Chapter+Ten


 * __ CHAPTER 10 __**** : Meiosis and Sexual Reproduction **

1. There are two main methods by which new cells are formed. A. ASEXUAL REPRODUCTION involves the formation of a new cell where all the genetic information in the new cell comes from ONE parent, therefore offspring will be identical to each other and the original parent cell. This method of reproduction is used in the human body to produce new cells for tissue growth, or to replace damaged or worn-out cells. Liver cells produce liver cells, skin cells produce skin cells, etc. This is also known as _CLONING. (see chapter 9) B. SEXUAL REPRODUCTION involves the formation of a new cell where the genetic information in the new cell comes from two parent cells. Each parent cell contributes ½ of the genetic information. This method of reproduction requires that the germ cells located in the gonads (ovaries and testes) become gamete cells (egg and sperm) by a process is called __and is followed by FERTILIZATION (the union of the genetic information from an egg and a sperm cell (gametes).__ __2. Most cells in the human body have__ __46_ chromosomes (individual strands of DNA).__ __A. On each chromosome are sections of genetic information that code for the__ __formation of a specific protein. These stretches of DNA are called__ GENES_. B. The chromosomes are divided into 23 “similar” pairs. They are similar because they are about the same size/length, the centromere is located in the same general location (middle or near end), and the genetic instructions are for the same basic physical characteristic or trait. C. These 23 pairs are often called HOMOGOLOUS chromosomes. The “pairs” are not identical however because one chromosome comes from each parent (hopefully your mom and dad don’t appear identical!). While both chromosomes may carry the sequence to create eye lashes, they may not necessarily carry the sequence for the same length of eye lashes. Homologous means they code for the same thing, but slight variations may occur. The “pairs of chromosomes” are NOT identical like the sister chromatids are. D. Most of the cells in the body are called somatic cells and formed from repeated mitotic divisions of that first cell that formed when an egg was fertilized by a sprem. Since that first cell contained 46 chromosomes (23 from each parent), all the cells produced by mitosis also have 46 chromosomes. Since the somatic cells have two sets of genetic instructions, they are also called __DIPLOID_ cells.__ __F. The gonads (ovaries and testes) produce gametes (eggs and sperm) that have only__ __23 chromosomes (½ the normal amount) so gametes are called__ HAPLOID_ cells. After fertilization, the resulting cell will again have 46 chromosomes.

3.** Meiosis ** is a process germ cells use to divide the total number of chromosomes in half to form cells with only half the normal number of chromosomes. As mentioned, this only occurs in the gonads (ovaries and testes) as they produce gametes (egg and sperm) cells prior to the final stage of sexual reproduction (fertilization). This process also occurs in several stages, however unlike mitosis, it sorts out and divides the chromosomes not once, but twice. A. ** Interphase ** __1. Although technically not part of meiosis, this stage is essentially the same as in Mitosis. It is the time between cell division for cell growth, duplication of needed organelles and “supplies” (including the DNA) to permit TWO cells to survive following cell division.__ __2. DNA exists in the nucleus as long thin strands called chromatin.__ __3. Individual chromosomes are not visible under the microscope during this phase, just a dark, round, mass of “stuff” in the center of the cell.__ __4. There will be a darkened area near but outside the nucleus. This area is called the centrosome, which contains the two pairs of centrioles that are used to help separate the chromosomes during the different phases of meiosis.__ __B.__** Prophase I ** _: 1. //T////his is the most complex phase and the one that is the most different from MITOSIS//. 2. Chromatin condenses, and sister chromatids become visible as in Mitosis. 3. However, in this phase of meiosis, the homologous chromosomes “somehow” find each other and bond together at their centromeres, forming a complex of two sister chromatids, so now there are a total of FOUR individual chromatids and the structure is called a TETRAD (tetra = 4).

