Big Ideas 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes. During meiosis, homologous chromatids exchange genetic material via a process called crossing over, which increases genetic variation in the resultant gametes. [See also 3.C.2] Fertilization involves the fusion of two gametes, increases genetic variation in populations by providing for new combinations of genetic information in the zygote, and restores the diploid number of chromosomes. Learning Objectives LO 3.7 The student can make predictions about natural phenomena occurring during the cell cycle. [See SP 6.4] LO 3.8 The student can describe the events that occur in the cell cycle. [See SP 1.2] LO 3.9 The student is able to construct an explanation, using visual representations or narratives, as to how DNA in chromosomes is transmitted to the next generation via mitosis, or meiosis followed by fertilization. [See SP 6.2] LO 3.10 The student is able to represent the connection between meiosis and increased genetic diversity necessary for evolution. [See SP 7.1] LO 3.11 The student is able to evaluate evidence provided by data sets to support the claim that heritable information is passed from one generation to another generation through mitosis, or meiosis followed by fertilization. [See SP 5.3] Concept 13.1 Offspring acquire genes from parents by inheriting chromosomes In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes What is a clone? In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents What would be the advantages of asexual reproduction? Sexual reproduction? Concept 13.2: Fertilization and meiosis alternate in sexual life cycles A karyotype is an ordered display of the pairs of chromosomes from a cell What are homologous chromosomes? How many chromosomes are in human somatic cells? How many autosomes are present in a karyotype? 1
The Variety of Sexual Life Cycles A diploid cell (2n) has two sets of chromosomes What is the diploid number for humans? Which cells in the human body are diploid? What is the haploid number for humans? Which cells in the human body are haploid? The alternation and fertilization is common to all organisms that reproduce sexually What is alteration of generations? This life cycle includes both a diploid and haploid multicellular stage Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid After chromosomes duplicate, two divisions follow: Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes Meiosis II (equational division) sister chromatids separate The result is four haploid daughter cells No chromosome replication occurs between the end I and the beginning II because the chromosomes are already replicated The Stages of Meiosis Meiosis 1 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n 1n) reduction division 2nd division separates sister chromatids (1n 1n) * just like mitosis * Figure 13.8a Figure 13.8b Metaphase I Anaphase I Telophase I and Cytokinesis I Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids ta Spindle (with kinetochore) Metaphase plate Sister chromatids remain attached Homologous Fragments chromosomes of nuclear envelope Microtubule attached to kinetochore Duplicated homologous Chromosomes line up chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2n 6 in this example. Homologous chromosomes separate Cleavage furrow Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming 2
Events Unique to Meiosis Crossing Over There are three unique events that all occur in meiosis l 1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes 3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate A Comparison of Mitosis and Meiosis DNA replication Number of divisions Number of daughter cells and genetic composition Role in the animal body MITOSIS Occurs during interphase before mitosis begins One Two, each diploid (2n) and genetically identical to the parent cell Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction MEIOSIS Occurs during interphase before meiosis I begins Two Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution What is the original source of genetic diversity? What are the three mechanisms that contribute to genetic variation? 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization 1. Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes Figure 13.10-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I 3
Figure 13.10-2 Figure 13.10-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 2. Crossing Over Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent Anaphase I 4
Anaphase I Anaphase I Anaphase II Anaphase II Daughter cells Recombinant chromosomes 3. 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 Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez The Evolutionary Significance of Genetic Variation Within Populations REVIEW.. Natural selection results in the accumulation of genetic variations favored by the environment Why is it rare for animals to reproduce asexually? 5
1. Which of the following would need to be present to produce a realistic signal to control the cell cycle? (More than one may be required.) a) a set of molecules that are sensitive to fluctuations in the abundance of regulators b) protein kinases c) molecules that activate kinases d) molecules that degrade activators e) All of the above. 2. Which of the following transmits genes from one generation of a family to another? a) DNA b) gametes c) somatic cells d) mitosis e) nucleotides 3. Fertilization is to zygote as meiosis is to which of the following? a) mitosis b) diploid c) chromosome d) replication e) gamete 4. Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are the two species so dissimilar? a) Privet chromosomes undergo only mitosis. b) Privet chromosomes are shaped differently. c) Human chromosomes have genes grouped together differently. d) The two species have appreciably different genes. e) Privets do not have sex chromosomes. 5. How and at what stage do chromosomes undergo independent assortment? a) meiosis I pairing of homologs b) anaphase I separation of homologs c) meiosis II separation of homologs d) meiosis I metaphase alignment e) meiosis I telophase separation 6. In this cell, what phase is represented? a) mitotic metaphase b) meiosis I anaphase c) meiosis I metaphase d) meiosis II anaphase e) meiosis II metaphase 6