Chapter 13 Meiosis

Fig. 13-1 Figure 13.1 What accounts for family resemblance?

I. Inheritance of Genes Genes = segments of DNA Genes are passed by gametes (sperm and eggs) Each gene has a specific location (locus) on a chromosome

II. Asexual vs Sexual Reproduction Asexual reproduction = one parent, genetically identical offspring, mitosis Sexual reproduction = two parents, offspring have combinations of genes

0.5 mm Parent Bud (a) Hydra (b) Redwoods Fig. 13-2 Figure 13.2 Asexual reproduction in two multicellular organisms Parent Bud (a) Hydra (b) Redwoods

III. Human Chromosomes Human somatic cells (body cells) have 23 pairs of chromosomes Karyotype = an ordered display of the pairs of chroms from a cell Homologous chromosomes = The two chroms in each pair (homologs) Chroms are same length and carry genes controlling the same traits

Fig. 13-3a APPLICATION Figure 13.3 Preparing a karyotype

TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Figure 13.3 Preparing a karyotype Sister chromatids Metaphase chromosome

Sex chromosomes are X and Y Human females have a pair of X chroms (XX) Human males have one X and one Y (XY) Autosomes = 22 pairs of chroms that do not determine sex

Each pair of homologous chroms has one chrom from each parent Diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46) In a cell in which DNA synthesis has occurred, each chromosome is replicated (two identical sister chromatids)

Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of Fig. 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Figure 13.4 Describing chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Gamete contains a single set of chroms, and is haploid (n) (monoploid) Humans, haploid number is 23 (n = 23) Each set of 23 = 22 autosomes + 1 sex chrom In an unfertilized egg (ovum), the sex chrom is X In a sperm cell, the sex chrom may be either X or Y

IV. Chromosomes in Human Life Cycle Fertilization = union of gametes Zygote = fertilized egg and has one set of chroms from each parent (2n) Produces somatic cells by mitosis and develops into an adult

At maturity, ovaries and testes produce haploid (n) gametes Gametes are only types of human cells produced by meiosis (23 chroms, 1 from each pair)

Multicellular diploid adults (2n = 46) Fig. 13-5 Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Figure 13.5 The human life cycle Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

V. Stages of Meiosis First cell division (meiosis I), homologous chroms separate Meiosis I = two haploid daughter cells with replicated chrom; reductional division Second cell division (meiosis II), sister chromatids separate Meiosis II = four haploid daughter cells with unreplicated chroms; equational division

Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes

Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Figure 13.8 The meiotic division of an animal cell Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Homologous chromosomes separate Cleavage furrow Fragments of nuclear envelope Microtubule attached to kinetochore

VI. Meiosis I Prophase I Occupies more than 90% of time needed for meiosis Chromosomes condense Synapsis = homologous chroms loosely pair up, aligned gene by gene

Crossing over = nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids Usually has one or more chiasmata, X-shaped regions where crossing over occurred

Metaphase I Tetrads line up at the metaphase plate, one chrom facing each pole Microtubules attached to the kinetochore of each chromosome of each tetrad

Prophase I Metaphase I Centrosome (with centriole pair) Centromere Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

Anaphase I Pairs of homologous chroms separate, sister chromatids remain attached

Telophase I and Cytokinesis Each half of the cell has a haploid set of chroms; each chrom still consists of two sister chromatids Cytokinesis forms two haploid daughter cells No chrom replication occurs between meiosis I and meiosis II

Telophase I and Cytokinesis Fig. 13-8c Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Figure 13.8 The meiotic division of an animal cell Homologous chromosomes separate Cleavage furrow

VII. Meiosis II Prophase II Spindle apparatus forms Chroms (composed of two chromatids) move toward the metaphase plate

Metaphase II 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 Kinetochores attach to microtubules

Prophase II Metaphase II Fig. 13-8e Figure 13.8 The meiotic division of an animal cell

Anaphase II Sister chromatids separate Sister chromatids of each chrom now move as two newly individual chroms

Telophase II and Cytokinesis Nuclei form, and the chroms begin decondensing Cytokinesis makes four daughter cells, each with a haploid set of unreplicated chroms Daughter cells are genetically distinct from the others and from the parent cell

Telephase II and Cytokinesis Fig. 13-8f Telephase II and Cytokinesis Anaphase II Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming

Meiosis - Andersen (9 min) Mitosis / Meiosis Simulation - Andersen (12 min)

VIII. Comparison of Mitosis and Meiosis Mitosis keeps the number of chromosomes the same, producing cells that are genetically identical to the parent Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent Mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Replicated chromosome Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase Metaphase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells Anaphase Telophase Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

Figure 13.9 A comparison of mitosis and meiosis in diploid cells Fig. 13-9b 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, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Figure 13.9 A comparison of mitosis and meiosis in diploid cells 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 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 chromosomes by half and introduces genetic variability among the gametes

Three events are unique to meiosis (all three occur in meiosis l): – Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information – At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Cell Cycle / Mitosis / Meiosis - Andersen (14 min)

IX. Genetic Variation in Offspring Mvmt of chroms during meiosis and fert gives variation Mutations (changes in DNA) are original source of genetic diversity Mutations create different versions of genes (alleles, A or a) Reshuffling of alleles during sexual reproduction produces genetic variation

A. Independent Assortment of Chromosomes Homologous chroms orient randomly at metaphase I of meiosis Each pair of chroms sorts maternal and paternal homologues into daughter cells independently of the other pairs Number of combinations possible is 2n, where n is the haploid number Humans (n = 23), there are more than 8 million (223) possible combinations

Possibility 2 Possibility 1 Two equally probable arrangements of Fig. 13-11-1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.11 The independent assortment of homologous chromosomes in meiosis

Possibility 2 Possibility 1 Two equally probable arrangements of Fig. 13-11-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.11 The independent assortment of homologous chromosomes in meiosis Metaphase II

Possibility 1 Possibility 2 Two equally probable arrangements of Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.11 The independent assortment of homologous chromosomes in meiosis Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

B. Crossing Over Produces recombinant chromosomes (combine genes inherited from each parent) Begins in prophase I, as homologous chroms pair up gene by gene Homologous portions of two nonsister chromatids trade places Variations come from combining DNA from two parents into a single chromosome

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-1 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure 13.12 The results of crossing over during meiosis

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-2 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-3 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis Anaphase I

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-4 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis Anaphase I Anaphase II

Recombinant chromosomes Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis Anaphase I Anaphase II Daughter cells Recombinant chromosomes

C. Random Fertilization Variation arises because any sperm can fuse with any ovum (unfertilized egg) Fusion of two gametes (each with 8.4 million possible combinations from ind assort) produces a zygote with about 70 trillion diploid combinations Crossing over adds even more variation

X. Evolution and Genetic Variation 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

You should now be able to: Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid Describe the events that characterize each phase of meiosis Describe three events that occur during meiosis I but not mitosis Name and explain the three events that contribute to genetic variation in sexually reproducing organisms Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings