Meiosis & Sexual Life Cycles Chapter 11.4 (Dragon Fly Bk) and Chapter 8.14 (Bird Bk) Meiosis & Sexual Life Cycles A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism. It starts at the conception of an organism until it produces its own offspring.

In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes. Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes. Homologous Chromosomes—46 chromosomes that make the 23 pairs These homologous chromosome pairs carry genes that control the same inherited characters.

We inherit one chromosome of each homologous pair from each parent. The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction. We inherit one chromosome of each homologous pair from each parent. The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set. Sperm cells or ova (gametes) have only one set of chromosomes - 22 autosomes and an X or a Y. A cell with a single chromosome set is haploid. For humans, the haploid number of chromosomes is 23 (n = 23).

By means of sexual intercourse, a haploid sperm reaches and fuses with a haploid ovum. These cells fuse resulting in fertilization. The fertilized egg (zygote) now has two haploid sets of chromosomes bearing genes from the maternal and paternal family lines. The zygote and all cells with two sets of chromosomes are diploid cells. For humans, the diploid number of chromosomes is 46 (2n = 46).

How are haploid cells formed? Gametes (sex cells), which develop in the gonads (sex organs), are not produced by mitosis. Instead, gametes undergo the process of meiosis in which the chromosome number is halved. Meiosis is similar to Mitosis except it goes through the cell division twice.

In males, Meiosis is called: Spermatogenesis—formation of sperm in the testes…testes form haploid sperm cells.

In females, Meiosis is called: Oogenesis—formation of eggs (ovum) in the ovaries. Ovaries form haploid ovum cells.

Let’s look at the basic process of Meiosis: Before meiosis occurs the cell duplicates its chromosomes and prepares for cell division. INTERPHASE I

Prophase I -Nucleus disappears, spindle fibers form, & the chromatid pairs form. -Chromatid pairs pair up with their homologous pair, forming a tetrad. -A tetrad contains 2 original homologous chromosomes and their duplicates. -Crossing-Over may occur at this time (genetic diversity) -Crossing-Over is a process where pieces of two homologous chromosomes, switch places and end up on the other chromosome. (1 cell)

Along the center of the cell. (1 cell) Metaphase I: Tetrads line up Along the center of the cell. (1 cell) Anaphase I: Tetrads split and move in opposite directions. During this phase the chromatid pairs move away from each other. (1 cell)

Telophase I: Two cells form each containing a complete set of chromosomes in the form of chromatid pairs. (2 cells) Interphase II: Very quick stage…chromosomes are not duplicated!

Prophase II: Process continues moving the chromatid pairs toward the center of the cell. (2 cells) Metaphase II: Chromatid pairs line up along the center of the cell. (2)

Anaphase II: Chromatid pairs split moving the individual chromosomes toward the opposite poles. (2 cells) Telophase II: Each of the two cells will begin to split in half & produce 2 more cells bringing the total to 4 cells. Each of these cells contain 1/2 the normal number of chromosomes. (4 haploid cells)

Fertilization and meiosis alternate in sexual life cycles. Fertilization restores the diploid condition by combining two haploid sets of chromosomes. Fertilization and meiosis alternate in sexual life cycles. Fig. 13.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Comparing Asexual & Sexual Reproduction In asexual reproduction, a single individual passes along copies of all its genes to its offspring. Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells. Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis. Fig. 13.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Sexual reproduction results in greater variation among offspring than does asexual reproduction. Two parents give rise to offspring that have unique combinations of genes inherited from the parents. Offspring of sexual reproduction vary genetically from their siblings and from both parents. Fig. 13.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Comparing Mitosis & Meiosis

Mitosis produces two identical daughter cells. Meiosis produces 4 very different cells. Fig. 13.8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings