BIOLOGY 101. CHAPTER 13: Meiosis and Sexual Life Cycles: Variations on a Theme

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1 BIOLOGY 101 CHAPTER 13: Meisis and Sexual Life Cycles: Variatins n a Theme

2 Meisis and Sexual Life Cycles: Variatins n a Theme CONCEPTS: 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes 13.2 Fertilizatin and meisis alternate in sexual life cycles 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.4 Genetic variatin prduced in sexual life cycles cntributes t evlutin

3 The Key Rles f Cell Divisin OVERVIEW: Meisis and Sexual Life Cycles The primary purpse f Meisis is t create haplid gametes, r egg and sperm that have nly 1 pair f 23 chrmsmes instead f 2 pairs f 23 chrmsmes (diplid) Anther critical functin f Meisis is t increase genetic diversity Genetic diversity is the crnerstne f natural selectin and evlutin Can yu explain hw the 2 primary purpses f meisis ccur?

ffspring with cded infrmatin in the frm f genes.

4 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes Parents endw their ffspring with cded infrmatin in the frm f genes. Yur genme is made up f the genes that yu inherited frm yur mther and yur father. Genes prgram specific traits that emerge as we develp frm fertilized eggs int adults. Genes are segments f DNA.

5 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes In plants and animals, reprductive cells called gametes (egg and sperm) transmit genes frm ne generatin t the next. After fertilizatin (fusin f a sperm cell and an egg), genes frm bth parents are present in the nucleus f the fertilized egg, r zygte.

6 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes Genes exist n chrmsmes and each gene has a specific lcatin, r lcus, n a specific chrmsme Fr example: The hemglbin gene is always n chrmsme 11, and in the same lcatin Humans have abut 23,000 genes

7 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes Images f the 46 human chrmsmes can be arranged in pairs in rder f size t prduce a karytype display. There are 46 chrmsmes because each parent dnates a set f 23 meaning that we have 2 f each chrmsme The tw chrmsmes in such a set, r hmlgus pair, have the same length, centrmere psitin, and staining pattern.

8 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes Smatic cells have tw cpies f each chrmsme One cpy is paternal, cming frm sperm One cpy is maternal, cming frm an egg The pairs f chrmsmes are called Hmlgus Chrmsmes 22 human chrmsme pairs are hmlgus these are called autsmes The 23 rd pair, the sex chrmsmes (X and Y), are nt hmlgus

9 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes Tw distinct sex chrmsmes, the X and the Y, are an exceptin t the general pattern f hmlgus chrmsmes in human smatic cells. The pattern f inheritance f the sex chrmsmes determines an individual s gender. Human females have a hmlgus pair f X chrmsmes (XX); males have ne X and ne Y chrmsme (XY).

10 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes When cells reprduce, they replicate chrmsmes during the S-phase f Interphase these cpies f chrmsmes are called Sister Chrmatids During the cell cycle hmlgus chrmsmes are cpied int Sister Chrmatids In Mitsis, these sister chrmatids are separated, leaving each new cell with 23 pairs f hmlgus chrmsmes Tw Hmlgus Chrmsmes Tw Sister Chrmatids

11 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes When cells reprduce, they replicate chrmsmes during the S-phase f Interphase these cpies f chrmsmes are called Sister Chrmatids Tw Sister Chrmatids During the cell cycle hmlgus chrmsmes are cpied int Sister Chrmatids In Mitsis, these sister chrmatids are separated, leaving each new cell with 23 (22+1) pairs f hmlgus chrmsmes Meisis separates hmlgus chrmsmes and sister chrmatids, leaving 4 new cells with nly ne set (23) chrmsmes Tw Hmlgus Chrmsmes

12 Backgrund: Genes and Inheritance 13.1 Offspring acquire genes frm their parents by inheriting chrmsmes The ccurrence f hmlgus pairs f chrmsmes is a cnsequence f sexual reprductin. We inherit ne chrmsme f each hmlgus pair frm each parent. The 46 chrmsmes in each smatic cell are tw sets f 23, a maternal set (frm yur mther) and a paternal set (frm yur father).

