Chapter 10 Powerpoint L — Presentation Transcript

Chapter 10 Powerpoint L — Presentation Transcript

• 1.  
• 2. Outline Reduction in Chromosome Number Meiosis Overview Homologous Pairs Genetic Variation Crossing-Over Independent Assortment Fertilization Phases of Meiosis Meiosis I Meiosis II Meiosis Compared to Mitosis Human Life Cycle
• 3. Meiosis: Halves the Chromosome Number Special type of cell division Used only for sexual reproduction Halves the chromosome number prior to fertilization Parents diploid Meiosis produces haploid gametes Gametes fuse in fertilization to form diploid zygote Becomes the next diploid generation
• 4. Homologous Pairs of Chromosomes In diploid body cells chromosomes occur in pairs Humans have 23 different types of chromosomes Diploid cells have two of each type Chromosomes of the same type are said to be homologous They have the same length Their centromeres are positioned in the same place One came from the father (the paternal homolog) the other from the mother (the maternal homolog) When stained, they show similar banding patterns Because they have genes controlling the same traits at the same positions
• 5. Homologous Chromosomes
• 6. Homologous Pairs of Chromosomes Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate A maternal copy from the mother A paternal copy from the father Many genes exist in several variant forms in a large population Homologous copies of a gene may encode identical or differing genetic information The variants that exist for a gene are called alleles An individual may have: Identical alleles for a specific gene on both homologs (homozygous for the trait), or A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)
• 7. Overview of Meiosis
• 8. Phases of Meiosis I: Prophase I & Metaphase I Meiosis I (reductional division): Prophase I Each chromosome internally duplicated (consists of two identical sister chromatids) Homologous chromosomes pair up – synapsis Physically align themselves against each other end to end End view would show four chromatids – Tetrad Metaphase I Homologous pairs arranged onto the metaphase plate
• 9. Phases of Meiosis I: Anaphase I & Telophase I Meiosis I (cont.): Anaphase I Synapsis breaks up Homologous chromosomes separate from one another Homologues move towards opposite poles Each is still an internally duplicate chromosome with two chromatids Telophase I Daughter cells have one internally duplicate chromosome from each homologous pair One (internally duplicate) chromosome of each type (1n, haploid)
• 10. Phases of Meiosis I: Cytokinesis I & Interkinesis Meiosis I (cont.): Cytokinesis I Two daughter cells Both with one internally duplicate chromosome of each type Haploid Meiosis I is reductional (halves chromosome number) Interkinesis Similar to mitotic interphase Usually shorter No replication of DNA
• 11. Genetic Variation: Crossing Over Meiosis brings about genetic variation in two key ways: Crossing-over between homologous chromosomes, and Independent assortment of homologous chromosomes 1. Crossing Over: Exchange of genetic material between nonsister chromatids during meiosis I At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues Holds homologues together Aligns DNA of nonsister chromatids Allows crossing-over to occur Then homologues separate and are distributed to different daughter cells
• 12. Crossing Over
• 13. Genetic Variation: Independent Assortment 2. Independent assortment: When homologues align at the metaphase plate: They separate in a random manner The maternal or paternal homologue may be oriented toward either pole of mother cell Causes random mixing of blocks of alleles into gametes
• 14. Independent Assortment
• 15. Recombination
• 16. Genetic Variation: Fertilization When gametes fuse at fertilization: Chromosomes donated by the parents are combined In humans, (223) 2 = 70,368,744,000,000 chromosomally different zygotes are possible If crossing-over occurs only once (423) 2 , or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible
• 17. Genetic Variation: Significance Asexual reproduction produces genetically identical clones Sexual reproduction cause novel genetic recombinations Asexual reproduction is advantageous when environment is stable However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous
• 18. Phases of Meiosis II: Similar to Mitosis Metaphase II Overview Unremarkable Virtually indistinguishable from mitosis of two haploid cells Prophase II – Chromosomes condense Metaphase II – chromosomes align at metaphase plate Anaphase II Centromere dissolves Sister chromatids separate and become daughter chromosomes Telophase II and cytokinesis II Four haploid cells All genetically unique
• 19. Meiosis I & II in Plant Cells
• 20. Meiosis versus Mitosis Meiosis Requires two nuclear divisions Chromosomes synapse and cross over Centromeres survive Anaphase I Halves chromosome number Produces four daughter nuclei Produces daughter cells genetically different from parent and each other Used only for sexual reproduction Mitosis Requires one nuclear division Chromosomes do not synapse nor cross over Centromeres dissolve in mitotic anaphase Preserves chromosome number Produces two daughter nuclei Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth 

nama ; dwi yudha kristianto
kelas ; XIA2
no.absen ; 12