Meiosis Lecture Notes PDF

32 Meiosis ILOs By the end of this lecture, students will be able to 1. Correlate chromosomal changes to different phases of meiosis. 2. Interpret role of crossing over in genetic variations. 3. Interpret the different types of chromosomal abnormalities. 4. Different...

32 Meiosis ILOs By the end of this lecture, students will be able to 1. Correlate chromosomal changes to different phases of meiosis. 2. Interpret role of crossing over in genetic variations. 3. Interpret the different types of chromosomal abnormalities. 4. Differentiate between aneuploidy and polyploidy. 5. Differentiate between balanced and unbalanced karyotypes. 6. Correlate the phenotypic outcome with types of chromosomal aberrations. Meiosis Meiosis is a special type of cell division resulting in the formation of gametes (spermatozoa or ova) whose chromosome number has been reduced from the diploid (2n) to the haploid (1n) number. Meiosis begins at the conclusion of interphase in the cell cycle. It produces the germ cells- the ova and the spermatozoa. This process has two crucial results: 1. Reduction in the number of chromosomes from the diploid (2n) to the haploid (1n) number, ensuring that each gamete carries the haploid amount of DNA and the haploid number of chromosomes. 2. Recombination of genes, ensuring genetic variability and diversity of the gene pool Meiosis is divided into two phases: I- Meiosis I, or reductional division (first event): Homologous pairs of chromosomes line up, members of each pair separate and go to opposite poles, and the cell divides; thus, each daughter cell receives half the number of chromosomes (haploid number). II- Meiosis II, or equatorial division (second event): The two chromatids of each chromosome are separated, as in mitosis, followed by migration of the chromatids to opposite poles and the formation of two daughter cells. These two events produce four cells (gametes), each with the haploid number of chromosomes and haploid DNA content. Page 1 of 3  In gametogenesis, when the germ cells are in the S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n but the chromosome number remains at 2n (46 chromosomes). Meiosis I 1. Prophase I: It begins after the DNA has been doubled to 4n in the S phase. Prophase of meiosis I lasts a long time. Homologous pairs of chromosomes approximate each other and condense. The most significant event in prophase I is formation of chiasmata (crossing over sites) as random exchange of genetic material occurs between homologous chromosomes. 2. Metaphase I: is characterized by lining up of homologous pairs of chromosomes, each composed of two chromatids, on the equatorial plate of the meiotic spindle. 3. Anaphase I: Homologous chromosomes migrate away from each other, going to opposite poles. 4. Telophase I: The chromosomes reach the opposing poles, nuclei are re-formed and cytokinesis occurs, giving rise to two daughter cells. Each cell possesses 23 chromosomes, the haploid (1n) number, but because each chromosome is composed of two chromatids, the DNA content is still diploid. Figure 2. Phases of meiosis d; double amount of DNA, s; haploid amount of DNA Page 2 of 3 Meiosis II (equatorial division) occurs without DNA synthesis and proceeds rapidly through four phases and cytokinesis to form four daughter cells each with the haploid chromosome number It is subdivided into prophase II, metaphase II, anaphase II, telophase II, and cytokinesis The chromosomes line up on the equator, the kinetochores attach to spindle fibers, followed by the chromatids migrating to opposite poles, and cytokinesis divides each of the two cells. Outcome of meiosis II: 1. Results in a total of four daughter cells from the original diploid germ cell. Each of the four cells contains a haploid amount of DNA and a haploid chromosome number. 2. The cells are genetically distinct because of reshuffling of the chromosomes and crossing over. Thus, every gamete contains its own unique genetic complement. Page 3 of 3