Meiosis and Sexual Reproduction

Transcription

1 Chapter Preview 1 Reproductio Asexual Reproductio Sexual Reproductio Chromosome Number 11 2 Meiosis Stages of Meiosis Comparig Mitosis ad Meiosis Geetic Variatio 3 Multicellular Life Cycles Diploid Life Cycle Haploid Life Cycle Alteratio of Geeratios Meiosis ad Sexual Reproductio Like may orgaisms, the daddy loglegs spider reproduces sexually. Meiosis is a process that forms the eggs ad sperm that make sexual reproductio possible for the daddy loglegs spiders. Why It Matters You kow that i sexual reproductio, a egg ad a sperm combie to form a ew orgaism. But how are eggs ad sperm produced? I this chapter, you will lear about a special type of cell divisio called meiosis. A mother daddy loglegs spider will watch over her ewly hatched youg for ie days. After ie days, the youg shed their ski ad leave the web to build their ow webs. 244

2 Sectio 1 Reproductio Key Ideas V I asexual reproductio, how does the offsprig compare to the paret? V I sexual reproductio, how does the offsprig compare to the paret? V Why are chromosomes importat to a orgaism? Key Terms gamete zygote diploid haploid homologous chromosomes Why It Matters Livig orgaisms produce offsprig. How closely the offsprig resemble their parets depeds o how the orgaism reproduces. Reproductio is the process of producig offsprig. Some offsprig are produced by two parets, ad others are produced by just oe paret. Some orgaisms look exactly like their parets, ad others look very similar. Whether a orgaism is idetical or similar to its paret is determied by the way that the orgaism reproduces. Asexual Reproductio I asexual reproductio, a sigle paret passes a complete copy of its geetic iformatio to each of its offsprig. V A idividual formed by asexual reproductio is geetically idetical to its paret. Prokaryotes reproduce asexually by a kid of cell divisio called biary fissio. May uicellular eukaryotes also reproduce asexually. Amoebas reproduce by splittig ito two or more idividuals of about equal size. Some multicellular eukaryotes, such as starfish, go through fragmetatio. Fragmetatio is a kid of reproductio i which the body breaks ito several pieces. Some or all of these fragmets regrow missig parts ad develop ito complete adults. Other aimals, such as the hydra show i Figure 1, go through buddig. I buddig, ew idividuals split off from existig oes. Some plats, such as potatoes, ca form whole ew plats from parts of stems. Other plats ca reproduce from roots or leaves. Some crustaceas, such as water fleas, reproduce by partheogeesis. Partheogeesis is a process i which a female makes a viable egg that grows ito a adult without beig fertilized by a male. V Readig Check What is fragmetatio? (See the Appedix for aswers to Readig Checks.) Figure 1 This hydra is i the process of reproducig asexually. The smaller hydra buddig from the paret is geetically idetical to the paret. SECTION 1 Reproductio 247

3 gamete (GAM eet) a haploid reproductive cell that uites with aother haploid reproductive cell to form a zygote zygote (ZIE GOHT) the cell that results from the fusio of gametes diploid a cell that cotais two haploid sets of chromosomes haploid describes a cell, ucleus, or orgaism that has oly oe set of upaired chromosomes homologous chromosomes (hoh MAHL uh guhs) chromosomes that have the same sequece of gees, that have the same structure, ad that pair durig meiosis Topic: Sexual ad Asexual Reproductio Code: HX81386 Figure 2 Two gametes, a egg ad a sperm, combie durig fertilizatio to form a zygote. V What types of cells produce gametes? Sexual Reproductio Most eukaryotic orgaisms reproduce sexually. V I sexual reproductio, two parets give geetic material to produce offsprig that are geetically differet from their parets. Each paret produces a reproductive cell, called a gamete. A gamete from oe paret fuses with a gamete from the other paret, as Figure 2 shows. The resultig cell, called a zygote, has a combiatio of geetic material from both parets. This process is called fertilizatio. Because both parets give geetic material, the offsprig has traits of both parets but is ot exactly like either paret. Germ Cells ad Somatic Cells Recall that the cells of a multicellular orgaism are ofte specialized for certai fuctios. Muscle cells, for example, cotract ad move your body. Cells that are specialized for sexual reproductio are called germ cells. Oly germ cells ca produce gametes. Other body cells are called somatic cells. Somatic cells do ot participate i sexual reproductio. Advatages of Sexual Reproductio Asexual reproductio is the simplest, most efficiet method of reproductio. Asexual reproductio allows orgaisms to produce may offsprig i a short period of time without usig eergy to make gametes or to fid a mate. But the geetic material of these orgaisms varies little betwee idividuals, so they may be at a disadvatage i a chagig eviromet. Sexual reproductio, i cotrast, produces geetically diverse idividuals. A populatio of diverse orgaisms is more likely to have some idividuals that survive a major evirometal chage. Chromosome Number Gees are located o chromosomes. V Each chromosome has thousads of gees that play a importat role i determiig how a orgaism develops ad fuctios. Each species has a characteristic umber of chromosomes. As show i Figure 3, mosquitoes have oly 6 chromosomes i each cell. Chimpazees have 48 chromosomes i each cell. Some fers have more tha 500! A orgaism must have exactly the right umber of chromosomes. If a orgaism has too may or too few chromosomes, the orgaism may ot develop ad fuctio properly. I humas, each cell has two copies of 23 chromosomes for a total of 46. Whe fertilizatio happes, two cells combie to form a zygote, which still has oly 46 chromosomes. Why is the umber the same? The gametes that form a zygote have oly oe copy of each chromosome, or oe set of 23 chromosomes. This reductio of chromosomes i gametes keeps the chromosome umber of huma somatic cells at a costat 46. V Readig Check What kid of cells do germ cells produce? 248 CHAPTER 11 Meiosis ad Sexual Reproductio

