Meiosis & Sexual Reproduction PDF

Because learning changes everything. ® Biology Sylvia S. Mader...

Because learning changes everything. ® Biology Sylvia S. Mader Michael Windelspecht Chapter 10 Meiosis & Sexual Reproduction Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.  Outline  10.1 Overview of Meiosis.  10.2 Genetic Variation  10.3 The Phases of Meiosis  10.4 Meiosis Compared to Mitosis  10.5 The Cycle of Life.  10.6 Changes in Chromosome Number and Structure 2  The Importance of Meiosis  Meiosis  It introduces an enormous amount of diversity.  There are more than 70 trillion different genetic combinations possible from the mating of two individuals.  Males and females differ in the way they form gametes.  In males, sperm production begins at puberty, but in the female, the process of producing eggs starts before birth and ends at menopause. 3  10.1 Overview of Meiosis  Meiosis  Special type of cell division  Used only for sexual reproduction  Chromosomes are replicated in the S phase & then halved prior to fertilization  Parents are diploid (2n)  Meiosis produces haploid (n) gametes  Gametes fuse in fertilization to form a diploid (2n) zygote  The zygote becomes the next diploid (2n) generation.  Important to conserve number of chromosomes.  If the events of meiosis go wrong, gametes will contain the wrong number of chromosomes. 4 Homologous Pairs of Chromosomes In diploid body cells, chromosomes occur in pairs. Humans have 23 different types of chromosomes. Diploid (2n) cells have two chromosomes of each type. Chromosomes of the same type are said to be homologous chromosomes (homologues). They have the same length. Their centromeres are positioned in the same place. One came from the father (the paternal homologue); the other from the mother (the maternal homologue). When stained, they show similar banding patterns. Homologous Chromosomes (1)  Homologous Chromosomes and Alleles Homologous copies of a gene may encode identical or different genetic information. The variants that exist for a gene are called alleles. An individual may have: Identical alleles for a specific gene on both homologues (homozygous for the trait), or. A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait). Example: a gene coding for short fingers on one homologue and a gene coding for long fingers at the same location on the other. AA or aa homozygous Aa heterozygous 7  Meiosis Is Reduction Division Meiosis involves two nuclear divisions: Meiosis I & Meiosis II. Meiosis I: Chromosomes are replicated prior to meiosis I.  Each chromosome consists of two identical sister chromatids. Homologous chromosomes pair up in synapsis.  Chromosomes may recombine or exchange genetic material. Homologous pairs align themselves against each other, side by side at the metaphase plate. The two members of a homologous pair separate. Each daughter cell receives one duplicated chromosome from each pair.  Chromosome number is reduced from 2n to n. 8 9 Meiosis II: DNA is not replicated between meiosis I and meiosis II. Sister chromatids separate and move to opposite poles. The four daughter cells contain one daughter chromosome from each pair. Each daughter chromosome consists of a single chromatid. The daughter cells are haploid. 10 Overview of Meiosis 11 Overview of Meiosis 12 13 14  Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division Second division Four haploid daughter cells centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n=2 n=2 MEIOSISI MEIOSISII Homologous pairs Sister chromatids separate, synapse and then separate. becoming daughter chromosomes. In plants the haploid daughter cells become spores 15  10.2 Genetic Variation  Genetic variation is essential for a species to evolve and adapt in a changing environment.  Meiosis brings about genetic variation in two key ways: 1. Crossing-over between homologous chromosomes, Exchange of genetic material between non-sister chromatids during meiosis I At synapsis, a (synaptonemal complex) appears between homologous chromosomes. 2. Independent assortment of homologous chromosomes align at metaphase plate , then separate in a random manner and are distributed to different daughter cells. 16  Crossing-Over of Nonsister Chromatids - Crossing-over: exchange of genetic material between nonsister chromatids of a bivalent in Meiosis I. - Chiasmata: regions where nonsister chromatids are attached. - After crossing-over => sister chromatids are no longer identical.  Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 18  Genetic Variation  Independent assortment  When homologous chromosome pairs align at the metaphase plate:  They separate in a random manner  The maternal or paternal homologue may be oriented toward either pole of mother cell  Causes random mixing of blocks of alleles into gametes 19  Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.  