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Meiosis Animation Meiosis • A second type of cell division designed to make haploid cells called gametes • Gametes include sperm and egg • Meiosis occurs in 2 parts • Meiosis 1 and Meiosis 2 • Meiosis 1 is a reductive division where homologous chromosomes line up together on a spindle fiber • Therefore, # spindle fibers required = half of diploid number = haploid number Meiosis Tutorial: The Biology Project! • Structure formed when chromosomes come together is called a tetrad • A tetrad consists of 2 replicated chromosomes or 2 chromatids • Chromatids are chromosomes that are still attached to their sister strand via a centromere • When centromere eventually splits and chromatids are free of their sister chromatid, both are called chromosomes…. • this does not happen until anaphase 2 Excellent Narrated Movie of Meiosis Main Difference between Mitosis and Meiosis • During prophase 1, homologous (similar) chromosomes come together • This process is known as synapsis • During this time the chromosomes are so close together that crossing over can occur • This leads to genetic variation since sections of the DNA are exchanged mixing maternal and paternal genes Prophase 1 • • • • Nuclear membrane disintegrates Spindle fibers form Chromosomes become distinct Homologous (similar) chromosomes come together • This process is called synapsis Stages of Meiosis Always ask yourself “What’s going on here?” Stages of Meiosis Metaphase 1 • Homologous replicated chromosomes line up on the same spindle fiber • Individual chromatids in the replicated chromosomes do not separate yet • Not until Anaphase 2 • In human cell, since 2n = 46 • 23 pairs line up • Therefore 23 spindle fibers are needed • If 2n = 10, 5 spindle fibers are required for this stage Stages of Meiosis Anaphase 1 • Homologous replicated chromosomes separate to opposite poles • Centromeres do not split yet Stages of Meiosis Telophase 1 • • • • • • Chromosomes arrive at opposite poles Cytoplasm pinches in Cytokinesis occurs in animal cells Equatorial plate forms in plant cells 2 haploid cells form Chromosomes may become chromatin and enter an interphase 2 in some cells Stages of Meiosis Prophase 2 • • • • New spindle fibers form Nuclear membrane dissolves Chromosomes become visible If 2n = 10, then only five spindle fibers needed…why? • Half of the chromosomes are in the other cell Stages of Meiosis Metaphase 2 • Chromosome line up on new spindle fibers • Chromosomes here are made up of 2 chromatids • If 2n = 46, 23 chromosomes are in each cell produced from telophase 1 • This means that 23 spindle fibers must be present • If 2n = 8, 4 spindle fibers needed now Stages of Meiosis Anaphase 2 • Centromere splits • Chromatids (now called chromosomes) migrate to opposite poles of the cell Stages of Meiosis Telophase 2 • Chromatids (now called chromosomes) arrive at poles • Cytoplasm starts to pinch in • Cytokinesis occurs • End result is 4 haploid cells • 2 from each of the 2 cells produced at the end of telophase 1 Stages of Meiosis Notice the difference Stages of Meiosis End result of Overview of Meiosis 1 Overview of Meiosis 1 Overview of Meiosis II Meiosis in males and females • In males called Spermatogenesis • Involves production of 4 equal sized cells each with same amount of cytoplasmic organelles • In females called Oogenesis • First division unequal One cell is a polar body and smaller • Second division produces another smaller cell called a polar body and then the Egg • Egg contains the majority of cytoplasmic organelles Oogenesis Spermatogenesis Meiosis Produces Sperm Cells Structure of Human Sperm Abnormal Meiosis • When homologous chromosomes fail to separate during anaphase of meiosis… • Abnormal gametes form with abnormal numbers of chromosomes (trisomy 21 etc..) • Process called non-disjunction • Karyotyping used to detect disorders • Chromosomes are arranged according to 3 characteristics – Centromere position – Banding pattern – Size Non-disjunction animation Sources of Variation 1. Gamete success 2. Crossing over depends on distance between genes and length of chromosome 3. Independent Assortment the way one homologous pair lines up has nothing to do with the way another homologous pair lines up! Animation of Crossing Over Another great animation!! Homologous pair of chromosomes Independent Assortment Detecting genetic Disorders • Ultrasound locates position of fetus • Amniocentesis uses a needle to withdraw some amniotic fluid from around fetus • Fluid is in uterus • Fetal cells can be analysed • Chorionic Villus Sampling (CVS) draws cells from outer membrane of embryo • CVS can be used as early as 8 weeks of pregnancy Animal Life Cycle Plant Life Cycle Alternation of Generations Sexual Stage Alternates with Asexual Plant and Animal Life Cycle Meiosis Oogenesis Spermato-genesis Sperm Egg (Ovum) Zygote Mitosis Embryo Fetus Child Moss Life Cycle Demonstrates Alternation Of Generations Alternation of Generations • Moss Plant • Fern Plant • Sexual stage alternates with an asexual stage • Sexual stage involves fertilization by union of 2 gametes (needs water) • Asexual stage involves spore production • One cannot exist without the other • Mitosis & Meiosis occurs • Sexual & Asexual stage separate • Same • Same • Same • Same • Sexual & Asexual stage on same plant Fern Life Cycle Alternation of generations Alternation of Generations in Fern Plants Cell Division Tutorials/Questions