Cell Division Grade 9 PDF

CELL DIVISION Biology – grade 9  Learning Objetive : To understand the stages of cell division (mitosis and meiosis) DNA DNA is located in the nucleus and controls all cell activities including cell division Long and thread-like DNA in a non-dividing cell is called chromat...

CELL DIVISION Biology – grade 9  Learning Objetive : To understand the stages of cell division (mitosis and meiosis) DNA DNA is located in the nucleus and controls all cell activities including cell division Long and thread-like DNA in a non-dividing cell is called chromatin Doubled, coiled, short DNA in a dividing cell is called chromosome Consists of 2 parts: chromatid and centromere o 2 identical “sister” chromatids attached at an area in the middle called a centromere o When cells divide, “sister” chromatids separate and 1 goes to each new cell  Chromatin to chromosomes illustration: Chromatin Coils up into Duplicates chromosomes itself Why does DNA need to change More efficient division from chromatin to chromosome? Chromosome number Every organism has its own specific number of chromosomes Examples: Human = 46 chromosomes or 23 pairs Dog = 78 chromosomes or 39 pairs Goldfish = 94 chromosomes or 47 pairs Lettuce = 18 chromosomes or 9 pairs  All somatic (body) cells in an organism have the same kind and number of chromosomes Examples: Human = 46 chromosomes Human skin cell = 46 chromosomes Human heart cell = 46 chromosomes Human muscle cell = 46 chromosomes Fruit fly = 8 chromosomes Fruit fly skin cell = 8 chromosomes Fruit fly heart cell = 8 chromosomes Fruit fly muscle cell = 8 chromosomes Cell Division Cell Division — process by which a cell divides into 2 new cells MITOSIS MEIOSIS Why do cells need to divide? 1.Living things grow by producing more cells, NOT because each cell increases in size 2.Repair of damaged tissue 3.If cell gets too big, it cannot get enough nutrients into the cell and wastes out of the cell Mitosis – division of the nucleus into 2 nuclei, each with the same number of chromosomes Mitosis occurs in all the somatic (body) cells Why does mitosis occur? So each new daughter cell has nucleus with a complete set of chromosomes Cell Cycle -- series of events cells go through as they grow and divide Cell grows, prepares for division, then divides to form 2 daughter cells – each of which then begins the cycle again  The cell cycle ▪ The cell cycle is an ordered sequence of events that extends ◦ from the time a cell is first formed from a dividing parent cell ◦ until its own division. © 2012 Pearson Education, Inc.  Interphase ◦ 3 phases ◦ G1 phase= cells do most of their growing ◦ Increase in size and synthesize new proteins and organelles ◦ S phase= chromosomes are replicated and the synthesis and DNA molecules takes place ◦ Usually if a cell enters S phase and begins replication, it completes the rest of the cycle ◦ G2 phase= many of the organelles and molecules required for cell division are produced ◦ Shortest of the 3 phases of interphase Interphase—period of cell growth and development DNA replication (copying) occurs during Interphase During Interphase the cell also grows, carries out normal cell activities, replicates all other organelles The cell spends most of its life cycle in Interphase  4 phases of nuclear division (mitosis), directed by the cell’s DNA (PMAT) Prophase Metaphase—(Middle) Anaphase—(Apart) Anaphase—(Apart) Telophase—(Two) Prophase Chromosomes coil up Nuclear envelope disappears Spindle fibers form Metaphase—(Middle) Chromosomes line up in middle of cell Spindle fibers connect to chromosomes Anaphase—(Apart) Chromosome copies divide Spindle fibers pull chromosomes to opposite poles Telophase—(Two) Chromosomes uncoil Nuclear envelopes form 2 new nuclei are formed Spindle fibers disappear Cytokinesis — the division of the rest of the cell (cytoplasm and organelles) after the nucleus divides In animal cells the cytoplasm pinches in In plant cells a cell plate forms After mitosis and cytokinesis, the cell returns to Interphase to continue to grow and perform regular cell activities Summary: Cell Cycle Interphase Mitosis (PMAT) Cytokinesis When cells become old or damaged, they die and are replaced with new cells  Chromosome Appearance & Phase Important Events Location DNA copies itself; chromatin DNA replication, cell grows and Interphase replicates organelles Chromosomes coil up Nuclear envelope disappears, Prophase spindle fibers form Chromosomes line up in the Spindle fibers connect to Metaphase middle chromosomes Chromosome copies divide Spindle fibers pull chromosome Anaphase and move apart copies apart to opposite poles Chromosomes uncoil back Nuclear envelopes reform, 2 new Telophase into chromatin nuclei are formed, spindle fibers disappear Chromatin Division of the rest of the cell: Cytokinesis cytoplasm and organelles Meiosis ◦ Production/formation of gamete ◦ Basis of sexual