eiosis Section 1 Chromosomes and Chromosome Number Reading Preview Essential Questions

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1 Section 1 Reding Preview Essentil Questions I How does the reduction in chromosome number occur during meiosis? I Wht re the stges of meiosis? I Wht is theimportnce of meiosis in providing genetic vrition? Review Vocbulry chromosome: cellulr structure tht contins DNA New Vocbulry gene homologous chromosome gmete hploid fertiliztion diploid meiosis crossing over Multilin egloss Figure 1 Homologous chromosomes crry genes for ny given trit t the sme loction. The genes tht code for erlobe type might not code for the exct sme type of erlobe. eiosis MAIN ~ Melosis produces hploid gmetes. Rel-World Reding Link Look round your biology clss. You might nou~ tht the students in your clss do not look the sme. They might be different heights nd hve different eye color, hir color, nd other fetures. This vriety chrcteristics is result of two sex cells combining during sexul re du Chromosomes nd Chromosome Number Ech student in your biology clss hs chrcteristics pssed on tot by their prents. Ech chrcteristic, such s hir color, height, or eye color, is clled trit. The instructions for ech trit re locted on chromosomes, which re found in the nucleus of cells. The DNA on chromosomes is rrnged in segments clled genes tht control the production of proteins. Ech chromosome consists of hundreds of genes, ech gene plying n importnt role in determining the chrc teristics nd functions of the cell. Homologous chromosomes Humn body cells hve 6 chrom somes. Ech prent contributes 23 chromosomes, resulting in 23 pl of chromosomes. The chromosomes tht mke up pir, one chromo. some from ech prent, re clled homologous chromosomes. As shown in Figure 1, homologous chromosomes in body cells hve the sme length nd the sme centromere position, nd they crry genes tht control the sme inherited trits. For instnce, the gene for erlo type will be locted t the sme position on both homologous chromo~ somes. Although these genes ech code for erlobe type, they might code for the exct sme type of erlobe. A pir of hbmologous chromosomes 270 Chpter 10 Sexul Reproduction nd Genetics

2 HPl0~ nd diploid cells In order to mintin the sme chromo 50me number from genertion to genertion, n orgnism produces metes, which re sex cells tht hve hlf the number of chromo ~mes. ~Jthough the number of chromosomes vries from one species 0not~~&~ in humns ech gmete contins 23 chromosomes. The symboi,~ cn be used to represent the number of chromosomes in mete A cell with n number of chromosomes is clled hploid cell. ~iploid comes from the Greek word hploos, mening single. The process by which one hploid gmete combines with nother hplo 1 gmete is clled fertiliztion. As result of fertiliztion, the cell now will contin totl of chromosomes n chromosomes from the femle prent plus n chromosomes from the mle prent. A cell tht contins number of chromosomes is clled diploid cell. Notice tht n lso describes the number of pirs of chromosomes in n orgnism. When two humn gmetes combine, 23 pirs of homolo gous chromosomes re formed. MeiOSiS I Gmetes re formed during process clled meiosis, which is type of cell division tht reduces the number of chromosomes; therefore, it is referred to s reduction division. Meiosis occurs in the reproductive structures of orgnisms tht reproduce sexully. While mitosis mintins the chromosome number, meiosis reduces the chromosome number by hlf through the seprtion of homologous chromosomes. A cell with number of chromosomes will hve gmetes with n number of chromo o. somes fter meiosis, s illustrted in Figure 2. Meiosis involves two con rs secutive cell divisions clled meiosis I nd meiosis II. FoI~DAB Incorporte in ormtion from this section into your Foldble. Femle (diploid) Mle (diploid) Grows into dult mle or dult femle ( I Meiosis \ Mle Femle gmete (hploid) n Meiosis Zygote (diploid gmete Fertiliztion ) (hploid) Figure 2 The sexul life cycle in nimls involves meiosis, which produces gmetes. When gmetes combine in fertiliztion, the number of chromosomes is restored. Describe wht hppens to the num ber of chromosomes during melosis. Section 1 Meiosis 271

