Class Note for ECOL 320 at UA
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Date Created: 02/06/15
Sexual Reproduction and Meiosis Unlabelled picture from web My guess is that it shows late arraphase of meiosis chromosomes blue kinetochores red microtubules green Arrow shows lagging chromosome Maize chromosomes in prophase pachytene of meiosis I stained with DAPI Eukaryotic Life Cycles Asexual life cycle mitosis Sexual life cycle meiosis syngamy EUKARYCITIC LIFE CYCLES N Rmeiosis72hl mitosis mitosis mitosis mitosi ltOSlS mitosis NZsyngamy QN syngamy 2 General most common eg Sam armm es Predominant PM 99 animal l l Rmeiosis N 2N mitosig mitogi mitosis mitosis mitosis styng my y 2 N QN Predominantly haploid eg mms Aselual COMMON 99 Some fungi N 2N j N meiosis 2N m J g H O 2 Ln SeX as the geneticist sees it When doing genetics Parental genotypes l A b a B l Recombinant genotypes Meiosis Meiosis Starts With DNA replication so in each diploid cell each chromosome replicates to produce two chromatids these stay tightly bound together Meiosis 2 divisions meiosis I and II or first and second meiotic divisions No DNA replication between M I and II Meiosis I 4 l Propnase leptotene Propnase pachylene Prophase dwlotener Zygotene Chromosomes chromandsvrsrble d a qnem Chmsmata condense homologues crossmgrover occurs form and termmhze synapse a an XXJ Anapnase Frbers Metaphase Homologous k metochores attach to Comm ampP Telopnase Nuclear chromosome bers from Omologous membranes form opposlte poles ne OChOYeS 0 around chromosomes Oppos e poles cylok mesls Ahm miosis I Nehronnsonns 2N chromm39tk 2C DNA Meiosis II There is no true interphase between meiosis I and II and no DNA replication Meiosis II is like mitosis Metaphase chromosome fibers attach so as to connect sister kinetochores to opposite poles Anaphase sister kinetochores and chromatids move to opposite poles segregate new After meiosis II N chromosomes C DNA Alleles have segregated Reductional and Equational Divisions Miwsis and meiosis 11 are called equational L divisions in which sism kinetochores and chromatids segregate Meiosis I is called an reductional division in which sister kinetochoies remain attached while homologous kinetochores segregate L How to Different Chromosomes Behave at Meiosis Kinetochores of different chromosomes segregate independently and randomly In 12 of cells kinetochoi39es from mammal parent go w same p01e and paternal kinewchoies go w opposite p01e in meiosis I Probability 5 AB and ab are parental genotypes In 12 of cells kinetochoi39es from mammal parent segregate w opposite poles and chromosomes from paternal parent do the same in meiosis I Piobability 05 A17 and AB are recombinant genotypes What will be the genotypes of the gametes produced by a large sample of cells undergoing meiosis AB 14 Ab 14 aB 14 ab 14 The alleles of both genes will segregate 12 A 12 a 12 B 12b The two alleles will segregate independently of each other Meiosis Summary Homologous chromosomes synapse and recombine in prophase of meiosis I Homologous kinetochores and centromere sequences segregate in anaphase of meiosis 1 Sister kinetochores and centromeres segregate in anaphase of meiosis II as in mitosis 2N gt N 2C gt 1C The two copies of each gene segregate Different kineetochores segregate independently therefore different chromosomes and different pairs of alleles will segregate independently Fate of Products of Meiosis Many protists unicellular eukaryotes algae and fungi all four products of meiosis survive In many cases they are bound together as a tetrad which permits very powerful genetic experiments Chlamydomonas zygospore with 4 haploid Yeast ascus with 4 ascospores nuclei o C 5 1 339 c I 0 7100 X i lMega39spore Ami x39i icrospores 5 2 7 rs t l Animals and plants Male animals and plants all four products of meiosis survive and differentiate into sperm or pollen but do not stay together in tetrads Female animals cytokinesis after both meiotic divisions is very unequal producing a large cell and a tiny one called a polar body These polar bodies disintegrate The big cell at the end of meiosis differentiates into the egg Which nucleus survives is purely random Female plants only one of the four products of meiosis becomes an egg as in animals Of the four nuclei produced by meiosis one nucleus divides mitotically and only one product of those divisions becomes the egg The other products of these mitoses and sometimes other products of meiosis become various kinds of accessory cells Special terminology for cells undergoing meiosis in animals not clear in book spermatogonia Ml gt primary spermatocytes Mll gt secondary spermatocytes differentiate gt sperm oogonia MI gt primary oocyte Mll gt secondary oocyte differentiate gt ovum polar body polar body Plants are even more complicated see text p 143 You probably won t need to know the plant terminology for exams or quizzes but you may need to look it up to answer homework questions All four cell products of meiosis in males differentiate into spermatozoa or pollen But only small random sample actually participate in fertilization In female animals or plants only one product of meiosis becomes an egg But which product survives is normally a matter of chance so this makes no difference in heredity and only a small random sample of products of meiosis participate in fertilization SUMMARY 0 Meiosis consists of 2 divisions I and II 0 In prophase of meiosis I homologous chromosomes each With 2 chromatids synapse and undergo crossing over exchange segments 0 In metaphase of MI homologous kinetochores attach by chromosome fibers to opposite poles 0 In anaphase of MI homologous kinetochores migrate to opposite poles dragging chromosomes behind them Each chromosome still consists of 2 chromatids 0 The kinetochores of different chromosomes attach to poles independently of each other 0 There is no interphase and no DNA replication between MI and M11 0 M11 is like mitosis sister kinetochores attach to and migrate to opposite poles 0 Only a small random sample of the products of meiosis participate in fertilization in animals and plants The genetic results are 0 Alleles segregate 0 Alleles on different chromosomes segregate independently 0 Recombinant genotypes are produced
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