Cell Division Part 2

Sexual Reproduction Most common way for eukaryotic organisms to produce offspring Organisms make gametes or spores with one set of all genes Two gametes fuse during fertilization to generate a new organism - zygote Spores germinate to develop either a gametophyte (plants) or new haploid organism (fungi)

Gametes Simple eukaryotes are isogamous gametes are morphologically similar Most eukaryotes are heterogamous gametes are morphologically different Sperm small and motile Oocytes or ova (Eggs) large and nonmotile Store nutrients & developmental information Microspores (Pollen) Macrospores (Ovules)

Meiosis Produces Haploid Gametes Gametogenesis or Sporulation Chromosome number reduced to ½ original 2n  n give a single set of all parental chromosomes Chromosomes sorted to assure each chromosome (& its genes) is represented

genetically identical genetically distinct Mitosis vs Meiosis Makes 2 genetically identical diploid daughter cells Makes 1-4 genetically distinct haploid gametes

Meiosis After a cell has gone through G1, S, & G2 Two successive divisions Meiosis I & II Each subdivided into Prophase (I & II) Metaphase (I & II) Anaphase (I & II) Telophase (I & II)

Meiosis Prophase I subdivided into Leptotena Zygotena Pachytena Diplotena Diakinesis

Periods of Prophase I Figure 3.11 A total of 4 chromatids tetrads A total of 4 chromatids Figure 3.11 A recognition process

Synaptonemal Complex

Periods of Prophase I 2 bivalents A tetrad A physical exchange of chromosome pieces

Stages of Meiosis I

Tetrads organized along the metaphase plate Homologous chromosomes aligned side by side (tetrads) One homolog linked to one pole The other homolog is linked to the opposite pole Assortment of homologues is independent Figure 3.13

Stages of Meiosis I Homologous pairs of sister chromatids separate Centromeres remain between sister chromatid pairs

Meiosis Telophase I of meiosis I is followed by meiosis II Each daughter cell has only one member of each homologous pair Each homolog = 2 recombinant sister chromatids genetic content is still 2n Meiosis II reduces genetic content to n

1 of each type of chromosome (n) in each gamete Stages of Meiosis II 1 of each type of chromosome (n) in each gamete

Spermatogenesis Production of sperm In testes Diploid spermatogonium cell divides mitotically  two cells One remains a spermatogonial cell Other becomes 1o spermatocyte 1o spermatocyte goes through meiosis I and II

Spermatogenesis Figure 3.14 (a) Meiois II  four haploid spermatids Meiois I  two 2 spermatocytes Figure 3.14 (a)

Oogenesis Production of egg cells (ova) In ovaries Early in development, diploid oogonia produce diploid 1o oocytes In humans ~ 106 1o occytes/ovary produced before birth

Mammalian Oogenesis 1o oocytes initiate meiosis I Then stop in prophase I until sexual maturity (puberty) At puberty, 1o oocytes are activated & progress through meiosis I Cell division is asymmetric  1 large 2o oocyte 1 small polar body

2o oocyte enters meiosis II arrests in metaphase II until ovulation Mammalian Oogenesis 2o oocyte enters meiosis II arrests in metaphase II until ovulation If 2o oocyte is fertilized meiosis II is completed A haploid egg & a 2nd polar body produced haploid egg & sperm nuclei fuse to create a diploid zygote

Cell divisions in oogenesis is asymmetric Figure 3.14 (b)

y r r y Y Heterozygous diploid cell (YyRr) undergoing meiosis Y R R y Y y Y R r r R Meiosis I y y y Y Y y Y Y R R r r r r R R Meiosis II y y y y Y Y Y Y R R r r r r R R Ry : rY ry : RY

XY system 3-78 Humans have 46 chromosomes 44 autosomes 2 sex chromosomes Males contain one X & one Y chromosome termed heterogametic Females have two X chromosomes termed homogametic The Y chromosome determines maleness 3-78

Sitting on the toilet reading (SIT)

Real Y Chromosome Map http://www.ncbi.nlm.nih.gov/genome/guide/human/

Normal Human Karyotypes Female 46, XX Male 46, XY

Aneuploidies of the Sex Chromosomes 47, XXY 45, X Klinefelter syndrome Turner syndrome

ZW System sex chromosomes designated Z & W Males have two Z chromosomes are homogametic Females have Z & W chromosomes are heterogametic and some fish

XO System In some invertebrates (insects, nematodes) Males are XO and females are XX In some insects (fruit fly) Males are XY and females are XX Y chromosome does not determines maleness Sex determined by the ratio of the # of X chromosomes to the # of autosome pairs (X/A) If X/A =< 0.5, fly is male If X/A = >1.0, fly is female

Haplo-Diplo System 3-81 Bees and some relatives Males are haploid Known as drones Produced from unfertilized eggs Females are diploid Include worker bees and queen bees Produced from fertilized eggs 3-81