3 Centromere Sister chromtids A pir of homologous chromosomes Figure 3 The homologous chromosomes re physiclly bound together during synpsis in prophse I. Figure 4 The results of crossing over re new combintions of genes. Determine which chromtids exchnged genetic mteril. Interphse Recll tht.the cell cjrcle includes interphse prior to mito sis. Cells tht undergo meiosis lso go through interphse s prt of the cell cycle. Cells in interphse crry out vrious metbolic processes including the repliction of DNA nd the synthesis of protçins. ProphSe I As cell enters prophse I, the replicted chrôñ-iosonies become visible. As in mitosis, the replicted chromosomes c~ñsist of sister chromtids. As the homologous chromosomes condei~e, they begin tolorm pirs in process clled synpsis. The homologous chro. mosomes re held tightly together long their lengths, s illustrted ~ Figure 3. Notice tht in Figure 4 the pink nd green chiokb somes h~ exchnged segments. This exchnge occurs during synps~s. Crossing over is process during which chromosoml segments r& &ichnged between pir of homologous chromosomes. As prophse I continues, centrioles move to the cell s i~posite p0 Spindle fiber~ form nd bind to the sister chromtids t the ~entrome MetphSe I In the next phse of meiosis, the pirs of homologous mosomes line up t the equtor of the cell, s illustrted in Figure 5. In meiosis, the spindle fibers ttch to the centromere of ech homologo chromosome. Recll tht during metphse in mitosis, the if~dividul chromosomes, which consist of two sister chromtids, line up t~the cel1~ equtor. During metphse I of meiosis, the homologous chiomosomes line up s pirs t the cell s equtor. This is n importnt disiinction between mitosis nd meiosis.. AnPhSe i During nphse I, the homologous chromo~.omes sep rte, which is lso illustrted in Figure 5. Ech member.o~she pir is guided by spindle fibers nd moves towrd opposite poles of the cell. The chromosome number is reduced from to n when the homolo~ gous chromosomes seprte. Recll tht in mitosis, the sister chrom tids split during nphse. During nphse I of meiosis, however, es homologous chromosome still consists of two sister chrointids. TelophSe I The homologous chromosomes, consisting of two sister chromtids, rech the cell s opposite poles. Ech pole contiiis only on member of the originl pir of homologous chromosomes. Notice in Figure 5 tht ech chromosome still consists of two sister chrorntids joined t the centromere. The sister chromtids might not be identicl becuse crossing over might hve occurred during synpsis. in~prqph -t 272 Chpter 10 Sexul Reproduction nd Genetics

4 visu lizing Meiosis 5 Follow long the stges of meiosis I nd meiosis II, beginning with nterphse t the left. 0 Metphse I o Anphse I Chromosome centromeres ttch to 0 prophse spindle fibers. Homologous chromosomes piring of homologous chromosomes Homologous chromosomes line up t seprte nd move to occurs~ ech chromosome consists of the equtor. opposite poles of the cell. o Telophse I two chromtids. crossing over produces exchnge The spindles brek down. of genetic informtion. Chromosomes uncoil nd The nucler envelope breks down..~ ) form two nuclei. spindles form. ~~_>,c The cell divides. Interphse Chromosomes replicte. Chromtin condenses. t \ Equtor 3 Centrioles MEIOSIS I 0 Prophse II Chromosomes condense. Spindles form in ech new cell. Products Spindle fibers ttch to I Four cells hve chromosomes. formed. Ech nucleus contins MEIOSIS II ~ hploid number Equto of chromosomes. Metphsell Centromeres of chromosomes line up rndomly t the 0 Telophse II equtor of ech cell. Four nuclei form round chromosomes. Spindles brek down. Cells divide. Conce in Motion Animtion Anphse II Centromeres split. Sister chromtids seprte nd move to opposite poles. Section 1 Meiosis 273

5 ( AREFILS IN BIOlOGY Medicl Geneticist A medicl geneticist reserches how diseses re inherited, how to dignose genetic conditions, nd tretments for genetic diseses. Inquiry Lunch Lb Review Bsed on wht you hve red bout meiosis, how would you now nswer the nlysis questions? During telophse I, cytokinesis usully occurs, forming furrow pinching in niml cells nd by forming cell plte in plnt cells. Poll ing cytokinesis, the cells my go into interphse gin before the Sec0 set of divisions. However, the DNA is not replicted gin during this interphse. In some species, the chromosomes uncoil, the nucler mern brne reppers, nd nuclei re-form during telophse I. elosis II Meiosis is only hlfwy completed t the end of meiosis I. During pro. phse II, second set of phses begins s the spindle pprtus forms the chromosomes condense. During metphse II, the chromosomes positioned t the equtor by the spindle fibers, s shown in Figure 5. During metphse of mitosis, diploid number of chromosomes linen t the equtor. During metphse II of meiosis, however, hploid flum. ber of chromosomes line up t the equtor. During nphse II, the sist chromtids re pulled prt t the centromere by the spindle fibers, nd the sister chromtids move towrd the opposite poles of the cell. The chromosomes rech the poles during telophse II, nd the nucler me brne nd nuclei reform. At the end of meiosis II, cytokinesis occurs, resulting in four hploid cells, ech with n number of chromosomes,s illustrted in Figure 5. Reding Check Infer Why re the two phses of meiosis importnt for gmete formtion?.:iiata I Bsed on Rel Dt* rw Co clu&ons cjj How do motor proteins ffect cell divi sion? Mny scientists think tht motor proteins ply n importnt role in the movement of chro mosomes in both mitosis nd meiosis. To test this hypothesis, reserchers hve produced yest tht cnnot mke the motor protein clled Kr3p. They lso hve produced yest tht cnnot mke the motor protein clled Cikip, which mny think modertes the func tion of Kr3p. The results of their experiment re shown in the grph to the right. Think Criticlly 1. Evlute whether Cikip seems to be impor tnt for yest meiosis. Explin. 2. Assess whether Kr3p seems to be necessry for yest meiosis. Explin. 3. Conclude whether ll motor proteins seem to ply vitl role in meiosis. Explin. I ~ Dt nd Observtions I-, C w S I U x i0~ 1 x106 Motor Protein Effect on cell Division Yest tht cnnot mke Kr3p Norml yest Yest tht cnnot mke Cikip Hours in meiosis medium *Dt obtined from: Shnks, et l The Kr3-lntercting protein cilcip plp criticl role in pssge through meiosis I in Scchromyces ce,evisie. GenebcS 159: chpter io Sexul Reproduction nd Genetics

