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Meiosis & Sexual Reproduction Heyer 1 Meiosis & Sexual Reproduction Meiosis & Sex Cells Arise From Preexisting Cells I. Asexual (Mitotic) Reproduction a. Mitosis: production of two identical nuclei b. Cytokinesis: physical division of the cell into two II. Sexual (Meiotic) Reproduction a. Meiosis: production of four non-identical nuclei b. Cytokinesis: physical division of the cell c. Fertilization: fusion of two Sexual reproduction creates sex cells d. Syngamy: fusion of new combinations of alleles. two nuclei Diploid cells have Homologous chromosomes [Homologs]: homologous pairs of chromosomes: same loci, maybe different alleles. 1 set from mom, 1 set from dad. Meiosis: Reductive Division Sex = Meiosis + Syngamy — Reduces Chromosome Number in Half Meiosis has 2 consecutive divisions – Meiosis I: Homologous pairs separate – Meiosis II: Sister chromatids separate Each division has a prophase, metaphase, anaphase and a telophase Meiosis: Syngamy: 2n 1n 1n 2n Diploid haploid Haploid diploid Heyer 1 Meiosis & Sexual Reproduction Chromosomes Matched in Sexual Life Cycles Homologous Pairs (Homologs) Human somatic (body) cells – 23 pairs = 46 chromosomes – Homolog = same size, shape, centromere, and genes Pairs #1 - 22 = Autosomes – Both male and female Pair #23 = Sex Chromosomes – Determine gender – XX = female, XY = male Diploid life history Alternation of Generations Haploid life history (animals) (plants) (fungi) Human karyotype Somatic (body) cells are Diploid Meiosis I Gametes (sex cells) are Haploid Diploid (2n) Prophase I – Two of each kind of chromosome Chromosomes condense – Humans 2n = 46 Nuclear envelope dissolves Haploid (n) Spindle forms – One of each kind of Tetrads form chromosome – of paired homologs – Humans n = 23 – 2 dyads 1 tetrad Haploid gametes are produced by meiosis – “Crossing over” – Male gamete = sperm • homologous – Female gamete = egg (ovum) chromosomes The human life cycle swap portions Stages of meiosis Meiosis I Meiosis I Metaphase I Anaphase I Homologous pairs Homologous pairs line up on spindle separate equator Chromosomes are still duplicated Stages of meiosis Stages of meiosis Heyer 2 Meiosis & Sexual Reproduction Meiosis I Telophase I Cytokinesis, but chromosomes remain condensed 2 new daughter cells Dyads connected at centromeres Tetrads connected at chiasmata – Both are now haploid, – (sites of crossing over) but with duplicated At anaphase I: chromosomes – chiasmata separate No DNA replication – centromeres remain before next division connected Stages of meiosis Meiosis II (in both cells from first Meiosis II (cont.) meiotic division) Anaphase II Prophase II – Sister chromatids – follows Telophase I separate – Chromosomes already condensed from – Daughter meiosis I chromosomes – Centromes duplicate, unduplicated new spindle forms Telophase II Metaphase II – Total of 4 haploid – Chromosomes line up cells produced again Stages of meiosis • Like metaphase of mitosis Stages of meiosis Mitosis & Meiosis Compared Mitosis Meiosis Mitosis makes clones, but meiosis changes the Mitosis separates chromatids in a single division genome. event Meiosis I separates homologs Meiosis II separates chromatids Heyer 3 Meiosis & Sexual Reproduction Spermatogenesis: but Oogenesis: one primary spermatocyte → one primary oocyte → Sexual Reproduction Produces Genetic Variation Meiosis doesn’t Variation arises from always I. Independent chromosome assortment create in meiosis 4 cells. II. Crossing-over between homologous chromosomes in meiosis III. Random process of fertilization → 4 sperm → 1 ovum + 2 polar bodies Homologous Chromosomes Can Carry Different Versions of Genes Homologous chromosomes come Meiosis from different parents creates a Different versions of random same gene = Alleles subset of – Ex. Brown coat, white coat chromosomes. Different info on homologous chromosomes Independent Assortment Meiosis I creates new genomes 2 ways: New combinations of alleles on each chromosome. Crossing over New combinations of Homologs line up by chance in Metaphase I chromosomes. Results in many different possible chromosome combinations in gametes Independent n 23 2 possible combinations (2 = >8 mill. for humans) assortment Heyer 4 Meiosis & Sexual Reproduction In division I, homologous Crossing over occurs pairs form tetrads and during early prophase I crossing over occurs. division I division I synapsis Crossing over Early prophase I: • Homologous (nonsister) chromatids aligned and linked by protein zipper complex (synaptonemal complex) • Synaptonemal complex disappears when chromosomes chiasma fully condense Outcomes of Crossing over is crossing over highly organized, Recombinant ensuring that chromosomes homologous Sister chromatids no segments are longer identical. traded. Each chromatid has alleles both from mom and from dad. Heyer 5 Meiosis & Sexual Reproduction In division II, no-longer-identical sister chromatids are separated. In humans, typically 1–3 crossings-over per chromosome in a meiotic division. division II Alterations of Chromosome Dictyate oogenesis Number and Structure in mammals, birds, & chondrichthyes Errors in meiosis can lead to gametes with ⇐ Arrested at Prophase I • after synapsis, but before final condensation – Aneuploidy • In humans – from 20 weeks gestation until stimulated by • (Abnormal number of chromosomes) menstrual cycle ⇐ Arrested at Metaphase II – Alteration of chromosome structure • until fertilization occurs Aneuploidy Aberrations of Chromosome Structure Trisomy 21 = An extra copy of chrom. 21 Errors in Crossing-over Down Syndrome Deletions and – Most common birth defect duplications Chance increases with age Deletion and duplication of mother – “Cat-cry” syndrome, deletion in chrom 5 Down syndrome Translocation – CML, most common of leukemia cancers Translocation of CML Heyer 6 Meiosis & Sexual Reproduction one pair of the 23 are sex Random Fertilization chromosomes Fertilization is a random process A gamete from one individual unites with one Random from another individual fertilization 23 Given that a man can produce 2 genetically determines sex different sperm, and a woman can produce of the zygote 223 genetically different ova: XX = female One mating couple can produce a diploid zygote with any of >70 trillion combinations XY = male of chromosomes! (223 x 223) – XY is NOT a homologous (Not even counting variation from crossing- over!) pair After fertilization, somatic cells reproduce asexually (mitotic divisions) Differentiation: all cells genetically identical, but with different gene expression Heyer 7 .
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