Meiosis and Sexual Life Cycles Chapter 13 A. P. Biology Mr. Knowles Liberty Senior High School

Mitosis- Review

Mutations

Comparison of Asexual and Sexual Reproduction In asexual reproduction One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

In sexual reproduction: Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.

Leewenhoek’s Early Ideas of Sperm

Types of Cells Somatic (nonreproductive cells) – are diploid. Body cells. Human somatic cells have 46 chromosomes. Gametes (reproductive cells) – are haploid. Human sperm and ova have 23 chromosomes.

Gametes Fuse to Form a Zygote n 2n Syngamy n

During fertilization: These gametes, sperm and ovum, fuse, forming a diploid zygote. The zygote Develops into an adult organism.

Fertilization or Syngamy

Zygote Undergoes Mitosis

Zygote is Diploid (2n)

Why is the number of chromosomes in gametes ½ that of somatic cells? If not, after 10 generations the 46 chromosomes in human cells would increase to over 46 X 210.

Meiosis A process of cell division in which the number of chromosomes in certain cells is halved during gamete formation. A reduction division.

The Human Life Cycle Key Figure 13.5 Haploid gametes (n = 23) Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

The Variety of Sexual Life Cycles The three main types of sexual life cycles: Differ in the timing of meiosis and fertilization

Meiosis occurs during gamete formation In animals Meiosis occurs during gamete formation Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n 2n Zygote Haploid Mitosis (a) Animals

(b) Plants and some algae Exhibit an alternation of generations. The life cycle includes both diploid and haploid multicellular stages. MEIOSIS FERTILIZATION n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B

(c) Most fungi and some protists In most fungi and some protists: Meiosis produces haploid cells that give rise to a haploid multicellular adult organism The haploid adult carries out mitosis, producing cells that will become gametes MEIOSIS FERTILIZATION n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C

Sexual Life Cycle Reproduction that alternates between fertilization (a diploid state) and meiosis (a haploid state) – sexual reproduction. Sexual reproduction – provides DNA from both parents.

Some Sexual Life Cycles Unicellular Protists – individual cells function as gametesundergo meiosis and fuse with others. Plants- may produce haploid cells undergo mitosis and produce a multicellular haploid organism. Animals- set aside germ cells undergo meiosis and become haploid gametes.

The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II An overview of meiosis

Interphase and Meiosis I Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Figure 13.8

Telophase I, Cytokinesis, and Meiosis II TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8

Meiosis Two nuclear divisions. Labeled PMAT I and PMAT II. Prophase I is very different in meiosis than in mitosis.

Interphase I

Prophase I

Synapsis in Prophase I

Prophase I The duplicated homologous chromosomes condense and the ends of chromatids attach to the nuclear envelope. Homologous pairs align next to each other. The chromatids of one homologue align in precise register with the chromatids of the other in a process called – synapsis (zipping up a zipper).

Prophase I The DNA of chromatids unwind and base pair with the complementary strand on the the other homologue. The crossed-over sister chromatids form an X-shaped structure – chiasma. DNA from one homologue sister chromatid is exchanged with the other – Crossing Over.

Crossing Over During Prophase I

Prophase I The chiasmata hold the two pairs of sister chromatids together at the ends – Terminal Chiasmata. The homologous chromatids have exchanged DNA.

Prophase I

Metaphase I

Metaphase I Terminal chiasmata are still formed. The nuclear membrane breaks down and spindle fibers are forming. Only one side of each centromere faces outward toward the growing spindle fiber. The fibers can attach to kinetochore proteins on only one side of each centromere.

Metaphase I Alignment of each pair on the metaphase plate is random. Either homologue may be situated toward a given pole.

Metaphase I

Anaphase I

Anaphase I Spindle fibers contract and break the chiasmata and pull the centromeres toward the poles. The entire centromere of the homologue proceeds to one pole. (Different than mitosis). Each pole receives a haploid number of chromosomes, one member of each homologous pair.

Anaphase I

Anaphase I Because the orientation of the homologous chromosomes on the metaphase plate is random, genes on separate chromosomes are inherited independently – Independent Assortment.

Telophase 1

Telophase I Individual chromosomes cluster at the poles. Each chromosome is made of two chromatids that are not identical because of crossing over that occurred in Prophase I. Cytokinesis may or may not occur after Telophase I. Nuclei may not reform.

Telophase I

The Second Meiotic Division No DNA replication before this division (Different than mitosis). Undergo a simple mitotic division with the products from Telophase I. Results are four haploid cells. Nuclei are reorganized.

Prophase II

Metaphase II

Metaphase II

Anaphase II

Anaphase II

Telophase II

Telophase II

(before chromosome replication) Comparison of Mitosis and Meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6

A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l: Synapsis and crossing over- Homologous chromosomes physically connect and exchange genetic information. Tetrads on the metaphase plate- At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates Separation of homologues: At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate

Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution 3 Sources of Genetic Variation: 1. Reshuffling of genetic material in meiosis-Crossing Over. 2. Homologous pairs of chromosomes are oriented randomly at metaphase - Independent Assortment. 3. Random Fertilization of Gametes- Will produce a zygote with any of about 64 trillion diploid combinations

Crossing Over Crossing over: Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Independent Assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Figure 13.10

Evolutionary Significance of Genetic Variation Within Populations -Is the raw material for evolution by natural selection Mutations: Are the original source of genetic variation Sexual reproduction: Produces new combinations of variant genes, adding more genetic diversity

The Fate of Haploid Cells n Undergo Mitosis Multicellular Gametes sperm ova n n Haploid Plants Haploid Fungi

Meiotic divisions that result in gametes. Gametogenesis Meiotic divisions that result in gametes.

Oogenesis Spermatogenesis Oogonium (female germ cell, 2n) Spermatogonium (male germ cell, 2n) Germ cells committed to meiosis Primary Oocyte (2n) Primary Spermatocyte (2n) First Meiotic Division ___Before Ovulation____ Secondary Spermatocyte (n) Secondary Spermatocyte (n) Secondary Ooctye (n) First Polar Body (n) Second Meiotic Division ___After Fertilization____ Four Spermatids (n) Second Polar Body (n) Ovum (n) Four Spermatozoa (n)

Primary Spermatocytes Seminiferous Tubule Spermatids Mature Sperm

Rat Sperm

Fertilization or Syngamy

Zygote is Diploid (2n)

Cloning to Save an Endangered Species Enculeated Cow Egg Bateng Nucleus Surrogate Cow Frozen Bateng Cells Cloned Bateng

Microtubules and Motor Proteins-Spindle Fibers

Cloned Animals as of 4-04

What animal group hasn’t been cloned…yet? Primates Why?

Can haploid cells develop into functioning animals or plants? Bees Do It!

How to make a bee colony? Fertilized Egg, (2n) Female Worker, (2n) Meiosis Mitosis w/o Cytokinesis Unfertilized Egg, (1n) Queen, (2n) Male Drone, (1n)