Meiosis and Sexual Life Cycles Chapter 13

Overview Sexual reproduction depends on the cellular processes of meiosis and fertilization. Hereditary, Similarity, and Variation Living organisms are distinguished by their ability to reproduce their own kind.

Key Terms Heredity: Is the transmission of traits from one generation to the next Variation: Shows that offspring differ somewhat in appearance from parents and siblings Genetics: The scientific study of heredity and hereditary variation

Inheritance of Genes Offspring acquire genes from parents by inheriting chromosomes. Genes are the units of heredity found on segments of DNA Each gene in an organism’s DNA has a specific locus on a certain chromosome. We inherit one set of chromosomes from our mother and one set from our father.

Comparison of Asexual and Sexual Reproduction In asexual reproduction One parent produces genetically identical offspring by mitosis Gives rise to clones In sexual reproduction Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Parent Bud 0.5 mm

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 A number of characteristics need to be considered when discussing life cycles: Chromosome numbers in somatic cells and gametes Chromosome behavior through sexual life cycles

Sets of Chromosomes in Human Cells In humans each somatic cell has 46 chromosomes, made up of two sets. One set of chromosomes comes from each parent A karyotype is an ordered, visual representation of the chromosomes in a cell 5 µm Pair of homologous chromosomes Centromere Sister chromatids

Chromosomes Homologous chromosomes are the two chromosomes composing a pair They have same the characteristics and may also be called autosomes. Sex chromosomes are distinct from each other in their characteristics Are represented as X and Y and determine the sex of the individual, XX being female, XY being male Only small parts of the X and Y are homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X.

Comparison of Chromosome Number A diploid cell has two sets of each of its chromosomes In a cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids Unlike somatic cells gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity, the ovaries and testes produce haploid gametes by meiosis. During fertilization these gametes, sperm and ovum, fuse, forming a diploid zygote. The zygote develops into an adult organism. As a human develops into a sexually mature adult, mitosis of the zygote generates all of the somatic cells of the body.

The Human Life Cycle Key Haploid gametes (n = 23) Haploid (n) Ovum (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 There are three main types of sexual life cycles The common feature of all three cycles is the alternation of meiosis and fertilization Contributes to the genetic variation among offspring They differ in the timing of meiosis and fertilization

Life Cycle in Animals In animals, meiosis occurs during gamete formation. Gametes are the only haploid cells.

Haploid multicellular organism (gametophyte) (b) Plants and some algae Life Cycle in Plants MEIOSIS FERTILIZATION n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Plants and some algae Exhibit an alternation of generations The life cycle includes both diploid and haploid multicellular stages Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a spore gives rise to a multicellular individual without fusing with another cell. A spore divides mitotically to generate a multicellular haploid stage called a gametophyte. The haploid gametophyte makes gametes by mitosis. Fertilization among the haploid gametes results in a diploid zygote, which develops into the next sporophyte generation. The sporophyte generation produces a gametophyte as it offspring, and the gametophyte generation produces the next sporophyte generation.

Life Cycle of Fungi & 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

Reduction Division Meiosis reduces the number of chromosome sets from diploid to haploid Takes place in two sets of divisions, meiosis I and meiosis II

Interphase 1 2 Meiosis I Meiosis II 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

Two Stages of Meiosis Meiosis I Meiosis II Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

BioFlix Animation http://www.youtube.com/watch?v=kVMb4Js99tA

(with centriole pairs) 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

MEIOSIS II: Separates sister chromatids 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

Prophase I 1. Synapsis occurs, forming tetrads. 2. Crossing over occurs between homologous chromosomes in the tetrads. 3. Crossing over increases genetic variation. 4. Areas of crossing over form chiasmata. 5. The nuclear envelope disintegrates, allowing the spindle to attach to the homologs. Synapsis is the joining of homologous chromosomes along their length. This newly formed structure is called a tetrad and precisely aligns the homologous chromosomes gene by gene. Crossing over the DNA from one homolog is cut and exchanged with an exact portion of DNA from the other homolog. 4. Where crossing over has occurred (two or three times per homologous pair), crisscrossed regions termed chiasmata form, which hold the homologs together until anaphase. 5. After crossing over, the centrioles move away from each other, the nuclear envelope disintegrates, and the spindle microtubules attach to the kinetochores forming on the chromosomes that begin to move to the metaphase plate.

