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Chapter 12

Chapter 12- Meiosis.docx

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Department
Biology
Course
BIO152H5
Professor
Fiona Rawle
Semester
Fall

Description
Chapter 12- Meiosis Ultimate explanation: Evolutionary. Why something happens Proximate explanation: Mechanistic. How something happens Meiosis: gamete formation 12.1 How Does Meiosis Occur? Homologous chromosomes (homologs): two chromosomes of each type. Similar in size, shape, content (genes). Carry same genes but may carry different alleles. Karyotype: number and types of chromosomes present Diploid: two versions of each type of chromosome. Two alleles of each gene; one on each chromosome. 2n Haploid: one allele of each gene. One chromosome of each kind. n Haploid number (n): The number of distinct types of chromosomes in a given cell. Human’s haploid number is 23 Diploid number (2n): Number of chromosomes present in a diploid cell ex humans have 46 Combination of a number and n is termed the cell’s ploidy. Polyploid: 3 or more of each type of chromosome -Unreplicated and replicated chromosomes are each considered single chromosomes—even though the replicated chromosome comprises two sister chromatids Unreplicated chromosome: Achromosome that consists of a single copy Replicated chromosome: has been copied; consists of two linear structures joined at the centromere Sister chromatids: the chromosome copies in a replicated chromosome Non-sister chromatids: the chromosome copies in homologous chromosomes Tetrad: homologous chromosomes that are joined together -When two gametes fuse during fertilization, a full complement of chromosomes is restored. Resulting cell is diploid and called zygote. Each diploid receives both a haploid chromosome set from its mother and father. Life cycle: The sequence of events that occurs over the life span of an individual from fertilization to the production of offspring. Chiasma: each crossover forms an x-shaped structure PRIOR TO MEIOSIS: Chromosomes replicate, forming sister chromatids 1.Interphase: Chromosomes replicate in parent cell, in uncondensed state. MEIOSIS 1: Homologs separate 2.Early Prophase 1: Chromosomes condense, 3.Late Prophase 1: Crossing over of non sister nuclear envelope breaks up, spindle apparatus forms. chromatids (often multiple cross-overs between the Synapsis of homologous chromosomes. Spindle same chromatids). Mix of chromosome segments fibers attach to kinetochores at centromeres from maternal and paternal chromosomes 4.Metaphase 1: tetrads migrate to metaphase plate 5.Anaphase I: Homologs separate and begin and line up moving to opposite sides of cell MEIOSIS 2: 6. Telophase I and cytokinesis: Chromosomes move to opposite sides of cell, then cell divides 7. Prophase II: Spindle apparatus forms. Nuclear 8. Metaphase II: Chromosomes line up at the middle of envelope (if formed in Telophase I) breaks apart cell (metaphase plate). 9.Anaphase II: Sister chromatids separate, begin 10. Telophase II and Cytokinesis: Chromosomes moving to opposite sides of cell move to opposite sides of cell; nuclear envelope forms. then cell divides. Sister chromatids separate -Each point where crossing over occurs, the non-sister chromatids from ach homolog get physically brken at the same point and attached to each other,As a result, segments of maternal and paternal chromosomes are swapped. Feature Mitosis Meiosis Number of cell divisions One Two Number of chromosomes in Same Half daughter cells compared with parent cell Synapsis of homologs No Yes Number of crossing over events None One of more per pair of homologous chromosomes Mak
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