Lecture11Notes - Heredity Meiosis Principles of Biology...

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10/17/10 1 Meiosis Principles of Biology Lecture 11 Prof. David Fitch Heredity A defining feature of life Heredity = passing on features from one generation to another – sometimes agenetic (e.g. maternal components in egg cell) genetic Variation implies that heredity is not "perfect" – environmental – developmental genetic How does it work? This time: meiosis Sister chromatids AND homologous pairs of chromosomes align (tetrads) Meiosis I recombination occurs between homologs homologs are segregated to daughter cel s Meiosis II: sister chromatids are segregated to daughter cells Meiosis makes haploid gametes: 2n 1n Meiosis results in recombinant variation What is n? WHAT IS n ? A. 1 B. 2 C. 3 D. 4 E. 5 Mitosis vs. Meiosis Meiosis only in germ cells Result is haploid cells (1n) 2n 1n 1n Mother Cell diploid (homologs segregate) 1n 1n haploids 1n 1n haploids Crossing-over & segregation Mitosis-like sep. of copies Fertilization restores diploidy Sexual reproduction Fertilization = fusion of two haploid gametes to form a diploid zygote Development = mitotic cleavage n +n 2n 2n 2n n n DNA synthesis, mitosis
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10/17/10 2 Outline of concepts How are the haploid gametes produced? meiosis Fertilization restores diploidy What are the consequences of meiosis and fertilization? – Genetic – Evolutionary Are meiosis and fertilization essential for higher organisms? Overview of meiosis I. First, homologous chromosome pairs are segregated II. Second, sister chromatids are segregated Interphase Homologous pair of chromosomes in diploid parent cel Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cel with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cel s with replicated chromosomes Sister chromatids separate Haploid cel s with unreplicated chromosomes Meiosis I Meiosis I Details of meiosis Prophase I – longest phase (egg may arrest here until fertilization) – nuclear envelope degrades – spindle begins to form homologs synapse to form "tetrads" ( synaptonemal complex ) – CROSSING-OVER occurs (recombination) at chiasmata Details of Prophase I Substages of Prophase I Leptotene ("thin threads"): chromosomes start to condense Zygotene ("paired threads"): homologous pairs align Pachytene ("thick threads"): synaptonemal complex (SC) forms, crossing-over occurs
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Lecture11Notes - Heredity Meiosis Principles of Biology...

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