4 Haploid ad Diploid Cells A cell, such as a somatic cell, that has two sets of chromosomes is diploid. A cell is haploid if it has oe set of chromosomes. Gametes are haploid cells. The symbol is used to represet the umber of chromosomes i oe set. Huma gametes have 23 chromosomes, so = 23. The diploid umber i somatic cells is writte as 2. Huma somatic cells have 46 chromosomes (2 = 46). Chromosome Number of Various Orgaisms Orgaism Peicillium 1 4 Saccharomyces (yeast) 16 Mosquito 6 Housefly 12 Number (2) of chromosomes Homologous Chromosomes Each diploid cell has pairs of chromosomes made up of two homologous chromosomes. Homologous chromosomes are chromosomes that are similar i size, i shape, ad i kids of gees that they cotai. Each chromosome i a homologous pair comes from oe of the two parets. I humas, oe set of 23 chromosomes comes from the mother, ad oe set comes from the father. Homologous chromosomes ca carry differet forms of gees. For example, flower color i peas is determied by a gee o oe of its chromosomes. The form of this gee ca be Garde pea Cor Fer Frog Huma Oraguta Dog , Figure 3 white or purple. The cells of each pea plat will have two flower-color Differet species have differet umbers gees, oe o each of the chromosomes that carry the flower-color of chromosomes. gee. Both could be gees for white flower color, or both could be gees for purple flower color. Or oe gee could be for white color, ad the other could be for purple color. Autosomes ad Sex Chromosomes Autosomes are chromosomes with gees that do ot determie the sex of a idividual. Sex chromosomes have gees that determie the sex of a idividual. I humas ad may other orgaisms, the two sex chromosomes are referred to as the X ad Y chromosomes. The gees that cause a zygote to develop ito a male are located o the Y chromosome. Huma males have oe X chromosome ad oe Y chromosome (XY), ad huma females have two X chromosomes (XX). Key-Term Fold O the back of your keyterm fold, write a defiitio i your ow words for the key terms i this sectio. Sectio 1 V Review KEY IDEAS 1. Compare the offsprig i asexual reproductio with the paret. 2. Describe how the offsprig i sexual reproductio compares geetically with its paret. 3. Compare the umber of sets of chromosomes betwee a haploid cell ad a diploid cell. 4. Explai why chromosomes are importat for orgaisms. CRITICAL THINKING 5. Iferrig Relatioships Why are haploid cells importat i sexual reproductio? 6. Formig Reasoed Opiios Do you agree or disagree that homologous chromosomes occur i gametes? Explai. METHODS OF SIENCE 7. Evaluatig Hypotheses A studet states that orgaisms that reproduce asexually are at a disadvatage i a stable eviromet. If you agree with this hypothesis, ame oe or more of its stregths. If you disagree, ame oe or more of its weakesses. SECTION 1 Reproductio 249

