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How do we get more cells: Mitosis vs. Meiosis and What’s going on the rest of the time DNA has two forms: • Chromatin: unwound and stringy; information is accessible but difficult to move • Chromosomes: folded up chromatin; information is NOT accessible, but easy to move • DNA SPENDS MOST OF THE TIME AS CHROMATIN!!! Chromosome structure: Telomeres (ends) Centromere (middle) During replication each chromosome is duplicated Sister chromatids – two identical copies of chromosomes attached at the center There are different types of chromosomes • Each type has a different set of GENES on them. • Chromosomes that are the same type are called HOMOLOGOUS chromosomes Homologous chromosomes • Have the same set of genes but may have different ALLELES • An allele is a variation of a gene. Homologous chromosomes Gene: Hair color - allele: brown hair Gene: Hair color - allele: blonde hair Gene: Eye color - allele: brown eyes Gene: Eye color - allele: blue eyes What do we need to do before the cell can split? • Make another copy of DNA • Make more organelles (cell parts) • Cells Grow Cell Cycle Cell Cycle G1(Gap 1) – Cell growth and protein synth. S – DNA replication (copying) G2 (Gap 2) – growth, prepare for cell division M – Mitosis (nuclear division) and cytokinesis (cell division) Why do cells need to divide? • When we grow • To repair damage • Replace cells when they die Mitosis • Process by which body cells duplicate and split their nucleus • 4 stages • Followed immediately by cytokinesis 1. Prophase Duplicated chromatin folds up into chromosomes 1. Prophase • Centrosomes separate and move toward opposite poles 1. Prophase • Centrosomes start to form the Mitotic spindle –Made of microtubules (cytoskeleton) 1. Prophase • Nuclear membrane breaks down • Nucleolus disappears 2. Metaphase • Chromosomes line up along the middle of the cell metaphase plate • Spindle fibers attach to the centromeres of each chromosome 3. Anaphase • Sister chromatids separate and one chromatid of each chromosome moves toward opposite poles 4. Telophase • • • • Chromosomes unwind to chromatin Nuclear Envelope reforms Spindle breaks down Nucleolus reappears Cell Division is not yet Done!! • What have we made? – One Cell with Two complete Nuclei What is left to do? - Cytokinesis: Divide the cytoplasm and separate the cells Cytokinesis • Animal cells: Membrane pinches inward until it divides the cytoplasm into two equal parts –Cleavage Furrow Cytokinesis • Plant cell: Cell plate forms in the middle of the cytoplasm • Made of cellulose • Becomes cell wall NOW Cell division is done! • Now you have Two Identical daughter cells (also identical to parent!) Controlling Cell Division • Cells know when they need to divide –Complicated series of “stop” and “go” signals make sure cells only divide when needed Controlling Cell Division • Cells know when they don’t need to divide Loss of control • What happens?!? – cells divide uncontrollably –Pile up on top of each other –Form big balls of cells called????? »TUMORS!!! Tumors • Tumor cells do not respond to (or do not have) the body’s control signals –missing a “stop” signal –Hyperactive “go” signal Tumor vs. Cancer • Cells that grow out of control for tumors. • BUT tumors aren’t necessarily cancerous Tumor vs. Cancer • Benign tumors: grow out of control, but are isolated and don’t interfere with healthy cells. • Malignant tumors: grow out of control AND interfere with healthy cells. Tumor vs. Cancer • Malignant tumors are cancerous. Spreading Cancer • What if 1 cancer cell breaks off and enters the blood stream? –When it “lands” it will form a new tumor there –This is called metastasis Sexual vs. Asexual Reproduction DNA from how many parents? Asexual Sexual Offspring Example Chromosome Terms • Diploid: two of each type of chromosome (one from each parent) • Haploid: one of each chromosome • Most human cells are Diploid –We have 46 chromosomes • 22 pairs of body chromosomes • 2 sex chromosomes (XX or XY) • What would happen if we made Egg and Sperm cells through mitosis? – Fertilized egg would have 92 chromosomes! • What do we have to do when forming these cells? –Divide the DNA in half! Major differences between Mitosis and Meiosis • one cell goes through two divisions to make a total of 4 cells • cells created at the end are different from the cell they came from because of crossing over Meiosis • After the first division, cells are haploid and no longer diploid • Happens only in reproductive cells Genetic Variation: - diversity between individuals - created by crossing over and meiosis and sexual reproduction! Meiosis I: 1st division 1.Homologous chromosomes pair up to form tetrads 2.Crossing over occurs: homologous chromosomes swap info (blonde allele for brown allele) Meiosis I: 1st division Similar steps to mitosis but: 3. Cells end up haploid but chromosomes are still duplicated Meiosis II: 2nd division Just like mitosis - each cell from Meiosis 1 divides - Creates 4 cells total - all are different because of crossing over - all have ½ the original chromosomes Compare and Contrast Mitosis and Meiosis