reproduction ◦ Only germ cells undergo meiosis  Haploid gametes (n = 23) n Egg cell n Sperm cell Meiosis Fertilization Ovary Testis Diploid zygote 2n (2n = 46) Key Mitosis Haploid stage (n) Multicellular diploid adults (2n = 46) Diploid stage (2n)  How meiosis halves chromosome number… INTERPHASE MEIOSIS I MEIOSIS II Sister chromatids 1 2 3 A pair of A pair of homologous duplicated chromosomes homologous in a diploid chromosomes parent cell  MEIOSIS I: Homologous chromosomes separate INTERPHASE: Chromosomes duplicate Prophase I Metaphase I Anaphase I Centrosomes Spindle microtubules Sister chromatids (with centriole Sites of crossing over attached to a kinetochore remain attached pairs) Centrioles Spindle Tetrad Chromatin Sister Metaphase Nuclear Centromere envelope chromatids Fragments (with a plate Homologous of the kinetochore) chromosomes nuclear separate envelope  MEIOSIS II: Sister chromatids separate Telophase II Prophase II Metaphase II Anaphase II and Cytokinesis Sister chromatids Haploid daughter separate cells forming  Meiosis Leads to Genetic Diversity ◦ Three ways genetic diversity is increased by meiosis: 1. 2 parents contribute ½ of the genetic material to offspring 2. Crossing-over in Prophase I 3. Chromosome Alignment in Metaphase I ◦ Meiosis produces cells that are NOT identical, unique gametes  Interphase I  Similar to mitosis interphase.  CHROMOSOMES (DNA) replicate in the S phase  Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.  CENTRIOLE pairs also replicate.  Interphase I Nucleus and nucleolus visible. Nucleus chromatin cell membrane nucleolus  Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. Four phases: a. Prophase I b. Metaphase I c. Anaphase I Prophase I d. Telophase I  Prophase I Longest and most complex phase (90%). Chromosomes condense. Synapsis occurs - Homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).  Non-Sister Chromatids- HOMOLOGS Homologs contain DNA that codes for the same genes , but different versions of those genes Genes occur at the same loci  Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids  Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits. Each locus (position of a gene) is in the same LOCI position on homologues. Humans have 23 pairs of homologous chromosomes: a. First 22 pairs of autosomes b. Last pair of sex chromosomes Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal  Crossing Over Crossing over may occur between non-sister chromatids at sites called chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) Causes Genetic Recombination  Genetic Recombination nonsister chromatids Tetrad chiasmata: site of crossing over variation  Sex Chromosomes XX chromosome - female XY chromosome - male  MEIOSIS I Meiosis I Homologs separate  Prophase I Nucleus & Nucleolus disappear Spindle forms Chromosomes coil & Synapsis (pairing) occurs Tetrads form & Crossing over Occurs spindle fiber centrioles aster fibers TETRAD  Metaphase I ◦ Shortest phase ◦ Tetrads align on the equator. ◦ Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION  Metaphase I OR Homologs line up at equator or metaphase plate Formula: 2n Example: 2n = 4 then 1n = 2 thus 22 = 4 combinations  Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.  Anaphase I Homologs separate  Telophase I Each pole now has haploid (1n) set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed. Telophase I cytokinesis  MEIOSIS II Sister Chromatids Meiosis II Separate 56  Meiosis II  No Interphase II or very short  No DNA Replication Remember: Meiosis II is similar to mitosis  Prophase II Same as Prophase in mitosis Nucleus & nucleolus disappear Chromosomes condense Spindle forms  Metaphase II ◦Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator  Anaphase II Same as Anaphase in mitosis SISTER CHROMATIDS separate  Telophase II Same as Telophase in mitosis. Nuclei and Nucleoli reform, spindle disappears CYTOKINESIS occurs. Remember: FOUR HAPLOID DAUGHTER cells are produced. Called GAMETES (eggs and sperm) 1n Sperm cell fertilizes 1n egg to form 2n zygote Telophase II  Variation Also known as GENETIC RECOMBINATION Important to population as the raw material for NATURAL SELECTION. All organisms are NOT alike Strongest “most fit” survive to reproduce & pass on traits  Summary of Meiosis MEIOSIS 1 A reduction division Early prophase → late prophase → metaphase → anaphase → telophase MEIOSIS 2 A copying division Late prophase → metaphase → anaphase → telophase → cytokinesis At the end of meiosis four new, non-identical, hapoid cells are formed from one parent cell, each with half the original number of chromosomes.The gametes are not identical to the parent cell. Spermatogenesis n=23 human sex cell sperm n=23 n=23 2n=46 n=23 diploid (2n) n=23 haploid (n) n=23 Meiosis I Meiosis II Oogenesis Haploid n=23 (1n) human egg sex cell n=23 2n=46 Polar Bodies (die) diploid (2n) n=23 Meiosis I Meiosis II Comparison Comparison