6 by ~ importnce of Meiosis Review Personl Tutor fble i shows comprison of mitosis nd meiosis. Recll tht mitosis of only one set of division phses nd produces two identicl diploid dughter cells. Meiosis, however, consists of two sets of divisions 0d produces four hploid dughter cells tht re not identicl. Meiosis is 1~p0rt~1t becuse it results in genetic vrition. Tble I Mitosis nd Meiosis Concepts in Motion Interctive Tble one division occu during mitosis. Two sets of divisions occur during meiosis: meiosjs I nd meiosis II. ne up DNA repliction occurs during intemhse. DNA rè pliction occurs once before meiosjs I. EMun. Sister synpsis of homologous chromosomes does not occur. Synpsis of homologous chromosomes occurs during prophse I.. nd te Two identicl cells re formed per cell qicle. Four hploid cells (n) re formed per cell cycle. The dughter cells re geneticlly identicl. Mitosis occurs only in body cells. Mitosis is involved in growth nd repir. The dughter cells re not geneticlly identicl becuse of crossing over. Meiosis occurs only in reproductive cells. Meiosis is involved in the production of gmetes nd providing genetic vrition in orgnisms. Prophse Duplicted chromosome two sister chromtids) MITOSIS Prent cell MEIOSIS (before chromosome repliction) Chromosome repliction 4, 4 Crossing over I Chromosome repliction Prophse I Synpsis nd crossing over of homologous chromosomes Meiosis I Metphse Anphse Telophs /er chrom\ seprte during nphse Chromosomes line up t the equtor Dughter cells of mitosis c Homologous chromosomes seprte during nphse I; sister chromtids remin together Homologous pirs line up t the equtor Anphse I Metphse I itt. ~Telo~hse I Dughter cells 4) of meiosis I 4% Hploid c n=2 cc / Melosis II Dughter cells of meiosis II Chromosomes do not replicte gin; sister chromtids seprte during nphse II Section 1 Meiosis 275

7 Diploid prent SsYy Y y 57~ yy 5 Meiosis provides vrition Recll tht pirs of homologous chro mosomes lineup t the equtor during prophse I. How the chromo~ - line up t the equtor is rndom process tht results in gmetes with different combintions of chromosomes, such s the ones in Figure 6. Depending on how the chromosomes line up t the equtor, four gme~~ with four different combintions of chromosomes cn result. Notice tht the first possibility shows which chromosomes were o~1 the sme side of the equtor nd therefore trveled together. Different combintions of chromosomes were lined up on the sme side of the equtor to produce the gmetes in the second possibility. Genetic Vri. tion lso is produced during crossing over nd during fertiliztion, when gmetes rndomly combine. Y ~ () (p ) ~ Meiosis continues Possibility I Possibility 3 Potentil types of hploid gmetes Figure 6 The order in which the homologous pirs line up explins how vriety of sex cells cn be produced. y SQctLQn 1 Section Summry I DNA repliction tkes plce only once during meiosis, nd it results in four hploid gmetes. I Meiosis consists of two sets of divisions. I Meiosis produces genetic vrition in gmetes. Sexul eproduction v. Asexu Reproduction Some orgnisms reproduce by sexul reproduction, while others reproduce by sexul reproduction. The life cycles of still other orgn. isms might involve both sexul nd sexul reproduction. During sexul reproduction, the orgnism inherits ll of its chromodo~es from single prent. Therefore, the new individul is geneticlly iden. ticl to its prent. Bcteri reproduce sexully, wheres most]srotists 4 reproduce both sexully nd sexully, depending on environmentl conditions. Most plnts nd mny of the more simple nimli~n reproduce both sexully nd sexully, compred to more dvnced nimls tht reproduce only sexully. Why do some species reproduce sexully while others reproduce sexully? Recent studies with fruit flies hve shown tht the rte of ccumultion of beneficil muttions is fster when species reproduce sexully thn when they reproduce sexully. In other words, ~then reproduction occurs sexully, the beneficil genes multiply fstcr over time thn they do when reproduction is sexul. Assessment Understnd Min Ides 1. 4~ Anlyze how meiosis produces hploid gmetes. 2. Indicte how metphse us different from metphse in mitosis. 3. Describe how synpsis occurs. 4. Digrm cell with four chromosomes going through meiosis. 5. Assess how meiosis contributes to genetic vrition, while mitosis does no~ Think Criticlly 6. Compre nd contrst mitosis nd meiosis, using Figure 5 nd Tble 1,by creting Venn digrm. wnrrng t. Biology 7. lmgineyou re chromosome going through meiosis. Describe wht hppens to you nd theother chromosomes. S 276 chpter 10 Sexul Reproduction nd Genetics Assessment OiiI