1. Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 CHAPTER 15 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS Prepared by Brenda Leady, University of Toledo

2. 2 Eukaryotic chromosomes Typical chromosome contains a single, linear, double-stranded DNA molecule DNA must be folded and packaged Chromatin is the DNA-protein complex making up chromosomes

3. 3 Compaction DNA wraps itself around histone proteins Nucleosome is a repeating unit of DNA wrapped around an octamer of histone proteins Negative charges of DNA attracted to positive charges of histones Amino terminal tails of histone proteins protrude from octamer  Modification can control the degree of compaction Shortens length of DNA about sevenfold

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5. Noll Confirmed Kornberg’s Beads-on-a-String Model by Digestion of the Linker Region Beads-on-a-string model of nucleosome structure was originally proposed by Roger Kornberg in 1974 Noll reasoned that if the model was correct, the linker region of DNA would be more accessible to DNase-I than would the 146-bp region that is tightly bound to the histones He expected incubation with DNase-I to make cuts in the linker region and produce DNA pieces that would be approximately 200 bp in length

7. 7 30-nm fiber Nucleosome units are organized into a more compact structure that is 30 nm in diameter Shortens the nucleosome structure another sevenfold Structure has proven difficult to determine because the conformation of the DNA may be substantially altered when extracted from living cells

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9. 9 30-nm fibers interact with the nuclear matrix Compacts 30-nm fiber by participating in the formation of radial loop domains Anchored to nuclear matrix

10. 10 Each chromosome located in discrete nonoverlapping chromosome territory Different chromosomes are not substantially intertwined even when noncompacted

11. 11 Compaction is not uniform Heterochromatin  Highly compacted and transcriptionally inactive  Some localized regions in nondividing cells Euchromation  Less condensed and capable of gene transcription  Most chromosomal regions in nondividing cells Chromosomes compact further when the cell is preparing to divide  All euchromatin converted to heterochromatin  Most transcriptional activities cease during cell division

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13. 13 In metaphase chromosomes, highly compacted radial loops remain anchored to a scaffold formed from proteins in the nuclear matrix

14. Histone Code Controls Chromatin Compaction Particular amino acids in the amino terminal tails of histones are subject to several types of covalent modifications, including acetylation, methylation, and phosphorylation 2 effects  May directly influence interactions between nucleosomes  Provides binding sites – histone code hypothesis Pattern affects degree of chromatin compaction May play key role in accessing the information within the genome of eukaryotes

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16. 16 Mitotic cell division A cell divides to produce 2 new cells genetically identical to the original Original called mother, new cells called daughters Involves mitosis and cytokinesis Can be for asexual reproduction or for production and maintenance of multicellularity

17. 17 Cytogenetics Field of genetics involving microscopic examination of chromosomes Tightly compacted chromosomes have distinctive shapes and number of chromosomes visible with light microscope Chemical dye gives a distinctive banding pattern Karyotype

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19. 19 In many species, individual has 2 sets of chromosomes  Humans- 23 pairs for 46 total chromosomes  Diploid, 2n  Haploid, 1n – gametes have 1 member of each pair

20. 20 Preparation for cell division DNA replicated Sister chromatids - 2 identical copies with associated proteins Tightly associates at centromere Serves as attachment site for kinetochore used in sorting chromosomes

21. 21 Like mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle 2 key differences 1. Homologous pairs form a bivalent or tetrad 2. Crossing over

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25. 25 Meiosis vs. Mitosis Mitosis produces two diploid daughter cells that are genetically identical  6 chromosomes in 3 homologous pairs Meiosis produces four haploid daughter cells  Each daughter has a random mix of 3 chromosomes

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27. 27 Life Cycle Sequence of events that produces another generation of organisms For sexually reproducing organisms, involves an alternation between haploid cells or organisms and diploid cells or organisms

28. 28 Diploid-dominant species  Most animal species are diploid  Haploid gametes are a specialized type of cell Haploid-dominant species  Many fungi and some protists  Multicellular organism is haploid  Haploid cells unite to form diploid zygote, then proceeds immediately through meiosis to make 4 haploid spores

29. 29 Alternation of generations  Plants and some algae  Intermediate dominance  Multicellular diploid organism – sporophyte  Multicellular haploid organism – gametophyte  Among species, relative size of sporophyte and gametophyte varies

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31. 31 Variation in Chromosomes Chromosome composition within a given species tends to remain relatively constant  Humans - 2 sets of 23 chromosomes (total of 46)  Dog - 78 chromosomes (39 per set)  Fruit fly - 8 chromosomes (4 per set)  Tomato - 24 chromosomes (12 per set)

32. 32 Chromosomes identified by  Size  Location of centromere Short arm is p, long arm is q, short arms on top Metacentric – middle Submetacentric – off center Acrocentric – near end Telocentric – at the end  Banding pattern Giemsa stain gives G banding

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34. 34 Chromosomal mutations Deficiencies  Segment missing, deletion Duplications  Section occurs 2 or more times in a row Inversions  Change in direction along a single chromosome Translocations  One segment becomes attached to another chromosome  Simple or reciprocal

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36. 36 Changes in chromosome number Euploid - chromosome number that is viewed as the normal number  In a diploid organism, 2 sets is normal, Diploid 2n Polyploid – 3 or more sets of chromosomes  Triploid 3n  Tetraploid 4n

37. 37 Aneuploidy  Alteration number of particular chromosomes  Total number not an exact multiple of a set Trisomic  Normal 2 copies of a chromosome plus a 3 rd  2n+1 Monosomic  Missing one of normal copies of a chromosome  2n-1

38. 38 Nondisjunction Chromosomes do not sort properly during cell division During meiosis can produce gametes with too many or too few chromosomes

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40. 40 Interspecies breeding Alloploid organisms have at least one set of chromosomes from 2 or more different species Allodiploid - only one set of chromosomes from two different species Allopolyploidy - two or more complete sets of chromosomes from two or more different species Allotetraploid - two complete sets of chromosomes from two species for a total of four sets

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42. 42 Consequences Animals do not tolerate deviations from diploidy well – usually lethal  However, male bees (drones) contain a single set of chromosomes while female bees are diploid  Diploid and polyploid species of amphibians and reptiles Plants commonly exhibit polyploidy  30-35% of fern and flowering plant species are polyploid  Important in agriculture - Wheat example

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44. 44 Aneuploidy in all eukaryotic species usually has detrimental consequences Trisomic and monosomic individuals have an imbalance in the level of gene expression interfering with proper cell function

45. 45 Aneuploidy in humans About 5-10% of all fertilized human eggs result in an embryo with an abnormality in chromosome number Approximately 50% of all spontaneous abortions are due to alterations in chromosome number Can survive some abnormalities  Trisomies or abnormalities in sex chromosome number

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