Lecture 19: Dividing the Cell II - Meiosis Bio 93: DNA to Organisms Lecture 19: Dividing the Cell II - Meiosis What’s wrong with this karyotype?

Lecture 19: Learning Objectives 19.1 – Describe the features of chromosomes and diploid and haploid cells 19.2 – Explain what happens in each step of meiosis and how it reduces chromosome number 19.3 – Explain how meiosis contributes to genetic variation 19.4 – Predict how abnormal chromosome number or alterations in chromosome structure can lead to disorders

Lecture 19: Key terms Heredity Haploid Synapsis Variation Meiosis Chiasmata Genetics Zygote Independent assortment Genes Fertilization Gametes Meiosis I Random fertilization Nondisjunction Somatic cells Meiosis II Locus Prophase I Aneuploidy Monosomy Asexual reproduction Metaphase I Trisomy Sexual reproduction Anaphase I Karyotype Telophase I Polyploidy Deletion Homologous chromosomes Prophase II Duplication Metaphase II Sex chromosomes Anaphase II Inversion Autosomes Translocation Telophase II Diploid Crossing over

Down syndrome results from an extra copy of chromosome 21 How could a cell get an extra copy of a chromosome?

1 Which statement is TRUE? A – Skin cells go through both mitosis and meiosis B – Skin cells go through mitosis but not meiosis C – Skin cells go through meiosis but not mitosis 19.1

replication replication division division division “division” “reduction division” Bio 93 (Summer 2017), Copyright © 2017, Dr. Salme Taagepera. All rights reserved.

2 What is true about this cell? Select all that apply. A – The cell is 2n = 4 B – The cell is haploid C – The cell has gone through S phase D – The cell has two pairs of homologous chromosomes E – The cell is a gamete 2 Pair of homologous chromosomes Diploid – two sets of chromosomes (one from mom AND one from dad) Haploid – one set of chromosomes (one from mom OR one from dad) n = number chromosomes in a single set Sister chromatid mom dad

1 Is each cell diploid or haploid? 19.2 Is each cell diploid or haploid? How many haploid cells are on the screen? Click in a number!

How meiosis reduces chromosome number 19.2 Figure 13.7

Figure 13.10a Figure 13.10 MITOSIS MEIOSIS Prophase Duplicated chromosome Metaphase Anaphase Telophase 2n Daughter cells of mitosis Sister chromatids separate. Individual chromosomes line up. Chromosome duplication 2n = 6 Parent cell Chiasma MEIOSIS I Prophase I Homologous pair Metaphase I Anaphase I Telophase I II Pairs of homologous Homologs chroma- tids Daughter cells of meiosis II Daughter cells of meiosis I n Figure 13.10a Figure 13.10a A comparison of mitosis and meiosis (part 1: mitosis vs. meiosis art) Figure 13.10

Meiosis promotes genetic variation through: 1 1. Independent assortment 2. Crossing over Figure 13.11 3. Random fertilization 19.3

Breakdown of nuclear envelope What happens if the mitotic spindle does not form properly during meiosis? Breakdown of nuclear envelope Prophase I Metaphase I Two groups of mice, A and B. One is healthy, one is abnormal What can you conclude? A – The spindle is not forming properly in mice A B – The spindle is not forming properly in mice B Mice A Mice B 2 19.2

A normal mouse oocyte has 20 chromosomes A normal mouse oocyte has 20 chromosomes. As shown in the graph, an error could lead to a mouse oocyte with 21 chromosomes. If this oocyte is fertilized by a normal sperm, the zygote would have 41 chromosomes. Which of the following best explains what happened to result in this zygote? 2 19.4 Gray – wild type Green – mutant A – An inversion occurred leading to monosomy of a chromosome B – A non-disjunction occurred leading to trisomy of a chromosome C – A duplication occurred leading to trisomy of a chromosome D – A deletion occurred leading to monosomy of a chromosome

homologous chromosomes Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes n + 1 n n − 1 Figure 15.13 All gametes abnormal Half gametes abnormal Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II 19.4

Meiosis is an error-prone process Trisomy 21 Trisomy 13 Trisomy 18 Roughly 5% of all conceptuses are trisomic or monosomic. 80 - 90 % of trisomies and monosomies result from non-disjunction at maternal M I. Analysis of human gametes: 20% oocytes and 3% of sperm have incorrect number of chromosomes. The most common types of autosomal trisomy that survive to birth in humans are: Trisomy 21 (Down's syndrome) = birth defects, intellectual disabilities, characteristic facial features. Often involves heart defects, visual and hearing impairments. Trisomy 18 (Edwards syndrome) = 50% of babies who are carried to term will be born alive, 10%+ may survive to their first birthdays. Trisomy 13 (Patau syndrome) = 20% of children with Patau syndrome survive to their first birthday. D103 lecture 24 © 2013 Grant MacGregor & Christine Sütterlin – All rights reserved

Summary Number of chromosomes Bio 93 (Summer 2017), Copyright © 2017, Dr. Salme Taagepera. All rights reserved.