Sexual Reproduction Involves the production of specialized sex cells (gametes) These sex cells fuse their nuclei together (fertilization) producing an egg cell (zygote). Offspring are not identical to the parent The process of sexual reproduction involves two parents.   Both parents normally contribute one gamete or sex cell to the process.   This process assures that the genetic information given to the offspring will be obtained equally from each parent. These gametes are formed in specialized reproductive structures called gonads. Mrs. Degl

Gametes (sex cells): Males – Sperm Females – Ovum or Egg Gametogenesis is the process in which gametes are produced by meiosis. This happens in the gonads. Most organisms have either male of female gonads. However, there are some that have both. They are called Hermaphrodites. Generally, hermaphroditism occurs in the invertebrates, although it occurs in a fair number of fish, and to a lesser degree in other vertebrates. Meiosis is a process which involves reduction division creating gametes. Fertilization is the fusion of one sperm nuclei with one egg (ova) nuclei, producing a zygote. Haploid or Monoploid (n) represents a gamete, which has half of the number of chromosomes necessary. Once two gametes fuse together they form a zygote that is Diploid (2n) or 2 joined chromosomes. This is a set. Mrs. Degl

Meiosis occurs in diploid cells Meiosis occurs in diploid cells. The chromosomes duplicate once, and through two successive divisions, four haploid cells are produced, each with half the chromosome number of the parental cell. • Meiosis occurs only in sexually reproducing organisms. Depending on the organism, it may produce haploid gametes, which do not divide further but instead fuse to produce a diploid zygote; or it may produce haploid spores, which divide by mitotic cell cycles and produce unicellular or multi-cellular organisms. • In animals, the products of meiosis are the gametes. Mrs. Degl

Premeiotic Interphase Meiosis consists of two successive nuclear divisions, Meiosis I and Meiosis II. Each division consists of these stages: prophase, metaphase, anaphase, and telophase. Premeiotic Interphase Prior to meiosis, all chromosomes are duplicated in a process similar to chromosome duplication. Mrs. Degl

Meiosis 1: Prophase 1 Before Prophase I, the chromosomes have already duplicated. Now, they coil and become shorter and thicker. The duplicated homologous chromosomes pair, and crossing-over occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous consists of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma). The nucleolus disappears during prophase I. • In the cytoplasm, the meiotic spindle forms between the two pairs of centrioles as they migrate to opposite poles of the cell. • The nuclear envelope disappears at the end of Prophase I, allowing the spindle to enter the nucleus. Mrs. Degl

Meiosis 1: Metaphase 1 The centrioles are at opposite poles of the cell. • The pairs of homologous chromosomes, now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. • Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids). Mrs. Degl

Meiosis 1: Anaphase 1 (remember APART) Anaphase I begins when the two chromosomes separate and start moving toward opposite poles of the cell as a result of the action of the spindle. • Notice that in Anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after Metaphase in Meiosis I, whereas in mitosis they separate. Mrs. Degl

Meiosis 1: Telophase 1 Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. • A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis (separation) follows. Mrs. Degl

Meiosis 2: Prophase 2 Chromosomes do not replicate again. • The nuclear envelope breaks down, and the spindle apparatus forms. Mrs. Degl

Meiosis 2: Metaphase 2 Each of the daughter cells completes the formation of a spindle apparatus. • Single chromosomes align, much as chromosomes do in Mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. Mrs. Degl

Meiosis 2: Anaphase 2 The two sister chromatids separate and the spindle fibers move them to opposite poles. Mrs. Degl

Meiosis 2: Telophase 2 Cytokinesis (splitting) occurs again, producing a total of four daughter cells Mrs. Degl

Meiosis Summary Mrs. Degl

Meiosis produces gametes, while mitosis produces other cell types. The process of Meiosis produces gametes or sex cells.   While some parts of this cell division process are similar to the asexual cell division process of mitosis, there are several key differences.    Differences: Meiosis produces gametes, while mitosis produces other cell types.   The process of meiosis halves the chromosome number from the original parent cell in the four cells it forms.   It does this by having two cell divisions forming four cells, where mitosis has only one cell division forming two cells.    Similarity: Both processes start out with one doubling or replication of the chromosome material.  Mrs. Degl