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Cell Division A. Purpose of Cell Division To create new cells or offspring Copy all of the chromosomes/genes exactly to create body cells (mitosis) or vary the gene combinations and pass on half of the chromosomes to form sex cells (meiosis) B. Mitosis and Types of Asexual Reproduction in Organisms Also known as asexual reproduction or cloning Reproductive process involving only one parent resulting in two offspring that are genetically identical 1. Vegetative Propogation/Reproduction Occurs in plants New plant is produced from an existing plant structure Cutting off a branch will develop into a new plant 2. Sporulation: Spores (reproductive cells) are released from mold and develop into new mold cells 3. Budding: Yeast cell produce offspring by forming a little “bud” that pinches off 4. Regeneration: When a portion of the organism is lost but is replaced with an identical structure Ex. Starfish, earthworm, lizard 5. Binary Fission: Single-celled organisms copy its chromosomes and then divide producing 2 smaller offspring C. Phases of Mitosis Diploid (2n): Normal number of chromosomes for a particular species Chromosomes are paired = homologous (have the same genes) Human diploid number = 46 (23 pairs) Ex. A cell contains 4 chromosomes 1. Interphase: period between cell divisions growth and preparation for the next mitosis a. G1 phase: cell finished mitosis and is now growing b. S phase: chromosomes replicate (copy) [46 to 92] c. G2 phase: centrioles (animal cells only) and spindles form G1 phase: Growth S phase: Chromosomes double (4 to 8) G2 phase: Centrioles and spindles form 2. M phase (Mitosis) = PMAT 4 phases a. Prophase: The replicated DNA or chromatin coils up to form chromosomes The chromosomes find their identical match and are joined together by a centromere Each chromosome in this new pair is called a sister chromatid Centrioles move to opposite sides of the cell (animal only) Spindles attach the centriole to each centromere and will help separate the chromatid pairs in the next few phases b. Metaphase: Chromosomes (chromatid pairs) line up in the middle of the cell Each pair lines up under each other Each chromatid is attached to its own spindle fiber c. Anaphase: Sister chromatids separate Each individual chromosome moves to the opposite side of the cell d. Telophase: Reverse prophase Chromosomes uncoil to form chromatin The cell membrane pinches in Nucleus with nucleolus reappears Cytokinesis: separation of cytoplasm to form 2 new cell i. animal cells: cell membrane pinches in ii. plant cells: cell plate forms in the middle to separate the 2 nuclei and cytoplasm eventually becoming a new cell wall C. Regulating the Cell Cycle Proteins/Enzymes: control cell division Cancer: uncontrolled cell division (mitosis) Caused by mutations: changes in DNA causing a change in the protein that starts or stops cell division Due to: smoking, radiation (x-rays, sun), infections (bacteria, virus), chemicals (pollutants) D. Summary of Mitosis 1 cell forms 2 cells Genetically identical, clones, asexual reproduction, mitosis 1 diploid (2n) cell forms 2 diploid cells Occurs in all body cells except sex cells (gametes: egg/sperm) E. Meiosis/Sexual Reproduction Homologous Chromosomes: Paired chromosomes (have genes that code for the same trait) One from the female parent and one from the male parent Diploid Cells: Number in humans = 46 (23 pairs) half of from each parent Contain both homologous chromosomes These cells contain a complete set of genes to create traits All body cells are diploid except gametes Haploid/Monoploid Cells (n): Cells that only contain one of the homologous chromosomes Cells have half the pair Only gametes (sex cells= egg/sperm, oocyte or ovum/spermatocyte) Haploid (n) number in humans = 23 (unpaired chromosomes) 1. Meiosis: a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to become haploid 2 cell divisions Formation of sex cells/gametes Creates 4 haploid cells from 1 diploid cell Creates variation in gametes Ex. Cell with 2 chromosomes 2n = 2, n = 1 a. Meiosis I: 1. Interphase: DNA is copied 2. Prophase I: homologous chromosomes pair up to form a tetrad Tetrad: 2 sets of homologous chromosomes or 4 chromatids All have the same genes in the same position Crossing Over/Genetic Recombination: homologous chromosomes exchange portions of genetic information when in tetrad formation o Creates 4 different chromatids o Chromosomes #2 and #3 switch portions 3. Metaphase I: tetrads line up in the middle, not underneath 4. Anaphase I: tetrad separates and homologous chromosomes (now varied) move to opposite ends 5. Telophase I: 2 diploid cells are formed but they are not identical as the original diploid cell; variation in chromosomes due to crossing-over b. Meiosis II: second cell division to form 4 cells 1. Prophase II: no replication of chromosomes, spindle forms, chromosomes stay exactly as they were at the end of telophase I 2. Metaphase II: chromosomes line up in homologous pairs the same way as in mitosis (in the middle) 3. Anaphase II: the chromatids separate to opposite sides 4. Telophase II: 4 haploid cells are formed Not identical to each other because of crossing over Ensures that no 2 sperm or egg are identical Cytokinesis 2. Meiosis Explains Mendel’s Results When Mendel crossed 2 hybrid plants it resulted in 25% of the offspring inheriting the recessive trait Anaphase I explains how the segregation of alleles for the recessive trait at random can produce recessive offspring 3. Mistakes in Meiosis: Can be beneficial to plants creating new varieties or desired traits beneficial to farmers Most of the time it is harmful to humans a. Nondisjuction: failure for homologous chromosome pairs to separate properly during anaphase I Problems separating the tetrads Results in gametes with too many or too few chromosomes i. Trisomy 21 = Down Syndrome: Extra chromosome #21 = cell with 47 ii. Monosomy = Turner Syndrome: One less chromosome = cell with 45 4. Meiosis Summary: 4 haploid cell Variations, not identical 4 sperm, 1 egg + 3 polar bodies Diploid Diploid Variations sperm egg sperm polar body All Haploid sperm polar body sperm polar body Comparing Mitosis to Meiosis Mitosis Meiosis Asexual Reproduction Sexual Reproduction Cloning Gametogenesis Identical Cells Cells with variation – crossing over All body cells Tissue repair Gametes Only Sex cells (egg/sperm) 1 Diploid = 2 Diploid 1 Diploid = 4 Haploid 2n = 46 2n = 46 2n = 46 1 Cell Division n = 23 2 Cell Divisions