1. Meiosis and Sexual Life Cycles

2. HUMAN KARYOTYPE

3. Homologous Chromosomes are chromosome pairs of the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father. [1] They pair (synapse) during meiosis, or cell division that occurs as part of the creation of gametes. Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome. Each pair, however, can contain the same allele (both alleles for blue eyes) or different alleles (one allele for blue eyes and one allele for brown eyes) for each feature.

4. Allele : different forms of the same gene. For example, humans have genes for eye color. However, there exists an allele for brown eyes and an allele for blue eyes. Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (2 Xs), while males have an X and a Y chromosome.

5. I. Meiosis: Overview a) Mitosis produces two cells with the same number of chromosomes as the parent cell. Mitosis of a diploid cell (2n) produces two diploid daughter cells. b) Meiosis is a process of cell division in eukaryotes characterized by: two consecutive divisions: meiosis I and meiosis II no DNA synthesis (no S phase) between the two divisions. the production of four cells with half the number of chromosomes of the starting cell, e.g., 2n n

6. c) The fusion of two gametes produces a 2n zygote. d) The diploid number (2n) refers to the number of chromosomes that are found in normal body cells (somatic cells). e) The haploid or monoploid number (n) refers to the number of chromosomes that are found in sex cells (gametes).

7. II.The Steps of Meiosis (reduction division) a) Meiosis I Prophase I 1. Sister chromatid become visible and are paired by a chiassmatta to form a structure known as a tetrad. This process is known as synapsis. 2. Nuclear membrane begins to disintegrate. 3. The replicated centrosomes begin to migrate towards the poles and begin producing the spindle fibers.

8. Metaphase I 1. The tetrads (bivalents), each composed of two chromosomes (four chromatids) align at the equatorial plane (metaphase plate). 2. The centrosomes are at the poles with spindle fibers extending to the tetrads. The formation of tetrads during Meiosis I is know as synapsis. This is essential in that it allows for the process of crossing over which increases genetic variability of potential offspring.

9. Anaphase1. Tetrads separate equally as a result of a process known as disjunction. 2. Chromosomes, each with two chromatids, move to separate poles. 3. Each of the daughter cells is now haploid but each chromosome has two chromatids If the separation of tetrads does not occur equally, an uneven number of chromosomes will result in formed gametes. This process is know as non-disjunction. If these gametes are successfully fertilized, offspring will have either one too many or one too few chromosomes. Down Syndrome is a disease that results from this process. (3 – 21 st chromosomes for a total of 47 chromosomes).

10. Telophase 1. Nuclear envelopes reform and the cell enter meiosis 2. Cytokinesis 1. The parent cell splits into two haploid daughter cells.

11. b) Meiosis II 1. Meiosis 2 is similar to mitosis. 2. However, there is no "S" phase. 3. The chromatids of each chromosome are no longer identical because of recombination. 4. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid

12. III. Life Cycles a) Life cycles are a diagrammatic representation of the events in the organism's development and reproduction. b) When interpreting life cycles, pay close attention to the ploidy level of particular parts of the cycle and where in the life cycle meiosis occurs. c) The different examples of life cycles include diploid, haploid, and alternation of generations.

13. d) Diploid Life Cycle: Animals have a dominant diploid phase, with the gametic (haploid) phase being a relative few cells. NOTE: Most of the cells in your body are diploid; germ line (primary sex cells) diploid cells will undergo meiosis to produce gametes, with fertilization closely following meiosis.

14. e) Haploid Life Cycle: 1) The haploid life cycle occurs in fungi and some protists (algae). 2) The cells of the organisms are mostly haploid. 3) Haploid, somatic cells give rise to other haploid, somatic cells via mitotic cell division. The result of this process is growth. This phase is called the gametophyte generation. 4) Some haploid cells will give rise to specialized haploid cells known as gametes by mitosis. 5) The haploid gametes will undergo fertilization to produce a diploid zygote. 6) The diploid zygote will grow via mitosis. 7) The diploid sporophyte gives rise to haploid spores via meiosis staring the cycle anew.

15. Haploid Life Cycle

16. f) Alternation of generations: Plant life cycles have two sequential phases that alternate between diploid and haploid phases. 1. The sporophyte phase is diploid and is that part of the life cycle in which meiosis occurs. The sporophyte produces haploid spores by the process of meiosis. Spores are haploid cells produced as a result of meiosis that can germinate and grow into a haploid organisms via mitosis. In flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is sporophyte.

17. Pollen and ovaries contain the male and female gametophytes, respectively. NOTE: Plant life cycles differ from animal ones by adding a phase (the haploid gametophyte) after meiosis and before the production of gametes. 2. The gametophyte phase is haploid and is the part of the life cycle in which gametes are produced (by mitosis of haploid cells). The haploid gametes that are produced as a result of the mitotic cell division fertilize and produce a diploid zygote. The diploid zygote grows via mitosis to restore that sporophyte phase.

18. Alternation of Generations: illustrated by life cycle of the fern.

19. IV. Origins of Genetic Variation a) The intraspecies and interspecies differences in genetic information is an essential aspect of evolutionary adaptation and change. Genetic variation allows for the adaptation and the ultimate survival of certain organisms when environmental change occurs. There exists several mechanisms in biological systems to increase to the degree of genetic diverisity.

20. b) Sexual Reproduction as a mechanism for increased genetic diversity 1. The independent assortment of homologous chromosomes into gametes during the process of meiosis leads to great variation in genetic composition. The number of different gametes that can be produced as a result the independent assortment of homologous chromosomes as a result of meiosis can be determined by using 2 n whereas n is equal to the number of homologous pairs of chromosomes. Ex: Humans have 23 pairs of homologous chromosomes. The number of different possible gametes that can be produced from a single primary sex cell is equal to 2 23.

21. Independent Assortment and Genetic Variability

22. 2. Crossing Over During prophase I of meiosis I, homologous pairs come together to form a tetrad. Complementary sections of two adjacent nonsister chromatid trade places. This process increases the genetic diversity of offspring by creating unique gene combinations. Genes found on the same chromosome are usually inherited together. For example, humans with dark colored eyes usually have dark hair (and vive versa). Question: How do humans have dark hair and light eyes?

23. 3. Random Fertilization The random nature of fertilization further leads to genetic variation by recombining two sets of different genetic material in a unique way.