Chapter 28 Lecture Notes

Chapter 28 Lecture Notes - Chapter 28 The Reproductive...

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1 Chapter 28 The Reproductive Systems Number of Chromosomes in Somatic Cells and Gametes Human somatic cells contain 23 pairs of chromosomes, a total of 46 chromosomes, one member from each parent. – 22 pairs called autosomes and one pair called sex chromosome Homologous chromosomes: – The two chromosomes in each pair contain similar genes arranged in the same order. – Generally they look similar under the microscope Exception is the sex chromosomes X and Y in males Y is much smaller than X Somatic cells contain two sets of chromosomes, are called diploid cells The symbol n used to denote the number of different chromosomes in an organism n = 23 different chromosomes, diploid cells are 2 n Meiosis produce gametes that contain a single set of 23 chromosomes (haploid = 1 n ) Two different gametes, one from each parent, produce 2 n organism
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2 Meiosis Occurs in two successive stages Meiosis I • Begins once chromosomal replication is complete • Consists of four phases – prophase I, metaphase I, anaphase I, telophase I Meiosis II • Consists of four phases: – prophase II, metaphase II, anaphase II, telophase II Interphase (precedes meiosis I) – The chromosomes of the diploid parent cell replicate . – Each chromosome consists of two genetically identical (sister) chromatids attached at their centromeres Meiosis I Prophase I – The chromosomes shorten and thicken. – The nuclear envelope and nucleoli disappear. – The mitotic spindle forms. – Synapsis ( not seen in mitosis ) : • The two sister chromatids of each pair of homologous chromosomes pair off to form tetrad. – Crossing-over ( not seen in mitosis ) • Parts of the chromatids of the two homologous chromosomes may be exchanged with one another this result in: – daughter cells are genetically different from each other and from the parent cells » Recombination: the formation of new combinations of genes Genetic variation among humans is due to: • Recombination • Random assortment of chromosomes derived from both parents toward opposite poles
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3 Meiosis I Metaphase I – Homologous pairs of chromosomes line up along the equatorial plane of the cell – The pericentriolar material of a centrosome forms kinetochore microtubules that attach to the centromeres Anaphase I – Chromosomes pulled to opposite poles of the cell by the kinetochore microtubules attached to the centromeres – The centromeres do not split, and the paired chromatids held by a centromere remain together (they split in mitosis) Telophase I and cytokinesis – similar to telophase and cytokinesis of mitosis. Net effect of meiosis – daughter cell contains only one member of each pair of the homologous chromosomes present in the parent cell. Meiosis II
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This note was uploaded on 05/29/2011 for the course BIO 233 taught by Professor Salti during the Spring '11 term at Portland CC.

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Chapter 28 Lecture Notes - Chapter 28 The Reproductive...

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