MEIOSIS & SEXUAL REPRODUCTION 10.1 HALVING THE CHROMOSOME NUMBER Interphase Meisis: type f nuclear divisin that ccurs in sexually reprducing rganisms. Reduces chrmsme number frm diplid (2n) t haplid (n) number. Gamete: reprductive cell (sperm & egg). Zygte: frmed by fusin f gametes; always has diplid (2n) number f chrmsmes. Sexual reprductin: gametes & zygte frmatin. Hmlgus Pairs f Chrmsmes In diplid bdy cells, chrmsmes ccur as pairs. Each set f chrmsmes is a hmlgus pair; each member is a hmlgus chrmsme r hmlgue. Hmlgues lk alike; have same length & centrmere psitin. A lcatin n 1 hmlgue cntains gene fr the same trait that ccurs at this lcus n the ther hmlgue, althugh genes may cde fr different variatins f that trait. Alleles: alternate frms f a gene. Overview f Meisis Meisis invlves 2 nuclear divisins & prduces fur haplid daughter cells. Each daughter cell has half the n f chrmsmes fund in diplid parent nucleus. Page 1 f 6
Meisis I: Prir t meisis I, DNA replicatin ccurs, each chrmsme has 2 sister chrmatids. During meisis I, hmlgus chrmsmes pair synapsis. During synapsis, the 2 sets f paired chrmsmes lay alngside each ther as a bivalent. Meisis II: centrmeres divide & daughter chrmsmes (derived as sister chrmatids) separate. Chrmsmes in the 4 daughter cells have nly 1 chrmatid. In animal life cycle, daughter cells becme gametes that fuse during fertilizatin. 10.2 GENETIC VARIATION Genetic Recmbinatin Occurs as crssing-ver & independent assrtment. Due t GR, ffspring have a different cmbinatin f genes than their parents. In prkarytes, variatin ccurs nly due t mutatin. Crssing-ver Crssing-ver: exchange f genetic material b/w nn-sister chrmatids f a bivalent during meisis I. At synapsis, hmlgus chrmsmes are held in psitin by nucleprtein lattice (synaptnemal cmplex). After crssing-ver, lattice f synaptnemal cmplex breaks dwn, but hmlgues are temprarily held tgether by chiasmata. Hmlgues separate and are distributed t daughter cells. Chiasmata: regins where nn-sister chrmatids are attached due t crssing-ver. Page 2 f 6
Independent Assrtment (metaphase I) Hmlgues align independently (in a randm manner) at metaphase plate. In a cell with 3 pairs f chrmsmes: 23 r 8 cmbinatins. 10.3 PHASES OF MEIOSIS A. Prphase I Nuclelus disappears; nuclear envelpe fragments; centrsmes migrate away frm each ther; & spindle fibers assemble. Hmlgus chrmsmes underg synapsis t frm bivalents; crssing-ver ccurs. Chrmatin cndenses & chrmsmes becme visible. B. Metaphase I Fully frmed spindle Bivalents held tgether by chiasmata align independently at the metaphase plate at equatr f spindle. Kinetchres attach t kinetchre spindle fibers. Maternal & paternal hmlgues f each bivalent may be riented twards either ple. C. Anaphase I Hmlgues f each bivalent separate & mve tward ppsite ples. Each chrmsme still has 2 chrmatids. D. Telphase I Nuclear envelpe refrms & nucleli reappear. May r may nt be accmpanied by cytkinesis. E. Interkinesis Page 3 f 6
Similar t interphase but n DNA replicatin. F. Meisis II and Gamete Frmatin Metaphase II: haplid number f chrmsmes align at metaphase plate. Anaphase II: sister chrmatids separate at centrmeres; 2 daughter chrmsmes mve tward ples. Due t crssing-ver, each gamete cntains chrmsmes with different types f genes. At the end f telphase II & cytkinesis, there are 4 haplid cells. In animals, haplid cells mature & develp int gametes. In plants, daughter cells becme spres & divide t prduce haplid generatin; these haplid cells fuse t becme a zygte that develps int a diplid generatin. 10.4 MEIOSIS COMPARED TO MITOSIS Mitsis Meisis 1 nuclear divisin 2 nuclear divisins prduces 2 daughter cells daughter cells are diplid genetically identical ccurs in all tissues fr grwth & repair prduces 4 daughter cells daughter cells are haplid daughter cells nt genetically identical t each ther r parental cell ccurs at certain time in sexually reprducing rganisms Meisis I Cmpared t Mitsis DNA is replicated nly nce befre bth mitsis and meisis. 1 nuclear divisin in mitsis; 2 nuclear divisins in meisis. Page 4 f 6
Prphase I f meisis: hmlgus chrmsmes pair & underg crssing-ver. Metaphase I f meisis: bivalents align at metaphase plate; in mitsis individual chrmsmes align. Anaphase I in meisis: hmlgus chrmsmes (with centrmeres) separate & mve t ppsite ples; in mitsis, sister chrmatids separate & mve t ppsite ples. 10.5 HUMAN LIFE CYCLE Life cycle: all reprductive events between 1 generatin & next. In plants, there are 2 adult stages: diplid (sprphyte) & haplid (gametphyte). In human males, meisis is part f spermatgenesis (prductin f sperm) in testes. In human females, meisis is part f genesis (prductin f eggs) in varies. After birth, mittic cell divisin is invlved in grwth & tissue regeneratin f smatic tissue. Spermatgenesis and Ogenesis Spermatgenesis In the testes f males, primary spermatcytes with 46 chrmsmes underg meisis I t frm 2 secndary spermatcytes, each with 23 duplicated chrmsmes. Secndary spermatcytes divide (meisis II) t prduce 4 spermatids, with 23 daughter chrmsmes. Spermatids differentiate int sperm (spermatza). Meitic cell divisin in males always results in 4 cells that becme sperm. Page 5 f 6
Ogenesis In varies f human females, primary cytes with 46 chrmsmes underg meisis I t frm 2 cells, each with 23 duplicated chrmsmes. 1 f the cells, secndary cyte, receives almst all the cytplasm; ther cell (plar bdy) disintegrates r divides again. Secndary cyte begins meisis II & then stps at metaphase II. At vulatin, secndary cyte leaves vary & enters viduct where it meets a sperm. If a sperm enters secndary cyte, cyte is activated t cntinue meisis II t cmpletin. Result is mature egg & anther plar bdy, each with 23 daughter chrmsmes. Meisis prduces 1 egg & 3 plar bdies; plar bdies serve t discard unnecessary chrmsmes & retain mst f the cytplasm in egg. Cytplasm: surce f nutrients fr develping embry. Page 6 f 6