Meiosis and Sexual Life Cycles 10 Meiosis and Sexual Life Cycles

Aim: How can we describe gametes and how they are made Aim: How can we describe gametes and how they are made? Do Now: Where did your father get his Y chromosome from? How do you know? Homework: Complete end of chapter questions for chapter 10.

Overview: Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Genetics is the scientific study of heredity and variation 3

Inheritance of Genes Genes are the units of heredity and are made up of segments of DNA Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) 4

Most DNA is packaged into chromosomes For example, humans have 46 chromosomes in their somatic cells, the cells of the body except for gametes and their precursors Each gene has a specific position, or locus, on a certain chromosome Genes are very specific, each chromosome has a unique set of genes. 5

Comparison of Asexual and Sexual Reproduction In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Video: Hydra Budding 6

Concept 10.2: Fertilization and meiosis alternate in sexual life cycles A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism 7

Sets of Chromosomes in Human Cells Human somatic cells have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters 8

Figure 10.3c 5 m Figure 10.3c Research method: preparing a karyotype (part 3: results) 9

Human females have a homologous pair of X chromosomes (XX) The sex chromosomes, which determine the sex of the individual, are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The remaining 22 pairs of chromosomes are called autosomes 10

A diploid cell (2n) has two sets of chromosomes Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n  46) 11

Each replicated chromosome consists of two identical sister chromatids In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids 12

Key Maternal set of chromosomes (n  3) 2n  6 Paternal set of Figure 10.4 Key Maternal set of chromosomes (n  3) 2n  6 Paternal set of chromosomes (n  3) Sister chromatids of one duplicated chromosome Centromere Figure 10.4 Describing chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) 13

For humans, the haploid number is 23 (n = 23) A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y 14

Behavior of Chromosome Sets in the Human Life Cycle Fertilization is the union of gametes (the sperm and the egg) The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult 15

At sexual maturity, the ovaries and testes produce haploid gametes Gametes are the only types of human cells produced by meiosis rather than mitosis Meiosis results in one set of chromosomes in each gamete 16

Aim: How can we describe the life cycle of meiosis Aim: How can we describe the life cycle of meiosis? Do Now: Are somatic cells haploid or diploid? How about gametes?

Sexual Life Cycles Gametes are the only haploid cells in animals They are produced by meiosis and undergo no further cell division before fertilization Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

Key Haploid (n) n Gametes n Diploid (2n) n MEIOSIS FERTILIZATION Figure 10.6a Key Haploid (n) n Gametes n Diploid (2n) n MEIOSIS FERTILIZATION Zygote 2n 2n Figure 10.6a Three types of sexual life cycles (part 1: animal) Mitosis Diploid multicellular organism (a) Animals 19

Concept 10.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell, and are not genetically identical to the parent cell. 20

The Stages of Meiosis For a single pair of homologous chromosomes in a diploid cell, both members of the pair are duplicated The resulting sister chromatids are closely associated all along their lengths Homologs may have different versions of genes, each called an allele Homologs are not associated in any obvious way except during meiosis 21

duplicated chromosomes Meiosis II Figure 10.7 Interphase Pair of homologous chromosomes in diploid parent cell Chromosomes duplicate Duplicated pair of homologous chromosomes Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 10.7 Overview of meiosis: how meiosis reduces chromosome number Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes 22

Interphase Pair of homologous chromosomes in diploid parent cell Figure 10.7a Interphase Pair of homologous chromosomes in diploid parent cell Chromosomes duplicate Duplicated pair of homologous chromosomes Figure 10.7a Overview of meiosis: how meiosis reduces chromosome number (part 1: interphase) Sister chromatids Diploid cell with duplicated chromosomes 23

duplicated chromosomes Meiosis II 2 Sister chromatids separate Figure 10.7b Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Figure 10.7b Overview of meiosis: how meiosis reduces chromosome number (part 2: meiosis I and II) Haploid cells with unduplicated chromosomes 24

Meiosis halves the total number of chromosomes very specifically It reduces the number of sets from two to one, with each daughter cell receiving one set of chromosomes 25

Four new haploid cells are produced as a result In the first meiotic division, homologous pairs of chromosomes pair and separate In the second meiotic division, sister chromatids of each chromosome separate Four new haploid cells are produced as a result NOTE – this is not part of the text, but rather a quick summary of what is in Figure 10.8. Animation: Meiosis Video: Meiosis I in Sperm Formation 26

Figure 10.8 http://www.youtube.com/watch?v=qCLmR9-YY7o MEIOSIS I: Separates homologous chromosomes MEIOSIS II: Separates sister chromatids Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids Centromere (with kinetochore) Sister chromatids remain attached Centrosome (with centriole pair) Cleavage furrow Chiasmata Metaphase plate Spindle Sister chromatids separate Homologous chromosomes separate Fragments of nuclear envelope Figure 10.8 Exploring meiosis in an animal cell Microtubule attached to kinetochore Homologous chromosomes Haploid daughter cells forming 27

MEIOSIS I: Separates homologous chromosomes Figure 10.8a MEIOSIS I: Separates homologous chromosomes Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Sister chromatids Centromere (with kinetochore) Sister chromatids remain attached Centrosome (with centriole pair) Cleavage furrow Chiasmata Metaphase plate Spindle Figure 10.8a Exploring meiosis in an animal cell (part 1: meiosis I) Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore Homologous chromosomes 28

