chromosomes de-condense Nuclear membrane reforms In plants, cell plate forms, membrane and cell wall is built up to make 2 cells In animals, microtubule ring attached to the cell membrane forms Ring shrinks, pulls membrane with it until the membrane meets itself, and splits into 2 cells Control of the c ell cycle Cells from different tissues divide at different rates Critical that cell division is controlled, regulated Regulation is done by proteins What controls cell division? Internal to the nucleus: Telomeres Repeating DNA at the end of the chromosome, “caps” Gets shorter with every cell division About 70 cell divisions, cell is now too old, it dies External signals: Kinases and cyclins Growth factors and hormones Contact inhibition How can you test for external signaling molecules? Control of the cell cycle Molecule in the cytoplasm of a cell in M-phase can cause mitosis to begin in other cells Mitosis promoting factor (MPF) Found in all eukaryotes Human MPF can cause mitosis in yeast MPF 2 parts Cyclin dependent kinase (CdK) signals other proteins to start mitosis- CdK is always present CdK only works when it is bound to cyclin- cyclin concentrations vary When cyclin concentration gets high enough, M phase starts During anaphase, enzymes are made that destroy cyclin Negative feedback loop Cyclin starts the reactions that leads to its own destruction Stopping the cell cycle Mechanisms within the cell can stop the cell cycle is something is wrong Checkpoints G1 checkpoint Pass if….. Cell is big enough Enough nutrients Social signals are present No DNA damage Cell Cycle Checkpoints G2 checkpoint MPF MPF activation may be stopped if there is DNA damage Metaphase Checkpoint Ensures chromosomes are attached to spindle apparatus properly G1 checkpoint Protein P53 Normally inactive Turns on when DNA is damaged, oxidative stress, osmotic shock, etc… When activated, 2 options: P53 stops cell cycle, DNA repaired, cell cycle restarts P53 triggers apoptosis (cell death) Cellular and genetic stability Tumor suppressor gene “Guardian of the genome” What happens if p53 malfunctions? Cancer % of human tumors have defective p53 SIDENOTE------------------------------------------------------------------------------------------------------------------------- For women in the USA, which cancer leads to the most deaths? Lung and broncus For men in the USA, which cancer leads to the most deaths? Lung and broncus Indiana data New cancer case rates (per 100,000) Indiana= 454.0 USA= 465.1 Cancer death rates Indiana= 193.3 USA= 178.1 SIDENOTE------------------------------------------------------------------------------------------------------------------- Cancer Cancer= uncontrolled, unregulated cell division Immortal Density-independent growth, no contact inhibition/ social control Tumors Angiogenesis Promote growth of blood vessels so they can get bigger Metastasis Cancer cells can break off, invade other tissues Breakdown of checkpoints and repair systems Cancer related genes BRCA1, BRCA2 Repairs radiation induced breaks in DNA Malfunctions correlate with greater risk of breast and ovarian cancers Lifetime risk of general population for breast cancer= 12% Lifetime risk women with malfunctioning BRCA gene= 60% DNA repair mechanisms Defects in DNA repair are common in cancers Although knowing the specific defect is still difficult When these responses don’t work, get genomic instability =increased mutations Target DDR pathways for cancer treatment? Social control In multicellular organisms, part of the signal to divide comes from other cells Growth factors If you want to culture mammalian cells, you need to add serum Serum= liquid part of blood (cells are removed) Why? - Serum contains platelet derived growth factor (PDGF) PDGFs are formed by platelets Cells in blood that promote clotting Why would they also promote cell division? PDGFs bind to tyrosine kinase receptors in the cell membrane Signal transduction and enzyme-linked receptors Hormone binds to its transmembrane receptor protein In this example, receptor tyrosine kinase (RTK) Hormone binding created dimer, which gets a phosphate group from ATP Activates the RTK Activated RTK forms a bridge with Ras protein Ras is active by GDP GTP Activated Ras phosphorylates protein to activate it, which can then phosphorylate other proteins, etc… Phosphorylation cascade If original protein 1 activates 10 protein 2s, and each protein 2 activates 10 protein 3s Social control About 20-30% of cancers have defective Ras proteins What if Ras ignored the tyrosine kinase and was always on? Environmental factors Genes are not destiny!!! Environmental factors can cause or correlate with cancer: 30% of cancers caused by smoking 87% of lung cancers Sun exposure linked to skin cancers Radon gas