Fertilization and Crossing-Over Fertilization: union of male and female gametes. Chromosomes donated by the parents are combined. In humans, (233 )2 =  (423 )2 , or 4,951,760,200,000,000,000,000,000,000, genetically different zygotes are possible. Crossing-over may occur several times in each chromosome.  Significance of Genetic Variation Asexual reproduction produces genetically identical clones. Sexual reproduction causes genetic recombinations among members of a population. Asexual reproduction is advantageous when the environment is stable. However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous.  Some offspring may have a better chance of survival.  Example: If the temperature rises due to climate change, an animal with less fur or reduced body fat would have an advantage. 10.3The phases of Meiosis Prophase I  A spindle forms.  The nuclear envelope fragments.  The nucleolus disappears.  Each chromosome is duplicated (consists of two identical sister chromatids).  Homologous chromosomes pair up and physically align themselves against each other side by side (synapsis).  Synapsed homologues are referred to as a bivalent (two homologues) or a tetrad (four chromatids). Metaphase I  Homologous pairs are arranged at the metaphase plate.  Bivalents are aligned at the spindle independently of one another. Anaphase I  Homologous chromosomes of each bivalent separate from one another.  Homologues move towards opposite poles.  Sister chromatids do not separate.  Each is still a duplicated chromosome with two chromatids. Reduction of chromosome number from 2n to n. Telophase I  Daughter cells have one duplicated chromosome (n) from each homologous pair.  Interkinesis and Meiosis II Two haploid (n) daughter cells, each with one duplicated chromosome of each type. Interkinesis is similar to mitotic interphase, except. It is usually shorter. DNA replication does not occur.  Meiosis II:  Each Cell from meiosis II will give two other cells.  At the end of meiosis we get 4 cells genetically different from parent cell. 28  10.4 Comparison of Meiosis and Mitosis Mitosis:  One nuclear division.  2 daughter cells result, genetically identical to the parent cell. Meiosis:  Two nuclear divisions  4 daughter cells result, genetically different from the parent cell.  Meiosis Compared to Mitosis Meiosis Mitosis Requires two nuclear divisions Requires one nuclear division Chromosomes synapse and cross-over Chromosomes do not synapse nor cross- Centromeres survive Anaphase I over Centromeres dissolve in mitotic Halves chromosome number anaphase Produces four daughter nuclei Preserves chromosome number Produces daughter cells genetically Produces two daughter nuclei different from parent and each other Used only for sexual reproduction Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth  Similarities: Meiosis I and Mitosis An orderly series of stages is involved in the sorting and division of chromosomes. The stages include prophase, prometaphase, metaphase, and telophase. Spindle fibers play an active role in sorting chromosomes. Cytokinesis follows the end of the process to divide cytoplasm between daughter cells. 3  Meiosis I Compared to Mitosis Meiosis I Mitosis Prophase I Prophase Pairing of homologous No pairing of chromosomes chromosomes Metaphase I Metaphase Bivalents at metaphase Duplicated chromosomes at plate metaphase plate Anaphase I Anaphase Homologues of each Sister chromatids separate, bivalent separate, and becoming daughter duplicated chromosomes chromosomes that move to move to poles the poles Telophase I Telophase Two haploid daughter cells, Two diploid daughter cells, not identical to the parent identical to the parent cell cell 3  Meiosis II Compared to Mitosis Meiosis II Mitosis Prophase II Prophase No pairing of chromosomes No pairing of chromosomes Metaphase II Metaphase Haploid number of duplicated Diploid number of duplicated chromosomes at metaphase chromosomes at metaphase plate plate Anaphase II Anaphase Sister chromatids separate, Sister chromatids separate, becoming daughter becoming daughter chromosomes that move to chromosomes that move to the poles the poles Telophase II Telophase Four haploid daughter cells, Two diploid daughter cells, not genetically identical identical to the parent cell 3  10.5 The Human Life Cycle  Life cycle  All reproductive events that occur from one generation to the next simila generation.  In humans, meiosis occurs only during gamete formation.  Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.  Spermatogenesis The testes contains stem cells called spermatogonia.  Spermatogonia make primary spermatocytes that undergo spermatogenesis. Primary spermatocytes undergo meiosis I to form secondary spermatocytes. Secondary spermatocytes undergo meiosis II to form spermatids that differentiate to form sperm.  