8/21/2016 What is Meiosis? GENETICS A division of the nucleus that reduces • INHERITED: GENES ARE INHERITED FROM YOUR PARENTS. OFFSPRING RESEMBLE THEIR chromosome number by half. PARENTS. GENES CODE FOR CERTAIN TRAITS THAT ARE PASSED ON FROM GENERATION TO GENERATION. •Important in sexual reproduction • •Involves combining the genetic • HEREDITY #2: HEREDITY IS THE PASSAGE OF THESE GENES FROM GENERATION TO information of one parent with that of GENERATION. EACH GENE IS A SET OF CODED INSTRUCTIONS FOR A SPECIFIC TRAIT. • the other parent to produce a • CHROMOSOME THEORY: CHROMOSOMES THAT SEPARATE DURING MEIOSIS ARE THE SAME AS THE CHROMOSOMES THAT UNITE DURING FERTILIZATION. GENES ARE CARRIED genetically distinct individual ON THOSE CHROMOSOMES. Homologous Chromosomes Similar chromosomes that are found in pairs. The paired TERMINOLOGY chromosomes come from the mother and father. * Human body cells have 46 chromosomes each • DIPLOID - TWO SETS OF CHROMOSOMES (2N), IN HUMANS * Human body cells have 23 homologous pairs 23 PAIRS OR 46 TOTAL • HAPLOID - ONE SET OF CHROMOSOMES (N) - GAMETES OR Meiosis and Fertilization SEX CELLS, IN HUMANS 23 CHROMOSOMES • HOMOLOGOUS PAIR Important for survival of many species, because these processes • EACH CHROMOSOME IN PAIR ARE IDENTICAL TO THE OTHER ( result in genetic variation of offspring. CARRY GENES FOR SAME TRAIT) • ONLY ONE PAIR DIFFERS - SEX CHROMOSOMES X OR Y Meiosis A kind of cell division that results in gametes (sex cells) with half the number of chromosomes. Chromosomes Cell from parentsMEIOSIS PHASES HOMOLOGOUS PAIR Nucleus PMAT X2 Chromosomes from both Chromosomes parents are copied & replicate paired to exchange DNA • CONTAIN THE SAME TYPE Like chromosomes OF GENES IN THE SAME pair up LOCATION This creates a mix of new genetic material in Chromosomes swap • ARE ABOUT THE SAME SIZE the offspring’s cell sections of DNA • SHARE THE SAME SIZE Chromosome pairs divide CHROMATIDS Nucleus divides into Chromosomes divide, daughter cells daughter cells have single chromosomes and a new Daughter cell nucleus mix of genetic material divides again 1 8/21/2016 A. Gametes do not have homologous pairs, but instead they have only half the chromosomes from each pair. Gametes MEIOSIS are referred to as haploid (n). INTERPHASE B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n). WHAT’S HAPPENING C. In meiosis, specialized cells divide 2 times producing • ONLY HAPPENS ONCE! 4 daughter cells. • CELL IS GROWING • REPLICATING ITS DNA • CARRYING OUT CELL PROCESSES OF THE SPERM AND EGG CELL • CYTOKINESIS IS THE LAUNCHING POINT OF THE CELL DIVISION PROCESS IN BOTH MEIOSIS AND MITOSIS. Phases of Meiosis (2 Cell divisions – Meiosis I & Meiosis II Metaphase I Reduction Division) MEIOSIS I Spindle fibers attach to chromosomes and line them up in the middle of the cell. Prophase I Homologous chromosomes pair together to form tetrads Anaphase I The fibers pull the homologous chromosomes apart. This is called disjunction. Chromosomes move toward opposite sides During Prophase I, crossing over occurs between homologous of the cell. chromosomes and genes are exchanged causing genetic variation Telophase I MEIOSIS II During Telophase I and cytokinesis, the nuclear membrane forms Prophase II and the cell splits into two new cells. Spindle fibers form and attach to double stranded chromosomes Metaphase II **** The Chromosome number is reduced to half during Chromosomes line up in the middle of the cell Meiosis I forming haploid (n) daughter cells 2 8/21/2016 Anaphase II MEIOSIS Sister chromatids separate and move towards opposite sides of the cell Parent cell – chromosome pair Telophase II Chromosomes copied Cells divide, resulting in 4 haploid cells – gametes (sex cells) 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes SEXUAL REPRODUCTION Fertilization n Sex cells formed by meiosis The union of an egg and a sperm Requires 2 parents Offspring not identical Zygote n 2n Meiosis The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes. The process that results in the formation of gametes Somatic Cells Gametes All body cells except for sex cells that have diploid chromosomes Sex cells, egg or sperm, which have haploid chromosomes Haploid Diploid Haploid means HALF, as in the number of chromosomes, Diploid means they have chromosome found in pairs, written as n written as 2n 3 8/21/2016 MEIOSIS – WRITE THIS DOWN!! REVIEW MITOSIS & MEIOSIS • BOTH ARE FORMS OF NUCLEAR DIVISION, BOTH INVOLVE REPLICATION • IN ASEXUAL REPRODUCTION, A CELL DIVIDES TO FORM TWO • BOTH INVOLVE DISAPPEARANCE OF THE NUCLEUS, AND NUCLEOLUS, NUCLEAR MEMBRANE IDENTICAL CELLS. PLANTS AND PRODUCERS CAN DO THIS • BOTH INVOLVE FORMATION OF SPINDLE FIBERS • IN SEXUAL REPRODUCTION A SPECIALIZED CELL FROM A FEMALE • MEIOSIS PRODUCES DAUGHTER CELLS THAT HAVE 1/2 THE NUMBER OF CHROMOSOMES AS (EGG) MERGES WITH A SPECIALIZED CELL FROM A MALE (SPERM) THE PARENT. GO FROM 2N TO 1N. DURING A PROCESS CALLED FERTILIZATION TO FORM A ZYGOTE. • DAUGHTER CELLS PRODUCED BY MEIOSIS ARE NOT GENETICALLY IDENTICAL TO ONE ANOTHER. • MEIOSIS IS THE PROCESS THAT FORMS SEX CELLS (EGGS AND SPERM). • MEIOSIS IS ABOUT REDUCTION DIVISION YOU START WITH 46 IN THE PARENT CELL AND END ONE CYCLE OF MEIOSIS IN A MALE RESULTS IN FOUR SPERM, AND IN WITH 23 A FEMALE ONE CYCLE MAKES ONE EGG, AND THREE POLAR BODIES. • IN MEIOSIS CELL DIVISION TAKES PLACE TWICE BUT REPLICATION OCCURS ONLY ONCE. THE SEX CELLS MADE DURING MEIOSIS HAVE HALF AS MANY • YOU DON’T WANT CELLS DIVIDING ALL THE TIME….OTHERWISE IT DOESN’T HAVE TIME TO CHROMOSOMES AS DO BODY CELLS DO ITS REAL JOB. MOST OF A CELLS LIFE IS THEREFORE SPENT IN INTERPHASE • REMEMBER : BODY CELLS – DIPLOID 46 SEX CELLS – 23 HAPLOID • CANCER HAPPENS GENETICALLY, THROUGH RISK FACTORS, OR FROM AN OVERGROWTH OF CELLS THAT ESCAPE YOUR BODY’S NATURAL IMMUNE SYSTEM CONTROL SYSTEMS. 4.