Transcript Mitosis
Mitosis Cell Size Limits • Why aren’t we just one giant cell?? 3 limitations to cell size 1. Diffusion- is fast and efficient over short distances but slow and inefficient over long distances - cells would die long before nutrients could reach the organelles 3 limitations to cell size 2. DNA limits cell size - cells need A LOT of proteins to perform critical functions BUT there is a limit to how quickly DNA can be made into proteins. 3 limitations to cell size 3. Surface area-to volume ratio - as a cell’s size increases, its volume increases much faster than its surface area Reasons for cell division 1. maintaining optimum cell size 2. growth in multicellular organisms 3. asexual reproduction of single-celled organisms 4. to replace dead cells Chromatin vs. Chromosome • Chromatin = long strands of DNA wrapped around proteins (tangled up spaghetti) • Chromosome = coiled strands of DNA that look more like rope or X’s; becomes like this right before cell division Cell cycle • G1→G2 = interphase • S phase = DNA replication • G1 & G2 = growth 1 & 2, cell is growing and preparing for division • Which phase does a cell spend most of its time in? Mitosis • Mitosis = process in which the nucleus of a cell is divided into 2 nuclei, each with the same number and kinds of chromosomes as the parent cell (also called asexual reproduction) Interphase • First step of the cell cycle. • Interphase is the preparation phase. • Three stages: G1, Interphase and G2. • What happens during interphase? Mitosis • Mitosis is the second step of the cell cycle (asexual reproduction) • It is broken into four phases: prophase, metaphase, anaphase and telophase. Prophase • First stage of mitosis. • Nuclear envelope begins to dissappear • Chromatin coils up and becomes X-shaped chromosomes Metaphase • Chromosomes (X’s) line up in the middle of the cell. • Spindle fibers attach to the centromere on the chromosome. Anaphase • Spindle fibers begin to pull the chromosomes (X’s) apart. • Chromatid (V’s) now move toward opposite ends Telophase • Chromatid (V’s) reach the poles. • Nuclear envelope begins to reappear around the chromatid • In plant cells a cell plate appears down the middle to divide the cell. • In animal cells a cleavage furrow appears to separate the cell. Cytokinesis • Last stage of the cell cycle. • Cell has separated creating two new identical daughter cells. Cytokinesis • Cytokinesis= the process by which the cytoplasm divides, thus forming 2 distinct cells Results of Mitosis • 2 identical daughter cells • In unicellular organisms that is how it reproduces • In multicellular organisms, cell growth and division results in a group of cells that work together (all human cells undergo mitosis except for our sex cells). Skin cells undergo mitosis the most. cells tissue organ organ system ORGANISM Cancer • Enzymes control each phase of the cell cycle. • Cancer = result of uncontrolled cell division • Extra cells form masses called tumors • Causes of cancer are both genetic and environmental (smoke, exposure to UV rays, viral infections)