meiosis

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Inheritance of Genes Genes - the units of heredity (segments of DNA that specify a certain protein) Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) © 2011 Pearson Education, Inc. Heredity - transmission of traits from one generation to next

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Comparison of Asexual and Sexual Reproduction asexual reproduction - a single individual passes genes to its offspring without gametes clones - genetically identical individuals from the parent © 2011 Pearson Education, Inc. sexual reproduction - offspring have unique combinations of genes inherited from two parents, via gamete fusion MITOSIS - genetically identical daughter cells are clones MEIOSIS - gametes are NOT genetically identical daughter cells

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Chromosomes in Human Cells somatic cells (any cell other than a gamete) human somatic cells are diploid (2n) = has pairs of chromosomes homologous chromosomes - pairs of chromosomes Chromosomes in a homologous pair carry genes controlling the same characters © 2011 Pearson Education, Inc.

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Figure 13.3b Pair of homologous duplicated chromosomes Centromere Sister chromatids duplicated chromosome karyotype - ordered display of pairs of chromosomes from a cell humans cells - diploid number is 46 (2 n) = 2(23) = 46

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sex chromosomes - determine gender Human females (XX) Human males (X Y) 22 pairs of homologous chromosomes are called autosomes

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Figure 13.4 Key Maternal set of chromosomes ( n  3) Paternal set of chromosomes ( n  3) Key 2 n  6 Pair of homologous chromosomes Each pair of homologous chromosomes consists of one chromosome from each parent

Inheritance of Genes:

A gamete (sperm or egg) contains a single set of chromosomes, (not paired) and is haploid ( n ) For humans, the haploid number is 23 ( n = 23) © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. The gonads (ovaries and testes) produce haploid gametes Gametes are the only human cells produced by meiosis Meiosis results in one set of chromosomes in each gamete

Comparison of Asexual and Sexual Reproduction :

Key Haploid ( n ) Diploid (2 n ) Egg ( n ) Haploid gametes ( n  23) Sperm ( n ) Ovary Testis Mitosis and development Diploid zygote (2 n  46) Multicellular diploid adults (2 n  46) MEIOSIS FERTILIZATION In egg, the sex chromosome is X In sperm, the sex chromosome may be either X or Y

Chromosomes in Human Cells:

Fertilization - the union of gametes (the sperm and the egg) © 2011 Pearson Education, Inc. zygote grows and develops by producing somatic cells via mitosis zygote – a “fertilized egg” (or fused egg and sperm) has one set of chromosomes from each parent

Figure 13.3b:

Meiosis reduces the number of chromosome sets from diploid to haploid Just like mitosis…. meiosis is preceded by chromosome replication Meiosis takes place in two sets of nuclear/cell divisions meiosis I and meiosis II Resulting in four daughter cells (rather than the two daughter cells in mitosis) Each daughter cell is haploid (has only half as many chromosomes as the parent cell)…………they are NOT genetically identical © 2011 Pearson Education, Inc.

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The Stages of Meiosis: 2 divisions Meiosis I ( reductional division): homologs pair up and separate Meiosis II ( equational division) sister chromatids separate © 2011 Pearson Education, Inc. meiosis I occurs in 4 phases: Prophase I Metaphase I Anaphase I Telophase I and cytokinesis meiosis II occurs in 4 phases: Prophase II Metaphase II Anaphase II Telophase II and cytokinesis

Figure 13.4:

Figure 13.7-1 Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Interphase

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Figure 13.7-2 Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Meiosis I 1 Interphase

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Figure 13.7-3 Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Homologous chromosomes separate Sister chromatids separate Interphase Meiosis I Meiosis II 2 1

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Prophase I of meiosis I Nonsister chromatids held together during synapsis homologous chromosomes aligned gene by gene Pair of homologs Chiasma Centromere crossing over - chromatids exchange segments of DNA chiasmata - where crossing over occurred

Meiosis reduces the number of chromosome sets from diploid to haploid:

Figure 13.8a Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. Nonsister chromotids exchange genes…. Crossing Over

The Stages of Meiosis: 2 divisions:

Figure 13.8b Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. four daughter cells, each with a haploid set of chromosomes Each genetically distinct from the others and from the parent cell

Figure 13.7-1:

Prophase Duplicated chromosome MITOSIS Chromosome duplication Parent cell 2 n  6 Metaphase Anaphase Telophase 2 n 2 n Daughter cells of mitosis MEIOSIS MEIOSIS I MEIOSIS II Prophase I Metaphase I Anaphase I Telophase I Haploid n  3 Chiasma Chromosome duplication Homologous chromosome pair Daughter cells of meiosis I Daughter cells of meiosis II n n n n Comparison of Mitosis and Meiosis

Figure 13.7-2:

Prophase I : Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata. Metaphase I : Chromosomes line up as homologous pairs on the metaphase plate as tetrads. Anaphase I : Homologs separate from each other; sister chromatids remain joined at the centromere. The events unique to meiosis all occur in meiosis l:

Figure 13.7-3:

Genetic variation contributes to evolution © 2011 Pearson Education, Inc. 3 mechanisms of sexual reproduction and meiosis contribute to genetic variation : Independent assortment of chromosomes Crossing over Random fertilization

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Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I

Figure 13.8a:

Figure 13.10-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

Figure 13.8b:

Possibility 1 Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 The number of combinations possible when chromosomes assort independently into gametes is 2 n ( n is the haploid number) For humans ( n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes

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Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Crossing Over Crossing over produces recombinant chromosomes : new combinations of DNA inherited from each parent

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Figure 13.11-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Genetic variation contributes to evolution:

Random Fertilization - any sperm can fuse with any egg © 2011 Pearson Education, Inc. The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of 70 trillion possible combinations

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The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations © 2011 Pearson Education, Inc. Mutations : (changes in an organism’s DNA) - source of genetic diversity

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