Sister chromatids from “mom”  Tetrad

Homologous chromosomes

Sister chromatids from “dad”  4. Due to the close proximity of the 4 strands, some of the genes “swap places” between the homologous chromosomes in a process called __5. The two identical sister chromatids are now__ __no longer identical! There may be 4 slightly__ __different chromosomes connected. This is the__ __first cause of genetic variations seen in__ __organisms produced by sexual reproduction.__ __6. The other aspects of this phase are the same__ __as in Mitosis.__
 * Crossing Over_ **__.__

__C.__ ** Metaphase I ** : 1. This phase is about the same as in Mitosis, except instead of the sister  chromatids lining up in the middle of the cell, the ** homologous chromosomes **__line up in the middle of the cell.__ __2. Because of the random arrangement of the__ __tetrads and how the sister chromatids were__ __oriented in relationship to the centrioles,__ __the initial contact between the microtubules__ __and the tetrads are random. Some of the tetrads__ __may be oriented so the maternal (mom’s) sister__ __chromatids are on one side of the cell, while in__ __other tetrads they are oriented so they are on the__ __other side of the cell. Due to this “random__ __arrangement” of the homologous chromosomes,__ __the chances of two cells having the exact same__ __arrangement are very rare.__ __3. This phenomenon is called “random assortment” (homologous chromosomes in each tetrad are oriented in various ways) and is the second cause of genetic variations seen in organisms produced by sexual reproduction.__ __D.__** Anaphase I ** ___:__ __1. Again, this stage is very similar to Mitosis, except instead of the identical__ __sister chromatids separating from each other and moving to opposite sides of__ __the cell, the__ ** Tetrads ** _separate so the homologous chromosomes move to opposite sides of the cell. For each tetrad, regardless of how they are oriented, if the maternal (mom’s) sister chromatids moves one direction, the paternal (dad’s) sister chromatids will move in the opposite direction. 2. As a result of the “random assortment” during the prophase 1, either the maternal (mothers) OR the paternal (fathers) homologous chromosome for each tetrad that formed will end up at one side of the cell prior to cell division, not BOTH. The chromosomes that end up in the gamete cells at the end of meiosis will be a mix of chromosomes from the two parents. No gamete cell produced will have both homologous chromosomes. 3. This phenomenon is called “segregation” (homologous chromosomes in each tetrad must separate) and is the third cause of genetic variations seen in organisms produced by sexual reproduction. E. ** Telophase I ** _: 1. This phase is similar to mitosis except the cell divides forming two haploid daughter cells, each containing only one of the original homologous pairs of chromosomes (in the form of non-identical sister chromatids). 2. Now the process continues as in Mitosis, where the two non-identical sister chromatids separate from each other, and the cell divides again, forming a total of ** 4 ** daughter cells that are NOT identical to each other nor the original parent cell.

4. Sexual reproduction continues with the process of fertilization, where the gamete cells (egg and sperm) unite, combining the genetic information carried by each. Since each gamete cell is unique (containing various genes from each parent), the resulting cell will be unique. The random combination of gamete cells provides the fourth and final cause of genetic variation seen in sexual reproduction. 5. Formation of Gametes (egg and sperm): A. Formation of sperm: SPERMATOGENESIS Primary Secondary Spermatogonium Spermatocyte Spermatocyte Spermatid Mature Sperm  (diploid #) (diploid #) (haploid #) (haploid #)

Growth Meiosis I Meiosis II

B. Formation of egg: OOGENESIS 

Three Polar Bodies First Polar Body Non-viable Oogonium Primary Ooctye (haploid #) (haploid #) (diploid #) (diploid #)

Germ Cell

Secondary Oocyte (haploid #)

Ovum Mature Egg (haploid #)

Growth Meiosis I Meiosis II

= REVIEW ACTIVITIES: = 6. Circle one of two possible answers given between parentheses in each statement. 1. ( Meiosis / Mitosis) divides chromosomes into separate parcels not once but twice prior to cell division. 2. Sperm and eggs are sex cells known as (somatic/ gamete ) cells. 3. (Haploid/ Diploid ) cells possess pairs of homologous chromosomes. 4. ( Meiosis / Mitosis) produces cells that have one member of each pair of homologous chromosomes possessed by the species. 5. Identical alleles are found on (homologous chromosomes/ sister chromatids ). 6. Two attached DNA molecules are known as ( sister chromatids / homologous chromosomes). 7. Two attached sister chromatids represent (two/ one ) chromosome(s). 8. With meiosis, chromosomes proceed through (one/ two ) divisions to yield four haploid nuclei. 9. DNA is replicated during (prophase I of meiosis I/ interphase preceding meiosis I ). 10. Cytoplasmic division following meiosis I results in two (diploid/ haploid ) daughter cells. 11. During (meiosis I/ meiosis II ), the two sister chromatids of each chromosome are separated from each other, and each sister chromatid is now a chromosome in its own right. 7. Choose the most appropriate answer for each of the following. 2.** H ** paternal chromosomes 3. ** B ** early prophase I 4. ? non-sister chromatids 5. ** G ** late prophase I 6.** D ** crossing over 7. ** A ** metaphase I 8. ? function of meiosis 9. ** E ** maternal chromosomes || A. Random positioning of maternal and paternal chromosomes at the spindle equator occur at this time B. Chromosomes continue to condense and be come thicker rod-like forms at this time C. Reduction of the chromosome number by half occurs at this time D. Chromatids that break at the same places along their length and then exchange corresponding segments E. Twenty-three chromosomes inherited from your mother F. Chromatids that are identical and are attached at the centromere G. Each duplicated chromosome is in thin, threadlike form at this time H. Twenty-three chromosomes inherited from your father I. Process where non-sister chromatids break at corresponding sites and exchange segments || 8. Compare mitosis and meiosis by completing this chart. the parent cell ||
 * 1. ** F ** sister chromatids
 * //Mitosis// || //Meiosis// ||
 * 1. Produces 2 identical cells || Produces haploid daughter cells unlike
 * Involves one cell division || 2. Involves 2 cell divisions ||
 * Produces two daughter cells || 3. Produces 4 daughter cells ||
 * 4. ? || Homologous chromosomes pair up forming “tetrads” and then separate ||
 * Individual chromosomes line up in metaphase || 5. Tetrads line up at the same level ||
 * No crossing over occurs || 6. Crossing over occurs ||
 * 7. Needed for cell growth and repair in all somatic cells || Needed for sexual reproduction ||

9. Complete the following table by entering the word //mitosis// or the word //meiosis// in the blank adjacent to the statement describing one of these processes.


 * ===Description=== || ===Mitosis/Meiosis=== ||
 * 1. Involves one division cycle || Mitosis ||
 * 2. Functions in growth and tissue repair Mitosis ||  ||
 * 3. Daughter cells are haploid || Meiosis ||
 * 4. Occurs only in germ cells || Meiosis ||
 * 5. Involves two division cycles || Meiosis ||
 * 6. Daughter cells have one chromosome from each homologous pair || Meiosis ||
 * 7. Asexual reproduction by single-celled eukaryotic species || Mitosis ||
 * 8. Daughter cells have the diploid chromosome number || Mitosis ||
 * 9. Completed when four daughter cells are formed || Meiosis ||

nuclear membrane disintegrates || 10. The following table compares the similarities and differences between mitosis and meiosis. We will complete this table together in class as a final review. Tetrads line up at metaphase plate || homologous chromosomes separate, sister chromatids remain togethe r || Chromosome clusters arrive at spindle poles, new nuclear envelope forms chromosomes decondense || to spindle poles, nuclear envelope breaks up || chromosomes decondense ||
 * Who |||||| **MITOSIS** || **MEIOSIS** ||
 * What ||||||  ||   ||
 * Where |||||| in eukaryots || in cells used for sexual reproduction ||
 * When/Why |||||| for cell growth and repair || for sexual reproduction and production of gametes ||
 * How ||||||  ||   ||
 * 1 |||||||| (same in both) By cellular division  ||
 * 2 ||||  ||||   ||
 * 3 ||||  ||||   ||
 * Stages ||||  ||||   ||
 * Interphase |||||||| (same in both) ||
 * Prophase |||||||| (same in both) Chromosome replication
 * ||  |||||| (only in meiosis) tetrads form ||
 * || **MITOSIS** || **MEIOSIS** ||
 * Metaphase || Chromosomes line up at metaphase plate || M-1
 * Anaphase || sister chromatids separate || A-1
 * Telophase || cell divides, chromosomes uncoil, nucleus reforms || T-1
 * ||  || Part II ||
 * ||  || P-2 chromosomes condense, bipolar spindle forms and attaches chromosomes
 * ||  || M-2 chromosomes align midway between spindle poles ||
 * ||  || A-2 sister chromatids separate as they are pulled toward spindle poles ||
 * ||  || T-2 chromosome clusters arrive at spindle poles, new nuclear envelope forms,