Meiosis and Sexual Reproduction Chapter 10

10.1 Introducing Alleles Asexual reproduction produces genetically identical copies of a parent (mitosis) Sexual reproduction introduces variation in the combinations of traits among offspring (meiosis)

Meiosis and Homologous Chromosomes A nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species If meiosis did not occur, the chromosome number would continue to double with each new generation. Homologous Chromosomes Same length, shape, centromere in the same location, are in pairs (one from the mother and one from the father), separate from each other during meiosis I, have alleles for the same characteristics even though the gene expression may not be the same.

Genes and Alleles Genes are regions in an organism’s DNA that encode information about heritable traits In sexual reproduction, pairs of genes are inherited on pairs of chromosomes Alleles are different or alternate forms of the same gene Offspring of sexual reproducers inherit new combinations of alleles (ex. Eye color = hazel eyes (mother), blue eyes (father) Study Figure 10.2

Halving the Diploid Number A diploid cell has two nonidentical copies of every chromosome (except XY sex chromosomes) Humans have 23 pairs of homologous chromosomes Meiosis in germ cells halves the diploid number of chromosomes (2n) to the haploid number (n), producing haploid gametes aka sex cells (egg, sperm, spores, gametophytes)

Restoring the Diploid Number Human gametes (eggs and sperm) have 23 chromosomes – one of each type of homologous chromosomes. The diploid number (23 pairs or 46 chromosomes) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote

Two Divisions, Not One In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid cells (four daughter cells) In meiosis I, each duplicated homologous chromosome is separated from its partner In meiosis II, sister chromatids are separated

Meiosis I newly forming microtubules of the spindle breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I = crossing over plasma membrane one pair of homologous chromosomes B Metaphase I C Anaphase I D Telophase I Figure 10.5 Meiosis halves the chromosome number. Drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but cells of nearly all eukaryotes have more than two. The two chromosomes of the pair inherited from one parent are in purple; the two inherited from the other parent are in blue. Micrographs show meiosis in a lily plant cell (Lilium regale). Figure It Out: Are chromosomes in their duplicated or unduplicated state during metaphase II? Answer: Duplicated Stepped Art Fig. 10-5a, p. 158

Meiosis II There is no DNA replication between the two nuclear F Metaphase II G Anaphase II H Telophase II E Prophase II There is no DNA replication between the two nuclear divisions. Figure 10.5 Meiosis halves the chromosome number. Drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but cells of nearly all eukaryotes have more than two. The two chromosomes of the pair inherited from one parent are in purple; the two inherited from the other parent are in blue. Micrographs show meiosis in a lily plant cell (Lilium regale). Figure It Out: Are chromosomes in their duplicated or unduplicated state during metaphase II? Answer: Duplicated Stepped Art Fig. 10-5b, p. 159

Animation: Meiosis step-by-step

10.4 How Meiosis Introduces Variation in Traits Crossovers the random sorting or alignment of chromosomes genetic recombination of alleles fertilization in meiosis introduce novel combinations of alleles into gametes (egg and sperm), resulting in new combinations of traits among offspring

Crossing Over in Prophase I The process by which a chromosome and its homologous partner (two of the same chromosomes; one from the mother and one from the father) exchange heritable information in corresponding segments Homo = same

Animation: Crossing over

Segregation of Chromosomes into Gametes Random assortment (independent assortment) produces (8,388,608) possible combinations of homologous chromosomes Gametes (egg and sperm; 223 ) Fertilization (fusion or combining of the egg and sperm; (223)2 = Over 70 trillion different zygotes – the first cell that is formed after fertilization) are possible.

Animation: Random alignment

Figure 10.8 (a) Generalized life cycle for most plants. A pine tree is a sporophyte. (b) Generalized life cycle for animals. The zygote is the first cell to form when the nuclei of two gametes, such as a sperm and an egg, fuse at fertilization. Fig. 10-8b, p. 162

Gamete Formation in Animals Males Meiosis of primary spermatocytes produces four haploid spermatids Females Meiosis of a primary oocyte forms cells of different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum ; other three cells (polar bodies – have no function, are dumping places for excess genetic material, are produced only during meiosis.) Study figure 10.10

Sperm Formation in Animals

Egg Formation in Animals

More Shufflings at Fertilization Chance combinations of maternal and paternal chromosomes through fertilization produce a unique combination of genetic information Fertilization The fusion of two haploid gametes (sperm and mature ova - egg) resulting in a diploid zygote (the first cell of a new individual with 46 chromosomes.

Figure 10.11 Comparing mitosis and meiosis, beginning with a diploid cell containing two paternal and two maternal chromosomes. Fig. 10-11c, p. 164

Figure 10.11 Comparing mitosis and meiosis, beginning with a diploid cell containing two paternal and two maternal chromosomes. Fig. 10-11d, p. 165

Animation: Comparing mitosis and meiosis

Animation: Generalized life cycles

Animation: Meiosis

Video: Why sex?