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Chromosomes & Cell Division Cell Division The growth and splitting of cells into two new, identical cells called daughter cells. Meiosis Parent cell DNA replicates Chromosomes separate Mitosis • Cell division to produce identical diploid “daughter” cells • Growth and repair Chromatids separate Mitosis Meiosis • 2 sequential cell divisions to produce non-identical haploid cells (gametes) • Prepare for sexual reproduction Parent cell DNA replicates Chromatids separate Chromosomes • A typical human cell nucleus contains 46 chromosomes made of DNA. • You get 23 from mom and 23 from dad. DNA When DNA is being used for daily activities it is called chromatin. • Chromatin: the relaxed fibers of DNA. DNA “Packing” When it is time for mitosis, the DNA is copied and “packed” for the move. DNA “Packing” To “pack”, DNA molecules wrap around proteins called histones and form a rodshaped structure called a chromosome. Chromatin • Uncoiled DNA • Not visible under microscope Histones • Proteins that help DNA supercoil Sister Chromatids • Each half of the chromosome • Identical copies of DNA Centromere • Center point in chromosomes where the sister chromatids are connected A Chromosome Chromatid Centromere Sister Chromatids Chromosome Chromosome Segments Chromosomes are divided into segments called Gene genes. • A small segment of DNA that codes for a protein that results in a particular trait. • Alternate forms of a gene are called alleles. Chromosome Pairs • Chromosomes come in pairs. • Half of the pair came from your mother, the other from your father. • The two chromosomes when together are called sister chromosomes or homologous chromosomes. A Pair of Chromosomes Chromosome from Mother Chromosome from Father Homologous Chromosomes Diploid • 2 of each type of chromosome • One from father, one from mother • All body cells Haploid • 1 of each type of chromosome • Only in gametes (reproductive cells) Aneuploid • Wrong number of chromosomes • Extra or missing chromosomes Karyotype Cell Cycle • Life cycle of the cell. • Made up of the events from one mitosis to another. –Phases are: interphase, mitosis, and cytokinesis Interphase • DNA is uncoiled and “working” • Growth, DNA replication, cell prepares to divide • 90% of time is interphase Mitosis • Division of nucleus • Produces 2 identical “daughter” nuclei Cytokinesis • Division of cytoplasm into 2 identical “daughter” cells • Time for growth and development. • Cells spend most of their time in interphase. • The end of interphase is spent preparing for mitosis. – G1: growth 1, doubles in size and gets large enough to divide – S: synthesis phase, copies DNA (replication) – G2: growth 2, copies organelles and makes proteins, enzymes that will be needed for mitosis. Mitosis • Follows interphase • Divides the genetic material (DNA) only. • Steps involved: – Prophase – Metaphase – Anaphase – Telophase Reasons For Mitosis • Multicellular Organisms • Growth • Repair • Replacement of dead cells/ Maintenance • Maturation (full grown) • Unicellular organisms • Asexual reproduction Growth and Maintenance • Growth: an increase in size because of the increase in the number of cells. • Maintenance: replacing cells that were dead or damaged. • Chromosomes form • Nuclear membrane and nucleolus disappear • Centrioles move to opposite poles • Spindle fibers begin to form. • Fibers made by the centrioles that pull the chromosome copies apart during mitosis • “Middle” phase • The chromosomes are pushed and line up on the equator or the center of the cell. • Centromere holding the chromatids together splits. • Spindle fibers shorten and pull the chromatids apart. • • • • Chromosomes are at opposite poles. Centrioles and spindle fibers disappear. The chromosomes begin to uncoil. The nuclear membrane and nucleolus reappear. “Reverse Prophase” Cytokinesis • Follows mitosis • Splits the cytoplasm only. • Results in two identical daughter cells. • The beginning of the process is indicated by a cleavage furrow in an animal cell and a cell plate in a plant cell. Summary of Mitosis Prophase Metaphase • Chromatin supercoils into Chromosomes • Nuclear membrane disappears • Chromosomes line up along the middle of the cell • Spindle fibers begin to pull sister chromatids apart • Sister chromatids move along spindle fibers toward opposite ends of cell • Spindle fibers lengthen and elongate the cell by Anaphase pushing the poles further apart Telophase • Daughter chromosomes reach opposite poles of cell • Nuclear membranes form again