13 Offspring acquire genes frm their parents by inheriting chrmsmes 13.1 Like begets like, mre r less: a cmparisn f asexual and sexual reprductin Only rganisms that reprduce asexually can prduce ffspring that are exact cpies f them. In asexual reprductin, a single individual is the sle parent t dnate genes t its ffspring. An individual that reprduces asexually gives rise t a clne, a grup f genetically identical individuals. Members f a clne may be genetically different as a result f mutatin

14 Offspring acquire genes frm their parents by inheriting chrmsmes 13.1 Like begets like, mre r less: a cmparisn f asexual and sexual reprductin In sexual reprductin, tw parents prduce ffspring that have unique cmbinatins f genes inherited frm the tw parents. Unlike a clne, ffspring prduced by sexual reprductin vary genetically frm their siblings and their parents Genetic variatin is the rt f success fr all living rganisms Variatin insures that there are traits in a ppulatin that can survive a changing envirnment this is the basis f Natural Selectin

15 Fertilizatin and meisis alternate in sexual life cycles 13.2 Human cells cntain sets f chrmsmes The number f chrmsmes in a single set is represented by n. Any cell with tw sets f chrmsmes is called a diplid cell and has a diplid number f chrmsmes, abbreviated as 2n. Sperm cells r va (gametes) have nly ne set f chrmsmes 22 autsmes and an X (in an vum) r 22 autsmes and an X r a Y (in a sperm cell). A cell with a single chrmsme set is a haplid cell, abbreviated as n.

16 Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle The human life cycle begins when a haplid sperm cell fuses with a haplid vum. The unin f these gametes, culminating in the fusin f their nuclei, is fertilizatin. The fertilized egg (zygte) is diplid because it cntains tw haplid sets f chrmsmes bearing genes frm the maternal and paternal family lines. As a persn develps frm a zygte t a sexually mature adult, mitsis generates all the smatic cells f the bdy. Each smatic cell cntains a full diplid set f chrmsmes

Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle QUESTION: What wuld happen if gametes were prduced by mitsis?

17 Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle QUESTION: What wuld happen if gametes were prduced by mitsis? Hw many chrmsmes wuld each egg and sperm have? ANSWER: If gametes were prduced by mitsis, the fusin f gametes wuld prduce ffspring with fur sets f chrmsmes after ne generatin, eight after a secnd, and s n. This means that gametes cannt start ut with 2 pairs (n=46) f each chrmsme! Gametes must becme haplid (n=23), cntaining nly ne set (23) f chrmsmes The gametic set f 23 is a cmbinatin f bth the maternal and paternal chrmsmes

18 Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle QUESTION: What wuld happen if gametes were prduced by mitsis? Hw many chrmsmes wuld each egg and sperm have? ANSWER: If gametes were prduced by mitsis, the fusin f gametes wuld prduce ffspring with fur sets f chrmsmes after ne generatin, eight after a secnd, and s n. This means that gametes cannt start ut with 2 pairs (n=46) f each chrmsme! Gametes must becme haplid (n=23), cntaining nly ne set (23) f chrmsmes The gametic set f 23 is a cmbinatin f either the maternal r paternal chrmsmes This is why gametes cannt be prduced using mitsis! The prcess f Meisis is hw gametes are prduced Meisis starts with a diplid cell and ends with haplid gametes

Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle Gametes, which develp in the gnads (testes r varies), are nt prduced by mitsis.

19 Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle Gametes, which develp in the gnads (testes r varies), are nt prduced by mitsis. Gametes develp frm specialized diplid cells called germ cells in the gnads. Gametes underg the prcess f meisis, in which the chrmsme number is halved. Human sperm r va have a haplid set f 23 different chrmsmes, ne frm each hmlgus pair.

20 Human cells cntain sets f chrmsmes 13.2 Let s discuss the behavir f chrmsme sets in the human life cycle Gametes, which develp in the gnads (testes r varies), are nt prduced by mitsis. Gametes develp frm specialized diplid cells called germ cells in the gnads. Gametes underg the prcess f meisis, in which the chrmsme number is halved. Human sperm r va have a haplid set f 23 different chrmsmes, ne frm each hmlgus pair. Fertilizatin restres the diplid cnditin by cmbining tw haplid sets f chrmsmes

21 The Key Rles f Cell Divisin 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Many steps f meisis resemble steps in mitsis. Bth meisis and mitsis are preceded by the duplicatin f chrmsmes.

22 The Key Rles f Cell Divisin 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Many steps f meisis resemble steps in mitsis. Bth meisis and mitsis are preceded by the duplicatin f chrmsmes. In meisis, there are tw cnsecutive cell divisins, meisis I and meisis II, resulting in fur daughter cells. The first divisin, meisis I, separates hmlgus chrmsmes. The secnd divisin, meisis II, separates sister chrmatids. The fur daughter cells at the end f meisis have nly half as many chrmsmes as the riginal parent cell.

23 The Key Rles f Cell Divisin 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Many steps f meisis resemble steps in mitsis. Bth meisis and mitsis are preceded by the duplicatin f chrmsmes. In meisis, there are tw cnsecutive cell divisins, meisis I and meisis II, resulting in fur daughter cells. The first divisin, meisis I, separates hmlgus chrmsmes. The secnd divisin, meisis II, separates sister chrmatids. The fur daughter cells at the end f meisis have nly half as many chrmsmes as the riginal parent cell.

