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Lecture 7

BIOC15Fall2013 Lecture 7 and Lecture 8 Notes 2.13.29 PM.docx

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Biological Sciences
Karen Williams

BIOC15Fall2013 Lecture 7 and Lecture 8 Notes Lecture 7: Chromosomal basis of inheritance Homologous and Non-homologous Chromosomes o o humans have 22 pairs of chromosomes and 2 sex chromosomes (23 pairs total) o homologous chromosomes  chromosomes that match in shape, size and banding  contain the same linear gene sequence, each derived from each parent Centromere position o centromere  the region of the chromosome that binds the kinetochore and attaches to the chromosome to the spindle fibres o chromosome takes name from position of centromere o metacentric  in the middle o acrocentric  off centre o telocentric  at one end o see pages 327 and 336 o p region  smallest distance to the centromere in human chromosomes o q region  larger distance to the ceontromere o maternal chromosomes contain the maternal centromere and paternal chromosomes are those containing the paternal chromosome Cell Cycle: Mitosis o interphase = G1  S  G2 o During S phase, DNA replicates o During G1 and G2 is where most of the cell growth occurs o Chromosomes duplicate to form sister chromatids o Prophase: o Chromosomes condense and become visibile o Centrosomes move apart toward opposite poles and generate new microtubules o Nucleoli beign to disappear o Prometaphase: o Nuclear envelope breaks down o Microtubules fro the centrosomes invade the nucleus o Sister chromatids attach to microtubules from opposite centrosomes o Metaphase: o Chromosomes align on the metaphase plate with sister chromatids facing the opposite poles o Anaphase: o Centromeres divide o The now separated sister chromatids move to opposite poles o Telophase: o Nuclear membranes and nucleoli reform o Spindle fibres disappear o Chromosomes uncoil and become at angle of chromatin o Cytokinesis: o The cytoplasm divides, splitting into 2 elongated parent cell into two daughter cells wth identical nuclei 1 o Animal cell  cleavage furrow (contractile ring) is formed that separates the elongated cell into 2 daughter cells o Plant cell  cell plate formed which separates the cell into 2 daughter cells Meiosis and Mitosis o Mitosis  daughter cell is identical to parent cell and a 2n cell gives 2 x 2n daughter cells o See page 99 o Meiosis  parent cell 2n produces 4 x n gametes; forms new combinations of chromosomes in gametes recombinant chromosomes are formed during crossing over) Meiosis o Meiosis I o Prophase I  Leptotene  chromosomes begin to condense and thicken, becoming visible as discrete structures (sister chromatids not yet visible under microscope)  Zygotene  chromosomes cleary visible and begin pairing with homologous chromosomes along the synaptonemial complex to form a bivalent (tetrad)  Pachytene  full synapsis of homologs; recombination nodules appear along the synaptonemal complex  Diplotene  tetrads appear to pull apart slightly but remains connected at crossover sites (chiasmata)  Diakinesis  further condensation of chromatids; nonsister chromatids that have exchanged parts by crossing over remain closely associated by chiasmata o o Prometaphase I o Metaphase I  paired homologs attach to spindle fibres from opposite poles (maternal and paternal chromosomes face opposite poles) o Anaphase I  homologs move to opposite poles o Telophase I  nuclear envelope reforms o What is the difference between meiosis I and mitosis? o During meiosis homologous chromosomes pair and crossing over occurs which does not happen in mitosis 2 o o Meiosis II o Prophase II  chromosomes condense o Metaphase II  chromosomes align at the metaphase plate o Anaphase II  sister chromatids move to opposite spindle poles o Telophase II  nuclear membrane reforms and cytokinesis follows Meiosis – 3 significant results (page 99) o Haploid nuclei from the starting diploid cell o Each chromosome (maternal or paternal) has an equal probability of ending up in a given daughter cell after Metaphase I o Chiasmata occur during meiosis I  crossing-over between maternal ad paternal chromatid pairs can generate genetic recombination o o In humans, egg formation begins in fetal ovaries and arrests during the prophase of meiosis I o Fetal ovaries contain about 500 000 primary oocytes arrested in the diplotene substage of meiosis I o If the egg released is fertilized, meiosis is completed 3 o Only one of the 3 cells produced by meiosis serves as the functional gamete or the ovum  division is assymetric o o o human sperm form continuously in the testes after puberty o spermatogonia are located near the exterior of seminiferous tubules in testis o once they divide to produce the primary spermatocytes, the subsequent stages of spermatogenesis (meiotic division and maturation of spermatids into sperm) occur successively closer to the middle of the tubule o mature sperm are released into the central lumen of the tubule for ejaculation Gene segregation in Meiosis o Two carriers of Cystic Fibrosis (CF+/-) have a child together. What is the probability that the child will have CF? o Let’s assume red and green chromosomes = mother’s 7s chromosome o Lets assume yellow and blue are dad’s chromosome 7s o If the chromosome 7s with the defective gene are blue and red what is the probability that the child will have CF? o o so there is a 25% chance of having a child with CF (CF-/-) Mendel’s laws o see table 4.4 – How the chromosome theory of inheritance explains Mendel’s Laws 4 o o Law of Segregation  alternative trait factors that come together in the offspring separate again when the offspring produces gametes o The law of independent assortment  differences for one trait are inherited independently of differences for another trait Chromosome theory of inheritance o Sutton and Boveri proposed the chromosome theory of inheritance  idea that genes are on chromosomes o Some evidence for this theory: o Nuclei of egg and sperm are equal in size and contain coloured bodies that are visible during cell division o The number of chromosomes is consistent from cell to cell within an organism but varies between organisms o The behavior of the chromosomes at meiosis correlates with the behavior of Mendel’s hypothetical particles o See page 102 Drosophila o If I cross 2 red eyed flies and find white eyed male flies (II-4, II-6, III-4, III-8 and III-10) o What should I conclude about the pattern of inheritance of white eyes in fruit flies? o o occurs only in males, and parents of affected individual are not affected  X linked recessive Example: Hemophilia (X-linked recessive) o 5 o only males are affected o parents of affected males are not affected, but the mother is a carrier (heterozygous) Example: Hypophosphatemia (X-linked dominant) o o males and females are affected o all daughters of affected males are affected o the sons of affected males are not affected, as long as mother is normal X-linked recessive trait o appears in more males than females since a female will have 2 X copies, which would mean they would need 2 defective copies in order to display the phenotype versus one defected X copy in males o mutation does not pass from father to son because the father gives the son the Y chromosome not an X chromosome o an affected male passes the X linked mutation to all his daughters  carriers  one half of the sons will have the defective X, and thus the trait o trait often skips a generation as it passes through grandfa
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