Ch 13 Meiosis BIO1300 PDF

Ch. 13: Meiosis & Sexual Life Cycles In sexual reproduction, 2 parents give rise to offspring that have unique combinations of genes inherited from the 2 parents Living organisms are distinguished by their ability to reproduce their own kind – Genetics: scientific study of heredity an...

Ch. 13: Meiosis & Sexual Life Cycles In sexual reproduction, 2 parents give rise to offspring that have unique combinations of genes inherited from the 2 parents Living organisms are distinguished by their ability to reproduce their own kind – Genetics: scientific study of heredity and variation – Genes are units of heredity & are made up of segments of DNA – Each gene has a specific location called a locus (plural is loci) on a certain chromosome Heredity = transmission of traits (via genes) from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings © 2011 Pearson Education, Inc. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes Genes are passed to the next generation via reproductive cells called gametes (gametes are produced by meiosis) A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) Humans haploid number is 23 (n = 23) In unfertilized egg (ovum), the sex chromosome is X In sperm cell, sex chromosome may be either X or Y Gametes fuse to form a diploid (2n) zygote that divides by mitosis to develop into a multicellular organism In a cell post DNA synthesis, each chromosome is replicated Each replicated chromosome consists of 2 identical sister chromatids © 2011 Pearson Education, Inc. In this karyotype, each homologous pair of chromosomes has been replicated, hence you see sister chromatids….shown are 92 chromosomes as post S Phase Sex chromosomes are X & Y Human females = homologous pair of X chromosomes (XX) While Human males = (XY) Remaining 22 pairs of chromosomes are autosomes Each pair of homologous chromosomes is 1 from each parent Human Somatic Cells have 22 autosomes + sex chromosomes: Fig. 13.3 Diploid Cell (2n) = 2 sets of 23 = 46 As 1 Set from Mom & 1 from Dad Human Gamete/Sex Cells have: Haploid Cell (n) = 1 set of 23 = 23 The 2 chromosomes of a pair are homologous chromosomes Each homologue of a pair has the same sites (or loci) for the same genes, although the homologues may carry different alleles Alleles = alternate states (or versions) for a given gene at homologous loci Homologous chromosomes = maternal & paternal copies of same chromosome Sister chromatids = 2 replicas of a single chromosome held together at their centromeres after DNA replication  Sets of Chromosomes in Human Cells Fig. 13.4 Describing Chromosomes 2n = 6 and n = 3 here Small, Medium, Large Chromosomes…but…. 12 chromosomes shown as the 6 were duplicated at S phase of Interphase © 2011 Pearson Education, Inc. Concept 13.2: Fertilization and Meiosis alternate in human life cycles to maintain chromosome number Fertilization = union of gametes (when sperm fuses with egg) Fertilized egg is a zygote & has 1 set of chromosomes from each parent – Zygote produces somatic cells by mitosis & develops into an adult At sexual maturity, ovaries & testes produce haploid (n) gametes Gametes are the only types of human cells produced by meiosis, rather than mitosis – Oocytes: ovarian cells that produce eggs – Spermatocytes: testes cells produce sperm – Meiosis results in 1 set of chromosomes in each gamete Fig. 13.5 – Only diploid cells (2n) can undergo Human meiosis Life Cycle Concept 13.3: Meiosis reduces the # of chromosome sets from diploid to haploid (“reduction division”) Like mitosis, meiosis is preceded by the replication of chromosomes – Meiosis occurs in 2 sets of cell divisions (meiosis I & meiosis II) Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids (during S phase) – Sister chromatids are genetically identical & joined @ centromere – Homologous chromosomes are separated at the end of meiosis I Meiosis II – Sister chromatids composing each chromosome are separated 2 cell divisions of Meiosis result in 4 daughter cells, not 2 cells as in mitosis – Each daughter cell has only half as many chromosomes as parent cell © 2011 Pearson Education, Inc. Stages of Meiosis Fig. 13.7 After chromosomes duplicate, 2 divisions: Meiosis I (reductional division) Homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes Meiosis II (equational division) Sister chromatids separate Result is 4 haploid daughter cells with unreplicated chromosomes © 2011 Pearson Education, Inc. Fig. 13.8 Division in Meiosis I Synapsis & Crossing Over Occurs Chromosomes undergo Independent assortment Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene In crossing over, nonsister chromatids exchange DNA segments – Each pair of chromosomes forms a tetrad, a group of four chromatids – Each tetrad usually has one or more chiasmata, regions where crossing over occurred © 2011 Pearson Education, Inc. Metaphase I Tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome Chromosomes undergo independent assortment ie. not all blue go to one pole Anaphase I Pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole © 2011 Pearson Education, Inc. Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells with duplicated chromosomes – In animal cells, a cleavage furrow forms – In plant cells, a cell plate forms (start of new cell wall) No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated © 2011 Pearson Education, Inc. Division in Meiosis II also occurs in 4 phases Prophase II….Metaphase II….Anaphase II…Telophase II and cytokinesis Meiosis II is similar to Mitosis as sister chromatids separate Prophase II A spindle apparatus forms In late prophase II, chromosomes (each still composed of 2 sister chromatids) move toward the metaphase plate Metaphase II Sister chromatids are arranged at metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles Anaphase II -- sister chromatids separate The sister chromatids of each chromosome now separate & move as two newly individual chromosomes toward opposite poles Telophase II and Cytokinesis Telophase II, chromosomes arrive at opposite poles – Nuclei form, and the chromosomes decondense Cytokinesis separates the cytoplasm At the end of meiosis, there are 4 resulting cells – Each with a haploid set of unreplicated chromosomes – Each is genetically distinct from the others and from the parent cell © 2011 Pearson Education, Inc. Comparison of Mitosis & Meiosis Mitosis conserves the # of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the # of chromosomes sets from 2 (diploid) to 1 (haploid), producing cells that differ genetically from each other and from the parent cell 3 events are unique to Meiosis & all 3 occur in Meiosis l – Synapsis and Crossing Over in Prophase I: Homologous chromosomes align & non-sister chromatids exchange – At the Metaphase I plate, there are paired homologous chromosomes (tetrads) that undergo independent assortment, instead of individual replicated chromosomes – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Lab Video to demonstrate Mitosis & Meiosis: (11:53min bead demo video by a Mr. Anderson) http://www.youtube.com/watch?v=zGVBAHAsjJM&feature=youtu.be © 2011 Pearson Education, Inc.  MITOSIS MEIOSIS Parent cell MEIOSIS I Chiasma Crossing Over & Prophase Prophase I Independent Duplicated Chromosome duplication Chromosome duplication Homologous Assortment chromosome 2n = 6 chromosome pair Can Occur Sister Metaphase I Homologous Metaphase Chromatids Chromosomes Separate Separate Anaphase Anaphase I Telophase Telophase I Haploid n=3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells n n n n of mitosis Daughter cells of meiosis II SUMMARY Property Mitosis Meiosis DNA Occurs during interphase before Occurs during interphase before meiosis I begins replication mitosis begins Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, divisions anaphase, and telophase and telophase Synapsis of Does not occur Occurs during prophase I along with crossing over homologous between nonsister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many daughter cells identical to the parent cell chromosomes as the parent cell; genetically different and genetic from the parent cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes animal body zygote; produces cells for growth, repair, by half and introduces genetic variability among the and, in some species, asexual reproduction gametes Fig. 13.10  Mitosis & Meiosis Practice Questions Question #1 Which of the following best describes the kinetochore? a) a structure composed of several proteins that associate with the centromere region of a chromosome and that can bind to spindle microtubules b) centromere region of a metaphase chromosome at which the DNA can bind with spindle proteins c) array of vesicles that will form between two dividing nuclei and give rise to the metaphase plate d) ring of actin microfilaments that will cause the appearance of the cleavage furrow e) core of proteins that forms the cell plate in a dividing plant cell © 2011 Pearson Education, Inc. Question #2 Which of the following transmits genes from one generation of a family to another? Select the single best answer. a) DNA b) Gametes (contain both DNA & nucleotides) c) somatic cells d) mitosis e) nucleotides © 2011 Pearson Education, Inc.  Question #3 Fertilization is to zygote as meiosis is to which of the following? Justify your answer. a) mitosis b) diploid c) chromosome d) replication e) gamete © 2011 Pearson Education, Inc.  Question #4 At what stage do chromosomes undergo independent assortment? a) meiosis I pairing of homologs b) anaphase I separation of homologs c) meiosis II separation of homologs d) meiosis I metaphase alignment e) meiosis I telophase separation © 2011 Pearson Education, Inc.  Question #5 In this cell, what phase is represented? a) mitotic metaphase b) meiosis I anaphase c) meiosis I metaphase d) meiosis II anaphase e) meiosis II metaphase © 2011 Pearson Education, Inc.