Chapter 7- Meiosis and Sexual Reproduction

I. Meiosis A. Meiosis forms haploid cells   1. Gametes are haploid, containing one set of chromosomes. 2. Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells.

3. Meiosis involves two divisions of the nucleus meiosis I and meiosis II each division is subdivided into prophase, metaphase, anaphase, and Telophase stages.

4. Meiosis involves two divisions of the nucleus, meiosis I and meiosis II.   5. Before meiosis begins, the DNA in the original cell is replicated.

B. Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm. Although meiosis appears much more complicated than mitosis, it is really just two divisions in sequence, each one of which has strong similarities to mitosis.

Meiosis 1. Meiosis I, the first of the two divisions, is often called reduction division, since it is here that the chromosome compliment is reduced from 2N (diploid) to 1N (haploid).

2. Interphase in meiosis is identical to interphase in mitosis and there is no way, by simply observing the cell, to determine what type of division the cell will undergo when it does divide. Meiotic division will only occur in cells associated with male or female sex organs

  3. Prophase I- The chromosomes are visible, the nuclear envelope breaks down. Homologous chromosomes pair all along their length and then cross-over. Crossing over occurs when portions of a chromatid on one homologous chromosomes are broken and exchanged.

4. Metaphase I- Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis. In the latter, all the chromosomes line up on the metaphase plate in no particular order.

  5. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that chromatid arms may overlap and temporarily fuse resulting in crossovers

6. Anaphase I- During Anaphase I the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell.

7. Telophase I- A cleavage furrow typically forms at this point, followed by cytokinesis, but the nuclear membrane (envelope) usually is not reformed and the chromosomes do not disappear.

8. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs.

9. While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division.

D. Meiosis II-Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II and the latter begins with:

1.Prophase II- A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. 2. Metaphase II- All the chromosomes in the two cells align with the metaphase plate.

3. Anaphase II- The centromeres split, spindle fibers shorten, drawing the chromosomes toward each pole of the cell.

 4. Telophase II- A cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane (envelope). The chromosomes begin to fade, replaced by the granular chromatin characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.

5. In the case of male structures, all four cells will eventually develop into typical sperm cells. In the case of female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell which is usually much larger than a typical sperm cell.

Meiosis is preceded by interphase Meiosis is preceded by interphase. The chromosomes have not yet condensed.

The chromosomes have replicated, and the chromatin begins to condense

The chromosomes are completely condensed The chromosomes are completely condensed. In meiosis (unlike mitosis), the homologous chromosomes pair with one another

The nuclear membrane dissolves and the homologous chromosomes attach to the spindle fibers. They are preparing to go to opposite poles.

The chromosomes move to opposite ends of the cell.

The cell begins to divide into two daughter cells The cell begins to divide into two daughter cells. It is important to understand that each daughter cell can get any combination of maternal and paternal chromosomes.

The cell has divided into two daughter cells.

As in Meiosis I, the chromosomes line up on the spindle fibers.

The two cells each begin to divide The two cells each begin to divide. As in Meiosis I, the chromosomes move to opposite ends of each cell

With the formation of four cells, meiosis is over With the formation of four cells, meiosis is over. Each of these prospective germ cells carries half the number of chromosomes of somatic cells.

Crossing over

E. Meiosis in males The process by which sperm are produced is called spermatogenesis. 3. Sperm contain 23 haploid chromosomes.

F. Meiosis in Females The process by which a female produces eggs is called oogenesis. 2. An egg is called an ovum and it contains 23 haploid cells.

II. Sexual reproduction Similarity to Parents is determined by the type of reproduction 1. Asexual reproduction-Single parent passes on copies of ALL of its genes= creates clones.

2. Sexual reproduction-Two parents each form haploid reproductive cells which join to form an offspring.

Eukaryotes have three kinds of sexual life cycles.

Haploid- ½ the number of chromosomes, gametes (egg and sperm) are haploid. Diploid- double the number of chromosomes, fertilization creates diploid organisms.

Alternation of generation- go back and forth between haploid and diploid life cycles.

Sporophyte- Diploid phase in the life cycle where spores are produced. 5. Gametophyte- Haploid phase where gametes are produced.