5 Sectio 2 Meiosis Key Ideas V What occurs durig the stages of meiosis? V How does the fuctio of mitosis differ from the fuctio of meiosis? V What are three mechaisms of geetic variatio? Key Terms meiosis crossig-over idepedet assortmet Why It Matters Meiosis allows geetic iformatio from two parets to combie to form offsprig that are differet from both parets. Most cells that divide ad produce ew cells form two offsprig cells that have the same umber of chromosomes as the paret cell. How do haploid gametes form from a diploid germ cell? Meiosis is a form of cell divisio that produces daughter cells with half the umber of chromosomes that are i the paret cell. Stages of Meiosis Before meiosis begis, the chromosomes i the origial cell are copied. Meiosis ivolves two divisios of the ucleus meiosis I ad meiosis II. V Durig meiosis, a diploid cell goes through two divisios to form four haploid cells. I meiosis I, homologous chromosomes are separated. I meiosis II, the sister chromatids of each homologue are separated. As a result, four haploid cells are formed from the origial diploid cell. Figure 4 illustrates the steps of meiosis. Stages of Meiosis I 1 Prophase I Chromosomes codese. The uclear evelope breaks dow. 2 Metaphase I Pairs of homologous chromosomes move to the cell s equator. 3 Aaphase I Homologous chromosomes move to the cell s opposite poles. 4 Telophase I Chromosomes gather at the poles. The cytoplasm divides. Spidle Homologous chromosomes 250 CHAPTER 11 Meiosis ad Sexual Reproductio

6 Meiosis I Meiosis begis with a diploid cell that has copied its chromosomes. The first phase is prophase I. 1 Durig prophase I, the chromosomes codese, ad the uclear evelope breaks dow. Homologous chromosomes pair. Chromatids exchage geetic material i a process called crossig-over. 2 I metaphase I, the spidle moves the pairs of homologous chromosomes to the equator of the cell. The homologous chromosomes remai together. 3 I aaphase I, the homologous chromosomes separate. The spidle fibers pull the chromosomes of each pair to opposite poles of the cell. But the chromatids do ot separate at their cetromeres. Each chromosome is still made of two chromatids. The geetic material, however, has recombied. 4 Durig telophase I, the cytoplasm divides (cytokiesis), ad two ew cells are formed. Both cells have oe chromosome from each pair of homologous chromosomes. Meiosis II Meiosis II begis with the two cells formed at the ed of telophase I of meiosis I. The chromosomes are ot copied betwee meiosis I ad meiosis II. 5 I prophase II, ew spidles form. 6 Durig metaphase II, the chromosomes lie up alog the equators ad are attached at their cetromeres to spidle fibers. 7 I aaphase II, the cetromeres divide. The chromatids, which are ow called chromosomes, move to opposite poles of the cell. 8 Durig telophase II, a uclear evelope forms aroud each set of chromosomes. The spidle breaks dow, ad the cell goes through cytokiesis. The result of meiosis is four haploid cells. V Readig Check I what phase of meiosis is geetic material exchaged? meiosis a process i cell divisio durig which the umber of chromosomes decreases to half the origial umber by two divisios of the ucleus, which results i the productio of sex cells (gametes or spores) crossig-over the exchage of geetic material betwee homologous chromosomes durig meiosis Figure 4 Durig meiosis, four haploid cells are produced from a diploid cell. V What is the differece betwee aaphase I ad aaphase II? Stages of Meiosis II Keyword: HX8MEIF4 5 Prophase II A ew spidle forms aroud the chromosomes. 6 Metaphase II Chromosomes lie up at the equators. 7 Aaphase II Cetromeres divide, ad chromatids move to opposite poles. 8 Telophase II A uclear evelope forms aroud each set of chromosomes. The cells divide. SECTION 2 Meiosis 251