Metaphase I Homologous pairs of chromosomes line up at the metaphase plate. The microtubules from each pole attach to each member of the homologous pairs. Homologous pairs of chromosomes line up at the metaphase plate The microtubules from each pole attach to each member of the homologous pairs This is in preparation for pulling them to opposite sides of the cell.

Anaphase I The spindle apparatus helps to move the chromosomes. Sister chromatids stay connected and move together toward the poles. The spindle apparatus helps to move the chromosomes toward opposite ends of the cell Sister chromatids stay connected and move together toward the poles.

Telophase I and Cytokinesis The homologous chromosomes move until they reach the opposite poles. Each pole, then, contains a haploid set of chromosomes, with each chromosome still containing two sister chromatids. Cytokinesis is the division of the cytoplasm and occurs during telophase. A cleavage furrow occurs in animal cells, and cell plates (the forming new cell wall) occur in plant cells. Both result in the formation of two haploid cells.

Prophase II A spindle apparatus forms. Sister chromatids move toward the metaphase plate.

Metaphase II The chromosomes are lined up on the metaphase plate. The kinetochores of each sister chromatid prepare to move to the opposite poles.

Anaphase II The centromeres of the sister chromatids separate. Individual chromosomes move to opposite ends of the cell.

Telophase II and Cytokinesis The chromatids have moved all the way to opposite ends of the cell. Nuclei reappear. Cytokinesis occurs. Each of the four daughter cells has the haploid number of chromosomes and is genetically different from the other daughter cells and from the parent cell.

A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l 1. Synapsis and crossing over Homologous chromosomes physically connect and exchange genetic information. 2. Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates. 3. 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. How do sister chromatids stay together through meiosis I but separate from each other in meiosis II and mitosis? Sister chromatids are attached along their lengths by protein complexes called cohesions. In meiosis, sister chromatid cohesion is released in two steps. In metaphase I, homologs are held together by cohesion between sister chromatid arms in regions where DNA has been exchanged. At anaphase I, cohesions are cleaved along the arms, allowing homologs to separate. At anaphase II, cohesions are cleaved at the centromeres, allowing chromatids to separate.

(before chromosome replication) 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

Origins of Genetic Variation Among Offspring Three mechanisms that contribute to the genetic variation arising from sexual reproduction: Independent Assortment of Chromosomes Crossing Over Random Fertilization Genetic variation produced in sexual life cycles contributes to evolution Reshuffling of genetic material in meiosis produces genetic variation In species that produce sexually the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis. In independent assortment, each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs. In humans, the number of possible combinations is 8.4 million. Each gamete that you produce in your lifetime contains one of roughly 8.4 million possible combinations of chromosomes.

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

Crossing Over During prophase I, the exchange of genetic material on homologous chromosomes between nonsister chromatids occurs. In metaphase II when sister chromatids separate, each chromatid is unique, thus increasing variation. Produces recombinant chromosomes that carry genes derived from two different parents. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Recombinant Chromosomes: individual chromosomes that carry genes (DNA) derive from two different parents. In meiosis in humans, an average of one to three crossover events occur per chromosome pair, depending on the size of the chromosomes and the position of their centromeres.

Random Fertilization The random nature of fertilization adds to the genetic variation arising from meiosis. Each male and female gamete represents just one of 8 million possible chromosome combinations (independent assortment). The fusion of gametes will produce a zygote with any of about 64 trillion diploid combinations

Evolutionary Significance of Genetic Variation Within Populations Genetic variation is the raw material for evolution by natural selection Individuals best suited to the local environment leave the most offspring; transmitting their genes But… the environment can change!!!