MEIOSIS II: Separates sister chromatids Figure 10.8b MEIOSIS II: Separates sister chromatids Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Sister chromatids separate Figure 10.8b Exploring meiosis in an animal cell (part 2: meiosis II) Haploid daughter cells forming 29

Chromosomes begin to condense Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene 30

In crossing over, nonsister chromatids exchange DNA segments Each homologous pair has one or more X-shaped regions called chiasmata Chiasmata exist at points where crossing over has occurred. 31

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome 32

In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole 33

Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells 34

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated 35

Division in meiosis II also occurs in four phases: Prophase II Metaphase II Anaphase II Telophase II and cytokinesis Meiosis II is very similar to mitosis 36

In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate 37

Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles 38

In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles 39

Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing 40

At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell 41

Aim: How can we compare and contrast mitosis and meiosis Aim: How can we compare and contrast mitosis and meiosis? Do Now: Name and describe one unique event that occurs during meiosis.

A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell Meiosis includes two divisions after replication, each with specific stages 43

Three events are unique to meiosis, and all three occur in meiosis l Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information Homologous pairs at the metaphase plate: Homologous pairs of chromosomes are positioned there in metaphase I Separation of homologs during anaphase I 44

Daughter cells of meiosis II Figure 10.9a MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Prophase I Chromosome duplication Chromosome duplication Homologous chromosome pair Duplicated chromosome 2n = 6 Individual chromosomes line up. Pairs of chromosomes line up. Metaphase Metaphase I Anaphase Sister chromatids separate. Homologs separate. Anaphase I Telophase Telophase I Figure 10.9a A comparison of mitosis and meiosis in diploid cells (part 1: mitosis vs. meiosis art) Sister chromatids separate. Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II 45

SUMMARY Property Mitosis Meiosis DNA replication Figure 10.9b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, prometaphase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Figure 10.9b A comparison of mitosis and meiosis in diploid cells (part 2: mitosis vs. meiosis table) Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosome sets by half and introduces genetic variability among the gametes 46

Protein complexes called cohesins are responsible for this cohesion Sister chromatid cohesion keeps sister chromatids of a single chromosome attached during meiosis and mitosis Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids) 47

Aim: How can we examine mutations. Do Now: Are mutations good or bad Aim: How can we examine mutations? Do Now: Are mutations good or bad? Explain.

Meiosis I is called the reductional division because it halves the number of chromosome sets per cell from diploid (2n) to haploid (n) Meiosis II is called the equational division because the haploid cells divide to produce haploid daughter cells The mechanism of sister chromatid separation in meiosis II is identical to that in mitosis 49

Concept 10.4: Genetic variation produced in sexual life cycles contributes to evolution Mutations (changes in an organism’s DNA) are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces genetic variation 50

Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation Independent assortment of chromosomes Crossing over Random fertilization 51

Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs 52

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes 53

Possibility 2 Possibility 1 Two equally probable arrangements of Figure 10.10-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 10.10-1 The independent assortment of homologous chromosomes in meiosis (step 1) 54

Possibility 2 Possibility 1 Two equally probable arrangements of Figure 10.10-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 10.10-2 The independent assortment of homologous chromosomes in meiosis (step 2) 55

Possibility 1 Possibility 2 Two equally probable arrangements of Figure 10.10-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 10.10-3 The independent assortment of homologous chromosomes in meiosis (step 3) Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 56

Crossing Over Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene 57

In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA, producing chromosomes with new combinations of maternal and paternal alleles Animation: Genetic Variation 58

Prophase I of meiosis Nonsister chromatids held together Figure 10.11-1 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure 10.11-1 The results of crossing over during meiosis (step 1) 59

Prophase I of meiosis Nonsister chromatids held together Figure 10.11-2 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Synapsis and crossing over Chiasma Centromere TEM Figure 10.11-2 The results of crossing over during meiosis (step 2) 60

proteins holding sister chromatid arms together Anaphase I Figure 10.11-3 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Synapsis and crossing over Chiasma Centromere TEM Breakdown of proteins holding sister chromatid arms together Anaphase I Figure 10.11-3 The results of crossing over during meiosis (step 3) 61

proteins holding sister chromatid arms together Anaphase I Figure 10.11-4 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Synapsis and crossing over Chiasma Centromere TEM Breakdown of proteins holding sister chromatid arms together Anaphase I Figure 10.11-4 The results of crossing over during meiosis (step 4) Anaphase II 62

proteins holding sister chromatid arms together Anaphase I Figure 10.11-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Synapsis and crossing over Chiasma Centromere TEM Breakdown of proteins holding sister chromatid arms together Anaphase I Figure 10.11-5 The results of crossing over during meiosis (step 5) Anaphase II Daughter cells Recombinant chromosomes 63

Chiasma Centromere TEM Figure 10.11a Figure 10.11a The results of crossing over during meiosis (TEM) TEM 64

Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations 65

Crossing over adds even more variation Each zygote has a unique genetic identity 66

The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations 67

Asexual reproduction is less expensive than sexual reproduction Nonetheless, sexual reproduction is nearly universal among animals Overall, genetic variation is evolutionarily advantageous 68