7 Comparisos Write two seteces that compare ad two seteces that cotrast meiosis ad mitosis. Figure 5 Mitosis produces two diploid daughter cells that are idetical to the paret cell. Meiosis produces four haploid cells from a diploid cell. V What is the differece betwee aaphase i mitosis ad aaphase I i meiosis I? Comparig Mitosis ad Meiosis The processes of mitosis ad meiosis are similar but meet differet eeds ad have differet results. V Mitosis makes ew cells that are used durig growth, developmet, repair, ad asexual reproductio. Meiosis makes cells that eable a orgaism to reproduce sexually ad happes oly i reproductive structures. Mitosis produces two geetically idetical diploid cells. I cotrast, meiosis produces four geetically differet haploid cells. The haploid cells produced by meiosis cotai half the geetic iformatio of the paret cell. Whe two such cells, ofte a egg cell ad a sperm cell, combie, the resultig zygote has the same umber of chromosomes as each of the parets cells. If you compare meiosis ad mitosis, as show i Figure 5, you may thik that they are alike. For example, i metaphase of mitosis ad metaphase I of meiosis, the chromosomes move to the equator. However, there is a major differece that happes i a earlier stage. I prophase I of meiosis, every chromosome pairs with its homologue. A pair of homologous chromosomes is called a tetrad. As the tetrads form, differet homologues exchage parts of their chromatids i the process of crossig-over. The pairig of homologous chromosomes ad the crossig-over do ot happe i mitosis. Therefore, a mai differece betwee meiosis ad mitosis is that i meiosis, geetic iformatio is rearraged. The rearragig of geetic iformatio leads to geetic variatio i offsprig. Crossigover is oe of several processes that lead to geetic variatio. V Readig Check How are cells formed by mitosis differet from cells formed by meiosis i relatio to umber of chromosomes? Comparig Mitosis ad Meiosis Mitosis 2 idetical diploid cells 4 oidetical haploid cells Prophase Metaphase Aaphase Telophase Meiosis Meiosis II Prophase I Metaphase I Aaphase I Telophase I 252 CHAPTER 11 Meiosis ad Sexual Reproductio

8 Hads O 30 mi Crossig-Over Model You ca use paper strips ad pecils to model the process of crossig-over. Procedure 1 Use a colored pecil to write A ad B o two paper strips. These two strips will represet oe of the two homologous chromosomes show. 2 Use a secod colored pecil to write a ad b o two paper strips. These two strips will represet the secod homologous chromosome show. 3 CAUTION: Hadle scissors with care. Use your chromosome models, scissors, ad tape to demostrate crossig-over betwee two chromatids. B b B b A A a a Homologous chromosomes Aalysis 1. Determie what the letters A, B, a, ad b represet. 2. Makig Ifereces Explai why the chromosomes that you made are homologous. 3. Compare the umber of differet types of chromatids (combiatios of A, B, a, ad b) before crossig-over with the umber after crossig-over. 4. CRITICAL THINKING Aalyzig Iformatio How does crossig-over relate to geetic recombiatio? Geetic Variatio Geetic variatio is advatageous for a populatio. Geetic variatio ca help a populatio survive a major evirometal chage. For example, i the Arctic, if temperatures drop below average, those polar bears with gees that make thicker fur will survive. Polar bears without the gees for thicker fur may die out. The polar bears with the gees for thicker fur reproduce, ad the populatio grows. Now, suppose that all of the idividuals i the populatio have the same gees, but oe of the gees are for thicker fur. What do you thik will happe if the temperature drops below average? The etire populatio of polar bears may die out. Geetic variatio is made possible by sexual reproductio. I sexual reproductio, existig gees are rearraged. Meiosis is the process that makes the rearragig of gees possible. Fusio of haploid cells from two differet idividuals adds further variatio. V Three key cotributios to geetic variatio are crossig-over, idepedet assortmet, ad radom fertilizatio. Topic: Geetic Variatio Code: HX80658 ACADEMIC VOCABULARY exist to occur or be preset Crossig-Over Durig prophase I, homologous chromosomes lie up ext to each other. Each homologous chromosome is made of two sister chromatids attached at the cetromere. Crossig-over happes whe oe arm of a chromatid crosses over the arm of the other chromatid, as illustrated i the QuickLab. The chromosomes break at the poit of the crossover, ad each chromatid re-forms its full legth with the piece from the other chromosome. Thus, the sister chromatids of a homologous chromosome o loger have idetical geetic iformatio. V Readig Check How ca crossig-over icrease geetic variatio? SECTION 2 Meiosis 253