24 The Key Rles f Cell Divisin 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Nte that in Metaphase I f Meisis, it is the hmlgus chrmsmes that align!

25 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis I is preceded by interphase, in which the chrmsmes are duplicated t frm sister chrmatids Tw genetically identical sister chrmatids make up ne duplicated chrmsme. In cntrast, the tw chrmsmes f a hmlgus pair are individual chrmsmes that were inherited frm different parents. Hmlgus chrmsmes appear t be alike, but they may have different versins f genes, each called an allele, at crrespnding lci

26 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis Overview:

27 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis Overview: One purpse f Meisis I is t separate Hmlgus Chrmsmes (als genetic variatin) Meisis I cnsists f Prphase I, Metaphase I, Anaphase I and Telphase I

28 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis Overview: One purpse f Meisis I is t separate Hmlgus Chrmsmes (als genetic variatin) Meisis I cnsists f Prphase I, Metaphase I, Anaphase I and Telphase I

29 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis Overview: One purpse f Meisis I is t separate Hmlgus Chrmsmes (als genetic variatin) Meisis I cnsists f Prphase I, Metaphase I, Anaphase I and Telphase I One purpse f Meisis II is t separate Sister Chrmatids Meisis II cnsists f Prphase II, Metaphase II, Anaphase II and Telphase II

30 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 1. Synapsis and crssing ver in prphase I 2. Randm hmlg alignment n the metaphase plate in metaphase I 3. Separatin f parental hmlgs in anaphase I 4. The generatin f 2 new haplid cells in telphase I

31 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Crssing ver and synapsis ccurs during prphase I. PROPHASE I: Crssing ver begins very early in prphase I, as hmlgus chrmsmes pair lsely alng their lengths. Each gene n ne hmlg is aligned precisely with the crrespnding gene n the ther hmlg prcess is called Synapsis)

32 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Crssing ver and synapsis ccurs during prphase I. PROPHASE I: Crssing ver begins very early in prphase I, as hmlgus chrmsmes pair lsely alng their lengths. Each gene n ne hmlg is aligned precisely with the crrespnding gene n the ther hmlg prcess is called Synapsis) In a single crssver event, the DNA f tw nnsister chrmatids f a hmlgus pair, precisely align, frming a Tetrad The tw regins at the crssver pint are jined t the ther chrmatid.

33 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Crssing ver and synapsis ccurs during prphase I. PROPHASE I: Crssing ver begins very early in prphase I, as hmlgus chrmsmes pair lsely alng their lengths. Each gene n ne hmlg is aligned precisely with the crrespnding gene n the ther hmlg prcess is called Synapsis) In a single crssver event, the DNA f tw nnsister chrmatids f a hmlgus pair, precisely align, frming a Tetrad The tw regins at the crssver pint are jined t the ther chrmatid. Thus, a paternal chrmatid is jined t a piece f maternal chrmatid, and vice versa. Crssing ver creates genetic diversity because individual chrmsmes carry genes derived frm tw different parents

Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 1.

34 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 1. Synapsis and crssing ver During prphase I, duplicated hmlgs pair up in a prcess knwn as synapsis crssing ver ccurs at this time Synapsis and crssing ver nrmally d nt ccur during prphase f mitsis.

Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 2.

35 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 2. Randm Hmlg Alignment During metaphase I f meisis, 23 pairs f hmlgs (rather than sister chrmatids as in mitsis) line up n the metaphase plate. This alignment is randm, with a mix f maternal and paternal hmlgs n either side This randm alignment is called Independent Assrtment, and increases genetic diversity

36 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 2. Randm Hmlg Alignment During metaphase I f meisis, 23 pairs f hmlgs (rather than sister chrmatids as in mitsis) line up n the metaphase plate. This alignment is randm, with a mix f maternal and paternal hmlgs n either side This randm alignment is called Independent Assrtment, and increases genetic diversity

37 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 3. Separatin f parental hmlgs During anaphase I f meisis, the hmlgus chrmsmes are separated The sister chrmatids f each duplicated chrmsme remain attached. In anaphase f mitsis, by cntrast, the sister chrmatids are separated

Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 3.

38 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 We can cmpare mitsis and meisis Fur events unique t meisis ccur during meisis I: 3. Separatin f parental hmlgs During telphase I f meisis, cytkinesis als ccurs, creating tw new haplid cells The tw haplid cells still have sister chrmatids, s thse must be separated in meisis II Nuclear membranes briefly refrm, and spindles briefly disassemble

39 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis II Overview: The purpse f Meisis II is t separate Sister Chrmatids just like in Mitsis!

