University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

22

Pages:

26
Author:

ramramum4x

Chromosomal Basis of Inheritance Part 2 Ch. 3 1 Phases of Mitosis ! Mitosis is subdivided into five phases – Prophase – Prometaphase – Metaphase – Anaphase – Telophase ! ! Chromosomes are decondensed By the end of this phase, the chromosomes have already replicated Mitosis – But the six pairs of sister chromatids are not seen until prophase ! The centrosome divides ! Nuclear envelope Mitosis dissociates into smaller vesicles ! Centrosomes separate to opposite poles ! The mitotic spindle apparatus is formed – Composed of mircotubules (MTs) ! Pairs of sister Mitosis chromatids align themselves along a plane called the metaphase plate ! Each pair of chromatids is attached to both poles by kinetochore microtubules Mitosis Separation of sister chromatids allows each chromatid to be pulled towards spindle pole connected to by kinetochore microtubule Anaphase 27 ! ! ! Chromosomes reach Mitosis their respective poles and decondense Nuclear membrane reforms to form two separate nuclei In most cases, mitosis is quickly followed by cytokinesis – In animals " Formation of a cleavage furrow – In plants " Formation of a cell plate " Refer to Figure 3.9 Mitosis ! Mitosis and cytokinesis ultimately produce two daughter cells – having the same number of chromosomes as the mother cell ! The two daughter cells are genetically identical to each other – Barring rare mutations ! Thus, mitosis ensures genetic consistency from one cell to the next ! The development of multicellularity relies on the repeated process of mitosis and cytokinesis Meiosis produces haploid germ cells ! Somatic cells – – divide mitotically and make up vast majority of organism’s tissues ! Germ cells – – specialized role in the production of gametes " Arise during embryonic development in animals and floral development in plants " Undergo meiosis to produce haploid gametes " Gamete unites with gamete from opposite sex to produce diploid offspring 31 Meiosis ! Gametes are typically haploid – They contain a single set of chromosomes ! Gametes are 1n, while diploid cells are 2n – A diploid human cell contains 46 chromosomes – A human gamete only contains 23 chromosomes ! During meiosis, haploid cells are produced from diploid cells – Thus, the chromosomes must be correctly sorted and distributed to reduce the chromosome number to half its original value " In humans, for example, a gamete must receive one chromosome from each of the 23 pairs Meiosis Chromosomes replicate one time, nuclei divide twice 33 Stages of Meiosis ! Prophase I: Pairing of homologous chromosomes. ! Metaphase I: Alignment of paired chromosome at equator. ! Anaphase I: Homologous chromosomes move to opposite poles. ! Telophase I: Nuclear envelope reforms, 1 chromosome set. ! Interkinesis: Cell divides. No duplication of chromosomes. ! Prophase II: Chromosomes re-condense. ! Metaphase II: Chromosomes align at metaphase plate. ! Anaphase II: Centromeres divide, chromatids go to opposite poles. ! Telophase II: Chromosomes decondense, nuclear envelope reforms. ! Cytokinesis: Cytoplasm divides. 34 Meiosis I: Stages of Prophase I ! Prophase I: consists of multiple sub-stages: – Leptotene: Thickening of thin chromosomes. – Zygotene: Homologous chromosomes begin attaching to each other by a synaptosomal complex which exactly aligns the chromosomes. – Pachytene: Completion of the synaptosomal complex to form a bivalent chromosome structure. Crossing over occurs here. – Diplotene: disintegration of the synaptosomal complex, and slight seperation of homologous chromosomes. – Diakinesis: Further condensation (thickening) of chromatids. 35 Meiosis I– Prophase I 36 Meiosis I– Prophase I continued 37 Meiosis I: Crossing Over ! An event where homologous chromosomes exchange parts, creating a new combination of gene alleles. ! The exchange of genetic material between the two homologous chromosomes is termed Recombination. ! Example – Before Crossover: " Maternal Chromosome Genes: ABCD " Paternal Chromosome Genes: abcd – After crossing over: " Maternal Chromosome Genes: ABcd " Paternal Chromosome Genes: abCD 38 Meiosis I: Crossing Over 39 Meiosis I – Metaphase and Anaphase 40 Meiosis I– Telophase I and Interkinesis 41 Meiosis II– Prophase II and Metaphase II 42 Meiosis II– Anaphase II and Telophase II 43 Meiosis II- Cytokinesis 44 Meiosis contributes to genetic diversity ! Segregation of Alleles in Anaphase I ! Independent Assortment of nonhomologous chromosomes creates different combinations of alleles among chromosomes in Anaphase I ! Crossing-over between homologous chromosomes creates different combinations of alleles within each chromosome in Prophase I 45 Oogenesis in humans 46 Spermatogenesis in humans 47 Summary: Mitosis Vs Meiosis Mitosis Meiosis -one round of DNA synthesis -one round of DNA synthesis -one cell division -two successive cell divisions -produces two somatic cells -produces four germ cells -no independent assortment -independent assortment (anaphase I) -produces diploid cells -produces haploid gametes -daughter cells are genetically identical to mother cell -cells are genetically different from mother cell and each other -no crossing over -crossing over (in prophase I) -for growth, cell replacement and asexual reproduction -for sexual reproduction

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