9 Figure 6 The same cell is show twice. Because each pair of homologous chromosomes separates idepedetly, four differet gametes ca result i each case. Possibility 1 The arragemet of chromosomes i each of these cells is equally probable. Metaphase of meiosis I Possibility 2 Metaphase of meiosis II These gametes show differet possible combiatios. idepedet assortmet the radom distributio of the pairs of gees o differet chromosomes to the gametes Idepedet Assortmet Durig metaphase I, homologous pairs of chromosomes lie up at the equator of the cell. The two pairs of chromosomes ca lie up i either of two equally probable ways. This radom distributio of homologous chromosomes durig meiosis is called idepedet assortmet. The four haploid cells formed i possibility 1 i Figure 6 have etirely differet combiatios of chromosomes tha do the four cells made i possibility 2. I humas, each gamete receives oe chromosome from each of 23 pairs of homologous chromosomes. Each of the 23 pairs of chromosomes separates idepedetly. Thus, 2 23 (about 8 millio) gametes with differet gee combiatios ca be made from oe origial cell. Radom Fertilizatio Fertilizatio is a radom process that adds geetic variatio. The zygote that forms is made by the radom joiig of two gametes. Because fertilizatio of a egg by a sperm is radom, the umber of possible outcomes is squared. I humas, the possibility is , or 64 trillio, differet combiatios! Sectio 2 V Review KEY IDEAS 1. Summarize the differet phases of meiosis. 2. Explai how the fuctio of meiosis differs from the fuctio of mitosis. 3. Describe three mechaisms of geetic variatio. CRITICAL THINKING 4. Comparig Fuctios Compare the processes of crossig-over ad idepedet assortmet. How does each cotribute to geetic variatio? 5. Iferrig Coclusios Why might sexual reproducers better adapt to a chagig eviromet tha asexual reproducers? ALTERNATIVE ASSESSMENT 6. Word Problem If oe cell i a dog (2 = 78) udergoes meiosis ad aother cell udergoes mitosis, how may chromosomes will each resultig cell cotai? 254 CHAPTER 11 Meiosis ad Sexual Reproductio

10 Sectio 3 Multicellular Life Cycles Key Ideas V What is a diploid life cycle? V What is a haploid life cycle? V What is alteratio of geeratios? Key Terms life cycle sperm ovum Why It Matters Some life cycles are maily diploid, others are maily haploid, ad still others alterate betwee haploid ad diploid phases. All of the evets i the growth ad developmet of a orgaism util the orgaism reaches sexual maturity are called a life cycle. All orgaisms that reproduce sexually have both diploid stages ad haploid stages. Diploid Life Cycle Most aimals have a diploid life cycle. Figure 7 illustrates this type of life cycle. Most of the life cycle is spet i the diploid state. All of the cells except the gametes are diploid. A diploid germ cell i a reproductive orga goes through meiosis ad forms gametes. The gametes, the sperm ad the egg, joi durig fertilizatio. The result is a diploid zygote. This sigle diploid cell goes through mitosis ad evetually gives rise to all of the cells of the adult, which are also diploid. V I diploid life cycles, meiosis i germ cells of a multicellular diploid orgaism results i the formatio of haploid gametes. Figure 7 Humas ad most other aimals have a life cycle domiated by a diploid idividual. V What are the oly haploid cells i a diploid life cycle? Diploid Life Cycle Adult male Adult female Baby life cycle all of the evets i the growth ad developmet of a orgaism util the orgaism reaches sexual maturity sperm the male gamete (sex cell) ovum a mature egg cell Meiosis Sperm Egg Mitosis Fertilizatio Zygote 256 CHAPTER 11 Meiosis ad Sexual Reproductio

11 Meiosis ad Gamete Formatio Male aimals produce gametes called sperm. As Figure 8 illustrates, a diploid germ cell goes through meiosis I. Two cells are formed, each of which goes through meiosis II. The result is four haploid cells. The four cells chage i form ad develop a tail to form four sperm. Female aimals produce gametes called eggs, or ova (sigular, ovum). A diploid germ cell begis to divide by meiosis. Meiosis I results i the formatio of two haploid cells that have uequal amouts of cytoplasm. Oe of the cells has early all of the cytoplasm. The other cell, called a polar body, is very small ad has a small amout of cytoplasm. The polar body may divide agai, but its offsprig cells will ot survive. The larger cell goes through meiosis II, ad the divisio of the cell s cytoplasm is agai uequal. The larger cell develops ito a ovum. The smaller cell, the secod polar body, dies. Because of its larger share of cytoplasm, the mature Data ovum has a rich storehouse of utriets. These utriets ourish the youg orgaism that develops if the ovum is fertilized. Chromosome Combiatios 15 mi Whe a sperm ad egg fuse, two sets of chromosomes are combied. I this lab, you will model this cross betwee two sets of chromosomes. Procedure 1 Write F1F2 X M1M2 o a sheet of paper. F1 ad F2 represet the father s chromosomes. M1 ad M2 represet the mother s chromosomes. 2 Determie all of the possible chromosome combiatios i the zygote that forms from the fusio of the gametes with the chromosomes that you wrote i step 1. Aalysis 1. Calculate the umber of chromosome combiatios that are possible i the zygote. 2. CRITICAL THINKING Aalyzig Data List all of the possible chromosome combiatios. V Readig Check How may gametes are formed from oe female germ cell? Figure 8 Meiosis of diploid germ cells results i haploid gametes. Meiosis i Male ad Female Aimals Spermatogeesis Oogeesis Diploid germ cell Diploid germ cell Meiosis I Meiosis II Haploid cells Haploid cell Haploid polar bodies (All 3 will die.) Sperm Ovum SECTION 3 Multicellular Life Cycles 257

