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11/6/2016 Ch. 7: The Cellular Basis of Inheritance Fig. 12.15, p. 205 Slide 19 Sexual reproduction • A diploid organism produces haploid sex cells (gametes) • The union of two gametes from opposite sexes (fertilization) makes a new diploid individual • A special type of nuclear division called meiosis yields variations in offspring, which provides a species survival advantage 1 11/6/2016 • An overview of mitosis vs. meiosis • Mitosis produces – 2 identical diploid daughter cells for development, growth and repair (in multicellular organisms such as animals) Meiosis produces – 4 non-identical haploid gametes for sexual reproduction Meiosis • • Occurs in testes and ovaries Chromosome number is reduced by half • After the chromosome duplication that occurred during interphase, there are 2 nuclear divisions called meiosis I and meiosis II • A mixing of gene combinations occurs via crossing over and independent assortment (more on this soon) – Diploid cells produce haploid gametes (sperm cells and egg cells) – Each of these has its own prophase, metaphase, anaphase, and telophase – Remember, genes code for proteins that can effect various aspects of an individual’s biology (e.g. physical traits, physiological traits, metabolic traits, etc.) 2 11/6/2016 Synapsis • A duplicated chromosome consists of 2 sister chromatids • During metaphase I, synapsis occurs: a tight pairing of (physical connection between) homologous duplicated chromosomes – The 4 sister chromatids together are called a tetrad – Genes on homologous chromosomes are precisely aligned with each other… Tetrad Synapsis Sister chromatids Crossing over • Occurs during synapsis • Non-sister homologous chromatids exchange a few genes • May occur at several points for each pair • Significance: crossing over ultimately results in recombinant chromosomes and gametes with different gene combinations 3 11/6/2016 Independent assortment of chromosomes • Refers to the random orientation of paternal and maternal chromosomes at metaphase I • Further increases the number of gene combinations in gametes Genetic variability of offspring • There are 2 sources of variability from meiosis: – 1. Independent assortment • Consider the possible outcomes… – There are 2 possible arrangements (maternal/paternal) for each pair of chromosomes at metaphase I, and there are 23 pairs in humans » So there are 223 options for chromosome orientation at metaphase I » I.e., there are 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 8,333,608 possible orientations of your maternal and paternal chromosomes at metaphase I » Which means, once fertilization occurs, there are 223 x 223 = 70 trillion diploid combinations for the new individual! – 2. Crossovers • There are an average of 2-3 crossovers per chromosome, which increases the possible diploid combinations even more…140 to 200 trillion! 4 11/6/2016 Comparison of reproductive strategies: asexual vs. sexual • Asexual: – Two parents – Offspring are a random genetic mixture of the parents (and, by extension, the grandparents) – Each parent usually has different alleles (versions of the same gene) – Offspring inherit new combinations of alleles, resulting in the appearance of new traits • New traits mean changes in individual behavior, appearance, physiology, etc. A review of mitosis vs. meiosis – Single “parent” – Offspring are genetically identical to the parent and each other – Offspring inherit the same alleles (versions of the same gene) that the parent has, so there’s less chance for the appearance of new traits • Sexual: 5 11/6/2016 Errors in meiosis • Occur when chromosomes behave abnormally during meiosis • There are 2 types of errors: – Abnormal chromosome number – Chromosome structural rearrangements • A karyogram (= an organized, visual display/chart of a karyotype) provides information about chromosome number and structure Abnormalities in chromosome number • Include the duplication or loss of entire (whole) chromosomes • Are caused by nondisjunction: – Failure of chromosomes to separate during either meiosis I or meiosis II • The risk of nondisjunction increases with age of the parents (see the graph on the next page) 6 11/6/2016 Chromosome number terminology • Euploid: has the normal number of chromosomes • Aneuploid: has an abnormal normal chromosome number – Polyploid: has more than the correct number of complete chromosome sets (e.g. more than 2 sets for a diploid species) • Rare in animals • Common in plants – Monosomy: loss of one chromosome – fail to develop to birth – Trisomy: gain of one extra chromosome • Most fail to develop to birth, however… • Some trisomies can result in offspring that survive for several weeks to many years • Down syndrome (trisomy 21) is a wellknown example More on chromosome number: sex chromosomes in humans • Two X sex chromosomes (XX) results in a female; XY is a male • Males operate with only one X chromosome • So do females… 7 11/6/2016 X inactivation (females only) • • • Early in female mammal development, one of the two X chromosomes in each cell (either the maternally derived one or the paternally derived one) is randomly inactivated (condensed into a nonfunctional structure called a Barr body) Each cell expresses the genes on the remaining X chromosome E.g. if you see a “tortoiseshell” cat, you know it’s female! Chromosome structural rearrangements • Include: – – – – Deletions Inversions Translocations Partial duplications • Are usually caused by DNA breakage and/or errors during DNA replication and repair • Effects vary… 8 11/6/2016 Deletion • Is the loss of a chromosomal segment • E.g. cri-du-chat: – Deletion of most of the small arm of chromosome 5 – Causes nervous system abnormalities and characteristic physical features Inversion • Is the detachment, 180° rotation, and reinsertion of a chromosomal segment • Has little to no effect if the gene sequence is not altered • Is associated with primate evolution: chromosome 18 inversion – It’s one of the main differences between humans and chimpanzees, our closest genetic relatives 9 11/6/2016 Translocation • Is the detachment of a chromosomal segment followed by its reattachment to a different chromosome • May or may not have an effect (it depends on how the gene position changes with respect to its regulatory sequences) • Specific translocations have been associated with several cancers and schizophrenia Partial duplication • Is the duplication of a chromosomal segment • Subsequent mutations to the duplicated segment allows the original segment to work normally while the new one may code for a new protein, for example… – Antarctic icefish • The duplication of a digestive gene appears to have mutated to code for an antifreeze glycoprotein, allowing these fish to live in water below 0°C 10