40 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis II Overview: The purpse f Meisis II is t separate Sister Chrmatids just like in Mitsis!

41 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis II Overview: The purpse f Meisis II is t separate Sister Chrmatids just like in Mitsis! Prphase II: Nuclear membranes breakdwn and spindles reassemble Metaphase II is when sister chrmatids line up at the metaphase plate Anaphase II is when sister chrmatids separate Telphase II prduces 4 new haplid cells Nuclear membranes refrm, spindles disassemble and chrmsmes unpack

42 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Meisis reduces the number f chrmsme sets frm diplid t haplid Meisis II Overview: The purpse f Meisis II is t separate Sister Chrmatids just like in Mitsis! Prphase II: Nuclear membranes breakdwn and spindles reassemble Metaphase II is when sister chrmatids line up at the metaphase plate Anaphase II is when sister chrmatids separate Telphase II prduces 4 new haplid cells Nuclear membranes refrm, spindles disassemble and chrmsmes unpack

43 Meisis reduces the number f chrmsme sets frm diplid t haplid 13.3 Genetic Variatin is Generated During Meisis I Tw events in Meisis I create genetic diversity 1. Crssing in prphase I (creates new versins f parental chrmssmes) 2. Independent Assrtment in metaphase I (randmly separates parental hmlgs)

44 The Key Rles f Cell Divisin 13.4 Genetic variatin prduced in sexual life cycles cntributes t evlutin Meisis is ne mechanism t increase genetic diversity (creates new cmbinatins f alleles) Mutatins are the riginal surce f genetic diversity. These changes in an rganism s DNA create the different versins f genes knwn as alleles. Primitive life that did nt use sexual reprductin primarily rely n mutatin fr their genetic diversity Once different versins f genes arise thrugh mutatin, reshuffling f alleles during sexual reprductin results in each member f a species having a unique cmbinatin f traits Sexual Reprductin is anther tremendus increase in genetic diversity

45 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Sexual life cycles prduce genetic variatin amng ffspring The behavir f chrmsmes during meisis and fertilizatin is respnsible fr mst f the variatin that arises in each generatin. Three mechanisms cntribute t genetic variatin arising frm sexual reprductin: Crssing ver (prphase I) Independent assrtment f chrmsmes (metaphase I) Randm fertilizatin (sexual reprductin)

46 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Sexual life cycles prduce genetic variatin amng ffspring Crssing ver prduces recmbinant chrmsmes, which cmbine genes inherited frm each parent During meisis in humans, an average f ne t three crssver events ccur per chrmsme pair Crssing ver, by cmbining DNA inherited frm tw parents int a single chrmsme, is an imprtant surce f genetic variatin. Independent Assrtment: At metaphase I, nnidentical sister chrmatids srt independently frm ne anther, further increasing the number f genetic types f daughter cells that are frmed by meisis. Independent Assrtment Crssing Over

47 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Sexual life cycles prduce genetic variatin amng ffspring The randm nature f fertilizatin adds t the genetic variatin arising frm meisis

48 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Evlutinary adaptatin depends n a ppulatin s genetic variatin Charles Darwin recgnized the imprtance f genetic variatin in evlutin.

49 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Evlutinary adaptatin depends n a ppulatin s genetic variatin Charles Darwin recgnized the imprtance f genetic variatin in evlutin. A ppulatin evlves thrugh the differential reprductive success f its variant members. Thse individuals best suited (having the best allele cmbinatins) t the lcal envirnment leave the mst ffspring, transmitting their genes in the prcess. This natural selectin results in adaptatin, the accumulatin f favrable genetic variatins in a specific envirnment

50 Genetic variatin prduced in sexual life cycles cntributes t evlutin 13.4 Evlutinary adaptatin depends n a ppulatin s genetic variatin As the envirnment changes, the ppulatin may survive if sme members can cpe effectively with the new cnditins. Mutatins are the riginal surce f different alleles, which are then mixed and matched during meisis. Mre variatin cmes frm crssing ver and independent assrtment New and different cmbinatins f alleles may wrk better than thse that previusly prevailed. The ability f sexual reprductin t generate genetic diversity is ne f the mst cmmnly prpsed advantages t explain the evlutinary persistence f this prcess.

Mitosis and Meiosis Lecture Notes

Mitosis and Meiosis Lecture Notes Bilgy Mitsis and Meisis Lecture Ntes Name Per Learning Gals Quiz #6: December 6th Describe what happens during interphase Identify steps f mitsis/meisis by picture and functin Explain the diseases that