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Meiosis &

Meiosis & Sex Cells Arise From Preexisting Cells

I. Asexual (Mitotic) Reproduction a. : 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 [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 Number in Half

 Meiosis has 2 consecutive divisions – Meiosis I: Homologous pairs separate – Meiosis II: Sister chromatids separate

 Each division has a , 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)

somatic (body) cells – 23 pairs = 46 chromosomes – Homolog = same size, shape, centromere, and genes

Pairs #1 - 22 = – Both male and

Pair #23 = Sex Chromosomes – Determine – XX = female, XY = male Diploid life history Alternation of Generations Haploid life history () () (fungi) Human karyotype

Somatic (body) cells are Diploid Meiosis I (sex cells) are Haploid  Diploid (2n) Prophase I – Two of each kind of chromosome  Chromosomes condense – 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 = • homologous – Female gamete = (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 : 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

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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.

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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 , , & 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 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

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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 223 genetically different ova:  XX = female  One 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

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