MEIOSIS: Sexual Reproduction Ch. 9.7 – EGG SPERM

9.7 Why Do So Many Organisms Reproduce Sexually? shuffles genes to produce genetically unique offspring combines different parental alleles in a single offspring rate of mutations is very low – although they produce new alleles allows for much more rapid evolution new combinations of alleles better adapt organisms to changing environments

9.8 How Does Meiotic Cell Division Produce Haploid Cells? Meiotic cell division – specialized cell division process that produces haploid gametes Diploid cell with paired chromosomes produces haploid daughter cells with unpaired chromosomes  each gamete receives 1 member of each pair of homologous chromosomes

Diploid vs. Haploid  Homologous chromosomes: carry similar genetic information;  1pair of homologous chromosomes = 1 from mom and 1 from dad  Diploid (2 sets= 2n): contains 2 full sets of chromosomes (i.e. body cells)  Haploid (1 set = n): contains only 1 set of chromosomes (i.e. gametes)

Synapsis: when tetrads form in Prophase I Disjunction: tetrads separate Crossing over occurs Meiosis I: separates pairs of homologues, with each daughter nucleus receiving one Tetrads line up on equator Tetrads separate to poles

Meiosis II: (identical to Mitosis) separates the chromatids and directs 1 chromatid into each of two more daughter nuclei

MEIOSIS OVERVIEW Haploid n Dipoid 2n be.com/watch?v=to WK0fIyFlYhttps:// be.com/watch?v=to WK0fIyFlY – Amoeba sisters ucation.com/sites/ /student_vie w0/chapter28/animati on__stages_of_meiosi s.htmlhttp://highered.mhed ucation.com/sites/ /student_vie w0/chapter28/animati on__stages_of_meiosi s.html - brief description of each step

VARIATION & DIVERSITY among offspring 1.Random fertilization: Who’s gonna be the lucky one? 2. Crossing over: mutual exchange of corresponding chromatid sections btwn homologues with help of enzymes (genetic recombination) 3.Independent assortment: random separation of chromosomes during meiosis

Crossing Over: Prophase I: Homologous chromosomes pair up and exchange DNA Proteins bind chromatids of maternal & paternal homologues together so they align precisely / /student_view0/chapter30/ meiosis_with_crossing_over.html / /student_view0/chapter30/ meiosis_with_crossing_over.html animaltion of crossing over with a quiz at the end.

Independent Assortment

Meisosis in Females vs. Males 1 mature egg cell (females are born with all the eggs they need for life) 4 mature sperm cells (males make sperm throughout their entire life)

9.9 When Do Mitotic and Meiotic Cell Division Occur in the Life Cycles of Eukaryotes 1.2 haploid (n) cells from different parental organisms fuse during fertilization creating a diploid (2n) cell with new gene combinations 2.Meiotic cell division occurs  recreating haploid cells (gametes) 3.Mitotic cell division results in growth of multicellular bodies or in asexual reproduction

MITOSIS vs. MEIOSIS CHARACTERISTICMITOSISMEIOSIS type of reproduction asexualsexual # of divisions12 # of resulting cells 24 chromosome # in resulting cells 46 (diploid) 23 (haploid)

Change in Chromosome # problems occur if mutations cause cells to have more or less than 46 chromosomes Caused by non-disjunction – failure of chromosomes from separating in Anaphase Trisomy = +1 Monosomy = -1 Karyotype: photo of chromosomes that show chromosomes arranged by size; 1) detect abnormalities in chromosome # 2) determines sex

Autosomes Pairs 1-22 Sex Chromosomes Pair 23 Karyotype

Sex Chromosome Disorders Klinefelter’s Syndrome: (XXY) testes stay child-sized don’t produce enough male testosterone taller than average & lack facial hair some develop enlarged breasts (gynecomastia) usually sterile

Turner’s Syndrome: XO monosomy 1 in 5,000 females; minor problems always sterile AUTOSOMAL (body cells) DISORDERS Edwards Syndrome: Trisomy 18 (extra chromosome) severe mental retardation & physical problems than Downs

Down Syndrome (trisonomy 21): extra chromosome 21 mentally retarded round, flat, facial features more common in elder females (over 35) Results from non-disjunction (chromosomes don’t separate properly)