Oogensis 37 - Ovaries contain stem cells called oogonia, that produce primary oocytes. Primary oocyte following meiosis I, forms a secondary oocyte and a polar body. The secondary oocyte begins meiosis II but stops at metaphase II, leaves the ovary, and enters the uterine tube. If there is no sperm the secondary oocyte degenerates. If a sperm is present in the uterine tube, it will trigger secondary oocyte to complete Meiosis II  Spermatogenesis and Oogenesis in Mammals SPERMATOGENESIS OOGENESIS primary primary spermatocyte oocyte 2n 2n Meiosis I Meiosis I first polarbody secondary spermatocytes n secondary n oocyte Meiosis II n Meiosis II spermatids Meiosis II is completed n second after entry of sperm polarbody (fertilization) n Metamorphosis egg Fertilization and maturation n sperm Sperm nucleus n n fusion of sperm nucleus and zygote agg nucleus 2n  10.6 Changes in Chromosome Number & Structure  Euploidy is the correct number of chromosomes in a species.  Aneuploidy is a change in the chromosome number  Results from nondisjunction  Monosomy - only one of a particular type of chromosome, Turner syndrome XO  Trisomy - three of a particular type of chromosome Klinfelter syndrome XXY 39  Nondisjunction pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction Fertilization Zygote 2n + 1 2n + 1 2n - 1 2n - 1 2n 2n 2n + 1 2n - 1 a. b. 40  Changes in Chromosome Number & Structure  Trisomy occurs when an individual has three of a particular type of chromosome  The most common autosomal trisomy seen among humans is Trisomy 21  Down syndrome  Recognized by: short stature, eyelid fold, flat face, stubby fingers.  Linked to increase risk of leukemia, cataract & aging.  Extra copy of Gart gene increases amounts of purines ( A & G). 41  Trisomy 21 a. b. 42  Changes in sex chromosome number:  Results from inheriting too many or too few X or Y chromosomes  Nondisjunction during oogenesis or spermatogenesis  Turner syndrome (XO)  Female with a single X chromosome  Short, with broad chest and widely spaced nipples. Underdeveloped reproductive system.  Can be of normal intelligence and function with hormone therapy. Use in vitro fertilization using donor eggs. Barr body !! 43 a. Turner syndrome missing b. Klinefelter syndrome extra chromosome X a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz, chromosome X 44 http://klinefeltersyndrome.org; a, b(bottom): © CNRI/SPL/Photo Researchers, Inc  Klinefelter syndrome (XXY)  Male with underdeveloped testes and prostate; some breast overdevelopment.  Go through testosterone treatments but it will not reverse sterility due to incomplete testicle development.  Long arms and legs; large hands  Near normal intelligence unless XXXY, XXXXY, etc. 45 In humans, presence of SRY gene determines maleness. Deletion of SRY gene results in Swyer syndrome or “XY female”, they lack hormone testes –determining factor. Movement SRY gene to X chromosome produces XXmale= Le Chapelle syndrome. undersized testes, sterility, breast development. Poly-X female (XXX, XXXX) Super female= XXX tends to be tall and thin but not usually retarded XXXX are severely retarded Jacobs syndrome (XYY) – nondisjunction during spermatogenesis. tall, persistent acne, speech and reading problems. Fertile Changes in chromosome structure Due to various environmental agents;  Radiation  Organic molecules.  Viruses. Sometimes the breaks occur and they reconnect and sometimes they do not which leads to mutations.  Changes in chromosome structure include:  Deletion  One or both ends of a chromosome breaks off  Two simultaneous breaks lead to loss of an internal segment  Duplication  Presence of a chromosomal segment more than once in the same chromosome.  Translocation  A segment from one chromosome moves to a non-homologous chromosome  Sometimes it is balanced, otherwise unbalanced will lead to miscarraige & abortions. - Inversion  Occurs as a result of two breaks in a chromosome  The internal segment is reversed before re-insertion  Most individuals exhibit no abnormalities. 49  Types of Chromosomal Mutation a a a b b b b c c c c + a William’s syndrome=Ch. # 7 d d d e d e May or may not cause e e Cri du chat= # 5 f f f d visible abnormalities, g g g e depending on size. f g a. Deletion b. Duplication a a a a b b b l b l c m c m c d d n d n c Do not exhibit abnormalities d e o e o Balanced in the person e e f f f p f p carrying it. g q q g g g h r r h But the children will receive one normal & c. Inversion d. Translocation the other is not 50  Deletion a a b b + h deletion lost c c d d e e f f g g h #7 a. b. b: Courtesy The Williams Syndrome Association Williams syndrome, elastin protein is missing which causes CV problems and early aging. Turned up noses, wide mouth, small chin & large ears 51 Alagille syndrome, is due to a translocation between chromosomes 2 & 20 It leads to a congenital heart defect= Tetralogy of fallot Digital clubbing of fingers, Liver problems