Review of Terms Haploid cells (1n) with one copy of each chromosome Diploid cells (2n) with two copies of each chromosome Somatic non-germline cells Gametes sex cells (eggs and sperm)
Reductional division Meiosis I A diploid cell divides and forms two haploid cells In humans, the chromosome number is reduced from 46 to 23 chromosomes
Chromosomes coil Nuclear membrane breaks down Chromosomes pair Recombination may occur Prophase I newly forming microtubules Prophase I Fig. 2.13a
Metaphase I spindle equator one pair of homologous chromosomes Chromosome pairs line up on the midline Fig. 2.13b Metaphase I
Anaphase I Centromeres do not divide Chromosome pairs move to opposite poles Fig. 2.13c Anaphase I
Telophase I Nuclear membrane reforms Chromosomes uncoil After Cytokinesis Two haploid cells are produced Fig. 2.13d Telophase I
Interkinesis No DNA synthesis occurs
Meiosis II Begins with Haploid Cells Prophase II Chromosomes coil Spindle forms Nuclear membrane breaks down Each chromosome is composed of two sister chromatids attached at the centromere Prophase II Fig. 2.13e
Metaphase II Chromosomes line up on the midline and attach to spindle fibers Fig. 2.13f Metaphase II
Anaphase II Centromeres divide Sister chromatids move to opposite poles Fig. 2.13g Anaphase II
Telophase II Nuclear membrane reforms Chromosomes uncoil After Cytokinesis Four unique haploid cells are produced Fig. 2.13h Telophase II
Unique Haploid Gametes Random assortment of maternal and paternal chromosomes Number of possible chromosomal combinations = 2 23 or 8,388,608 Recombination between chromosome pairs increases the possible combinations
Summary of Mitosis and Meiosis Fig. 2.15
Members of chromosome pair Sister chromatids Sister chromatids Each chromosome pairs with its homologue Paired homologues separate in meiosis I Sister chromatids separate and become individual chromosomes in meiosis II Fig. 2-14, p.32
Members of chromosome pair Sister chromatids Sister chromatids Each chromosome pairs with its homologue Fig. 2-14a, p.32
Paired homologues separate in meiosis I Sister chromatids separate and become individual chromosomes in meiosis II Fig. 2-14b, p.32
Fig. 2-16, p.34
Fig. 2-16a, p.34
Fig. 2-16b, p.34
Fig. 2-16c, p.34
Fig. 2-16d, p.34
Meiosis
Recombination http://www.answersingenesis.org/home/area/magazines/tj/images/v17n1_demographics.jpg
Mitosis Meiosis 1. In somatic cells 2. Both haploid and diploid cells 3. 2n to 2n 1. In germ cells 2. Only diploid cells 3. 2n to n 4. One round of division 4. Two rounds of division
Mitosis Meiosis 5. Homologous chromosomes do NOT pair 6. No (or very rare) recombination 7. Sister chromatids attach to spindle fibers from opposite poles 5. Homologous chromosomes pair 6. Recombination occurs 7. During metaphase I, homologous chromosomes attach to opposite spindle fibers
Mitosis Meiosis 8. Centromeres divide In anaphase 8. Centromeres do NOT divide during Meiosis I Centromere divides In meiosis II
Why is meiosis important? 1. Meiosis results in the production of haploid cells, which when fused restore and maintain chromosome number. 2. Meiosis allows for the generation of gametes with different combinations of paternal and maternal chromosomes, generating genetic diversity. 3. Recombination during meiosis allows for further generation of genetic diversity.