Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 9 Sexual reproduction and Meiosis

Why sex? Asexual reproduction – replicate genetic material and split into 2 cells –Genetically identical offspring Sexual reproduction – fusion of sex cells produces next generation –Offspring genetically different Asexual reproduction cannot create or maintain diversity Sexual reproduction can

Meiosis Humans are diploid –2 full sets of chromosomes –1 set from each parent Somatic cells – most body cells do not participate directly in reproduction Germ cells – specialized to produce gametes – eggs and sperm – haploid (1n) Gametes fuse at fertilization to produce new diploid zygote

Alternation of generation –Meiosis occurs in diploid sporophyte generation to make haploid spores –Spores divide mitotically to produce haploid gametophyte generation which produces gametes by mitosis –Sperm fertilizes egg to form diploid zygote – sporophyte generation

Meiosis Single DNA replication Cell undergoes 2 divisions during meiosis –Meiosis I – reduces numbers of chromosomes by half Shuffling of genetic information –Meiosis II – produces 4 haploid nuclei

Meiosis I –23 pairs of human chromosomes 22 autosomes –Pair looks alike and carries same genes for same traits 1 pair of sex chromosomes – XX or XY –Not homologous but behave that way to pair in meiosis –Before first meiotic division, every chromosome duplicates and pairs with its counterpart

Meiosis I –Interphase – similar to that before mitosis with G 1, S, and G 2 phases –Prophase I – replicated chromosomes condense, homologs pair, crossing over occurs, spindle forms –Metaphase I – paired homologs line up and down the center of the cell –Anaphase I, Telophase I, Cytokinesis – homologs split and move to opposite poles

Meiosis II –second interphase in many species –Prophase II – chromosomes condense again –Metaphase II – chromosomes line up –Anaphase II – sister chromatids split –Telophase II – nuclear envelope reforms –Cytokinesis – total of 4 new cells

Meiosis generates variability 3 mechanisms 1.Crossing over – 2 homologs exchange genetic material resulting in new gene combinations

2.Random alignment of chromosome pairs in metaphase I – all possible combinations equally probable (2 23 possibilities in humans)

3.Random fertilization – any one of 223 possible sperm can unite with any one of 223 possible sperm for 70 trillion unique combinations (crossing over contributes more variability) –Identical twins result from one fertilization – monozygotic –Nonidentical (fraternal) twins – 2 eggs and 2 sperm - dizygotic

Differences between mitosis and meiosis Mitosis occurs in somatic cells throughout the life cycle – meiosis only in germ cells during some life stages Mitosis has one cell division yielding 2 daughters – meiosis has 2 cell divisions yielding 4 daughters Cytokinesis happens once in mitosis but twice in meiosis Only in meiosis do homologous pairs align and crossing over occurs – not in mitosis Mitosis yields identical daughter cells for growth, repair, asexual reproduction – meiosis yields genetically variable daughters for sexual reproduction

Errors in meiosis Polyploidy – one or more complete sets of extra chromosomes –Not viable in humans but 30% of flowering plants Nondisjunction – chromosomes fail to separate at 1 st or 2 nd meiotic division –Gamete with an extra or missing chromosome –Most embryos with incorrect chromosome numbers cease developing –Trisomy causes fewer problems than monosomy

Extra autosomes –Trisomy 21 – 3 copies of 21 st chromosome Most common cause of Down syndrome Likelihood of giving birth to child with trisomy 21 increases dramatically as a woman ages Can also come from sperm with extra 21 Most common because it is least likely to kill fetus –Trisomy 18 and 13 are next most common –Others likely occur but embryos fail to develop

Extra or missing sex chromosomes –XXX or triplo-X Extra X chromosome in a female 1 in female births –XXY or Klinefelter syndrome Male with extra X chromosome 1 in 500 – 2000 male births Symptoms vary widely –XYY or Jacobs syndrome Male with extra Y chromosome 1 in 1000 male births –XO or Turner syndrome More than 99% of XO fetuses do not survive to birth Infertile –YO – never reported without an X too much genetic information is missing

Smaller-scale abnormalities –Deletion – loss of one or more genes Cri du chat – deletion of several genes on chromosome 5 –Duplication – multiple copies of one or more genes Probably an important role in evolution – “spare” copy free to mutate –Inversion – nonhomologous chromosomes exchange parts Consequences depend on genes disrupted 95% of people with chronic myelogenous leukemia have a translocation that results in a protein that speeds up cell division and suppresses apoptosis

Gametes Spermatogenesis –Formation of sperm in the testes –Secondary spermatocytes undergo meiosis II to become 4 equal sized spermatids

Oogenesis –Formation of egg cells in the ovaries Meiosis I – primary oocyte divides into polar body and secondary oocyte Meiosis II – secondary oocyte divides into polar body and mature egg cell or ovum Tiny polar bodies have no role in reproduction Gametes

Alternation of generations –Diploid sporophyte produces haploid spores by meiosis –Spores germinate producing a multicellular gametophyte –Gametophyte produces eggs and sperm by mitosis –In moss, gametophyte free-living –In flowering plants, gametophyte microscopic