12 Haploid Life Cycle Diploid (2) Haploid () Figure 9 Some orgaisms, such as fugi, have haploid cells as a major portio of their life cycles. Haploid cells Meiosis 2 Zygote Haploid idividuals Fusio Mitosis Gametes Whe haploid cells divide by meiosis, multicellular haploid idividuals result. Two-colum otes Use two-colum otes to summarize the stages ad details of the haploid life cycle. Haploid Life Cycle The haploid life cycle, show i Figure 9, happes i most fugi ad some protists. Haploid stages make up the major part of this life cycle. The zygote, the oly diploid structure, goes through meiosis immediately after it is formed ad makes ew haploid cells. The haploid cells divide by mitosis ad give rise to multicellular haploid idividuals. V I haploid life cycles, meiosis i a diploid zygote results i the formatio of the first cell of a multicellular haploid idividual. Alteratio of Geeratios V Plats ad most multicellular protists have a life cycle that alterates betwee a haploid phase ad a diploid phase called alteratio of geeratios. I plats, the multicellular diploid phase i the life cycle is called a sporophyte. Spore-formig cells i the sporophyte udergo meiosis ad produce spores. A spore forms a multicellular gametophyte. The gametophyte is the haploid phase that produces gametes by mitosis. The gametes fuse ad give rise to the diploid phase. Sectio 3 V Review KEY IDEAS 1. Summarize the process i a diploid life cycle. 2. Describe what happes i a haploid life cycle. 3. Describe what happes to the polar bodies formed durig meiosis of a female diploid cell i aimal. 4. Explai the alteratio of geeratios life cycle. CRITICAL THINKING 5. Evaluatig Processes How does the formatio of sperm through meiosis of a diploid germ cell differ from the formatio of a ovum from a diploid germ cell? 6. Aalyzig Iformatio What type of cell or structure is the first stage of every sexual life cycle? WRITING IN SCIENCE 7. Lesso Pla Write a lesso pla that you ca use to teach a classmate the differece betwee a haploid ad a diploid life cycle. I your ow words, write a summary of each. Iclude diagrams with your explaatio. 258 CHAPTER 11 Meiosis ad Sexual Reproductio

13 Chapter 11 Summary Keyword: HX8MEIS Key Ideas Key Terms 1 Reproductio gamete (248) V A idividual formed by asexual reproductio is geetically idetical to its paret. V I sexual reproductio, two parets give geetic material to produce offsprig that are geetically differet from their parets. V Each chromosome has thousads of gees that play a importat role i determiig how a orgaism develops ad fuctios. zygote (248) diploid (249) haploid (249) homologous chromosomes (249) 2 Meiosis V Durig meiosis, a diploid cell goes through two divisios to form four haploid cells. V Mitosis produces cells that are used durig growth, developmet, repair, ad asexual reproductio. Meiosis makes cells that eable a orgaism to reproduce sexually ad it oly happes i reproductive structures. V Three key cotributios to geetic variatio are crossig-over, idepedet assortmet, ad radom fertilizatio. meiosis (250) crossig-over (251) idepedet assortmet (254) 3 Multicellular Life Cycles V I diploid life cycles, meiosis i germ cells of a multicellular diploid orgaism results i the formatio of haploid gametes. V I haploid life cycles, meiosis i a diploid zygote results i the formatio of the first cell of a multicellular haploid idividual. V Plats ad most multicellular protists have a life cycle that alterates betwee a haploid phase ad a diploid phase called alteratio of geeratios. life cycle (256) sperm (257) ovum (257) 260 CHAPTER 11 Meiosis ad Sexual Reproductio