BSCI222 – Lecture 14 (10/22/13, missed L.13)- Last time: chromosome variation- Variation in ploidy = variation in the number of complete chromosome sets.o Haploid (1N), diploid (2N), etc.o Autopolyploids versus allopolyploido Autopolyploidy happens by non-disjunction: DNA replicates, chromosome separate, but the cell never divides. Happened in one cell, thus “auto”. If 4N happens in a population of individuals, just a few of them, then their gametes are 2N after meiosis -> unite with a normal 1N gamete, making a triploid offspring. Triploids are usually sterile out in the wild; used a lot infish, for aquaculture. 3 copies of chromosomes makes it very hard to segregate that evenly into gametes. For every single chromosome, has its own random pattern of segregation, making the gametes highly unbalanced. (Could have 1N of chromosome A, 2N of chromosome B, 0 for C, 0 for D, 2N for E, etc.) The organism with such an unbalanced set of chromosomes will not usually survive.o Plants have undergone a lot of speciation, through both of these mechanisms.o Allopolyploids are made by hybridization: 2 species’ gametes fuse. Very difficult because of all the differences in chromosome structure for itall to properly pair in meiosis and properly separate. Typically make nonviable gametes. BUT, if nondisjunction happens, it leads to a doubling of all chromosomes, producing an allotetraploid, and then the chromosome pairing and separating into the gametes is balanced. During the history of domestication of modern wheat, had 2 cycles:- Einkorn wheat (AA, 2n = 14) and wild grass (BB, 2n = 14). Same number of chromosomes, slightly different structures. Mitotic nondisjunction -> balanced gametes. The resulting Emmer wheat (AABB, 4n = 28) joined with different wild grass (DD, 2N = 14), making a hybrid (ABD, 3N = 21), which (via mitotic nondisjunction) made bread wheat (AABBDD, 6N = 42).- The people didn’t know this was happening, was random. - Chapter 22: Developmental Genetics and Immunogenetics- Drosophila life cycle.o Drosophila egg is single-cell. After fertilization, have a single diploid nucleus in this cell and a huge mass of cytoplasm.o First thing that happens is multiple rounds of nuclear division and DNA replication and chromosomal segregation, but no cleavage of the cytoplasm. Thus multinucleate cell, called a syncytium (about 6000 nuclei). Molecules can diffuse through this single cell for the early stages of development. Have waves of division across the syncytium.o After all the nuclear division, the nuclei migrate out to the edges of the cell, making a skin of nuclei around the cytoplasm. The pole nuclei, at the end, will become the germ line.o Later, the actual cells form, about 100 cells from one end of the embryo to the other. o All the molecules in the egg and the DNA sequences encode all the information about what is head what is tail, what is up what is down.o Right at about 10 hours, the number and orientation of the body segments are established. Gastrulation. Have head, thoracic segments, and abdominal segments.Still a larvae worm. o The information and the cells that form the adult structures are segregated in the larvae. First have to specify the larvae, then later have to specify the adult. The early processes that pattern the embryo, also pattern the imaginal discs, which are segregated off and hold the patterns for the adult body. Small discs of tissue, which will ultimately grow and become that structure. (Wing, haltere, leg, etc.)o Overall, 3 main stages: 1. Establishment of the main body axes, 2. Determination of number and polarity of body segments, 3. