Mitosis & Meiosis

AHSGE Science Standards 6 Describe the roles of mitotic & meiotic divisions during reproduction, growth & repair of cells. 6 Describe the roles of mitotic & meiotic divisions during reproduction, growth & repair of cells. 6a Comparing sperm & egg formation in terms of ploidy. 6a Comparing sperm & egg formation in terms of ploidy. 6b Comparing sexual & asexual reproduction. 6b Comparing sexual & asexual reproduction.

I. DNA Replication A. Cells must be able to divide in order for the organism to grow, reproduce & repair itself. A. Cells must be able to divide in order for the organism to grow, reproduce & repair itself. B. Multicellular organisms are made of 2 kinds of cells: B. Multicellular organisms are made of 2 kinds of cells: –1. reproductive cells (sex cells) –2. somatic cells (body cells)

I. DNA Replication C. Both kinds of cells contain DNA, which is stored in the nucleus in the form of chromatin. C. Both kinds of cells contain DNA, which is stored in the nucleus in the form of chromatin. –1. Chromatin consists of long strands of DNA, jumbled up with proteins, that together form a kind of disorganized mass of genetic material in the nucleus. –2. When the cell is ready to divide, the chromatin coils & condenses to form chromosomes.

I. DNA Replication D. Reproductive cells (sex cells) have a single set, or haploid number (n), of chromosomes. D. Reproductive cells (sex cells) have a single set, or haploid number (n), of chromosomes. E. Somatic cells (body cells) have 2 sets, or a diploid number (2n), of chromosomes. E. Somatic cells (body cells) have 2 sets, or a diploid number (2n), of chromosomes.

I. DNA Replication F. When the cell divides, the chromosomes must be distributed between the newly produced cells. F. When the cell divides, the chromosomes must be distributed between the newly produced cells. –1. This means that the DNA must be able to copy itself through the process of replication.

II. The Cell Cycle A. The cell cycle is the sequence of stages through which a cell passes between 1 cell division & the next. A. The cell cycle is the sequence of stages through which a cell passes between 1 cell division & the next.

II. The Cell Cycle B. Most of the cell cycle is spent in interphase. See Figure 4.4 The Cell Cycle B. Most of the cell cycle is spent in interphase. See Figure 4.4 The Cell Cycle

II. The Cell Cycle –1. Interphase consists of 3 major parts:  a. G 1  b. S  c. G 2

II. The Cell Cycle 1. During the G 1 phase of interphase, the cell grows in size. 1. During the G 1 phase of interphase, the cell grows in size. 2. In the S phase, replication of the DNA containing the genetic material occurs, which gives the cell a double amount of DNA. 2. In the S phase, replication of the DNA containing the genetic material occurs, which gives the cell a double amount of DNA. 3. In the G 2 phase, the cell prepares for mitosis by replicating organelles & increasing the amount of cytoplasm. 3. In the G 2 phase, the cell prepares for mitosis by replicating organelles & increasing the amount of cytoplasm.

III. Mitosis A. Somatic cells (body cells) undergo a process called mitosis. A. Somatic cells (body cells) undergo a process called mitosis. –1. Mitosis is a type of cell division that generates 2 daughter cells with the identical components of the mother cell (parent cell). –2. Mitosis is the mechanism for asexual reproduction, which only requires 1 parent. –3. Mitosis also allows multicellular organisms to grow & replace cells.

III. Mitosis B. The stages of mitosis are: B. The stages of mitosis are: –1. Prophase –2. Metaphase –3. Anaphase –4. Telophase

III. Mitosis 1. Prophase 1. Prophase –a. The nucleus of the cell organizes the chromatin material into thread-like structures called chromosomes. –b. The centriole, in animal cells only, divides & moves to each end of the cell. –c. Spindles form between the centrioles.

III. Mitosis 2. Metaphase 2. Metaphase –a. The chromosomes attached at the center, or centromeres, line up on the spindle at the center of the cell.

III. Mitosis 3. Anaphase 3. Anaphase –a. Chromosomes separate at the center, & the spindles pull them toward either end of the cell –b. A nuclear membrane forms around the chromosomes as they disorganize.

III. Mitosis 4. Telophase 4. Telophase –a. Chromatin again forms from the chromosomes, & a cell membrane begins to grow across the center between the 2 new nuclei. See Figure 4.5 Stages of Mitosis

IV. Cytokinesis A. Cytokinesis, the division of the cell cytoplasm, usually follows mitosis. A. Cytokinesis, the division of the cell cytoplasm, usually follows mitosis. –1. Cytokinesis generally begins during the telophase of mitosis. –2. It finalizes the production of 2 daughter cells, each with approximately half of the cytoplasm & organelles as well as 1 of the 2 nuclei formed during mitosis.

IV. Cytokinesis B. The processes of mitosis & cytokinesis are together called cell division. B. The processes of mitosis & cytokinesis are together called cell division.

V. Meiosis A. Meiosis is a type of cell division necessary for sexual reproduction. A. Meiosis is a type of cell division necessary for sexual reproduction.

V. Meiosis B. It is limited to the reproductive cells in the testes, the sperm cells, & the reproductive cells in the ovaries, the eggs. B. It is limited to the reproductive cells in the testes, the sperm cells, & the reproductive cells in the ovaries, the eggs. C. Meiosis produces 4 reproductive cells or gametes. C. Meiosis produces 4 reproductive cells or gametes. –1. These 4 cells contain half the # (haploid) of chromosomes of the mother cell, & the chromosomes are not identical.

V. Meiosis D. There are 2 phases of cell division: D. There are 2 phases of cell division: –1. Meiosis I –2. Meiosis II E. Before meiosis begins, each pair of chromosomes replicates while the cell in its resting phase (interphase). E. Before meiosis begins, each pair of chromosomes replicates while the cell in its resting phase (interphase).

V. Meiosis F. During meiosis I, each set of replicated chromosomes lines up with its homologous pair. F. During meiosis I, each set of replicated chromosomes lines up with its homologous pair. –1. Homologous chromosomes are matched pairs of chromosomes.  a) They are similar in size & shape & carry the same kinds of genes.  b) They are not identical because each set usually comes from a different parent.

V. Meiosis G. The homo pairs of chromo can break & exchange segments during the crossing over process, a source of genetic variation. G. The homo pairs of chromo can break & exchange segments during the crossing over process, a source of genetic variation. H. They then separate. H. They then separate. I. The cell splits into 2 daughter cells, each containing 1 pair of the homo chromo. See Figures 4.6 & 4.7 I. The cell splits into 2 daughter cells, each containing 1 pair of the homo chromo. See Figures 4.6 & 4.7

V. Meiosis J. Interkinesis is the resting period before meiosis II begins. J. Interkinesis is the resting period before meiosis II begins. K. During meiosis II, 2 daughter cells divide again without replication of the chromosomes. K. During meiosis II, 2 daughter cells divide again without replication of the chromosomes. L. The result is 4 gametes, each having half the # of chromosomes of the mother cell. L. The result is 4 gametes, each having half the # of chromosomes of the mother cell.

V. Meiosis 1. In males, all 4 gametes produce a long whip- like tail. 1. In males, all 4 gametes produce a long whip- like tail.

V. Meiosis V. Meiosis 2. In females, 1 gamete forms an egg cell with a large supply of stored nutrients. 2. In females, 1 gamete forms an egg cell with a large supply of stored nutrients. –a. The other 3, called polar bodies, disintegrate.

V. Meiosis M. In humans, the body cells have 23 different pairs or a diploid (2n) # of 46 chromosomes total. M. In humans, the body cells have 23 different pairs or a diploid (2n) # of 46 chromosomes total. –1. Each egg & each sperm have 23 single or haploid (n) # of chromosomes. See Figure 4.8

Complete Section Review 1: Reproduction of Cells

VI. Asexual VS Sexual Reproduction A. Asexual reproduction by mitosis is a careful copying mechanism. A. Asexual reproduction by mitosis is a careful copying mechanism. –1. Some unicellular organisms, like the amoeba, & many plants reproduce asexually. –2. The offspring produced are always genetically identical to the parent.

VI. Asexual VS Sexual Reproduction B. Sexual reproduction by meiosis brings with it the enormous potential for genetic variability. B. Sexual reproduction by meiosis brings with it the enormous potential for genetic variability. –1. The # of possible chromosome combinations in the gametes is 2 n, where n is the haploid chromosome # & 2 is the # of chromosomes in a homologous pair.

VI. Asexual VS Sexual Reproduction –2. See Figure 4.9  a. When n=2, 4 distinct distributions are possible.  b. When n=3, 8 distinct distributions are possible. –3. If humans have a haploid # of n=23, then 2 23, or 8,388,608 distinct distributions are possible. –4. This is only the genetic variation that occurs before fertilization.

VII. Fertilization & Cell Differentiation A. The haploid gametes produced during meiosis are spermatozoa in males & ova in females. A. The haploid gametes produced during meiosis are spermatozoa in males & ova in females. B. During fertilization, these gametes fuse to form a new diploid parent cell called the zygote. B. During fertilization, these gametes fuse to form a new diploid parent cell called the zygote. –1. The zygote is 1 cell with a set of 2n chromosomes.

VII. Fertilization & Cell Differentiation C. Each parent contributes 1 homolog to each homologous pair of chromosomes. C. Each parent contributes 1 homolog to each homologous pair of chromosomes. D. It then begins the process of mitosis to grow in size becoming an embryo. D. It then begins the process of mitosis to grow in size becoming an embryo.

VII. Fertilization & Cell Differentiation E. The group of cells produced in the very early stages of the embryo’s growth are similar to the original zygote. E. The group of cells produced in the very early stages of the embryo’s growth are similar to the original zygote. –1. They are called embryonic stem cells.

VII. Fertilization & Cell Differentiation F. When the embryo reaches cells in size, this group begins to produce cells that are different from themselves F. When the embryo reaches cells in size, this group begins to produce cells that are different from themselves –1. This process is called cell differentiation.

VII. Fertilization & Cell Differentiation G. The cells become specialized & later become tissues. G. The cells become specialized & later become tissues. H. As each cell differentiates, it produces proteins characteristic to its specific function. H. As each cell differentiates, it produces proteins characteristic to its specific function. I. Stem cells have the capability to become any type of cell. I. Stem cells have the capability to become any type of cell.

Complete Section Review 2: Reproduction, Fertilization, & Cell Differentiation Complete Review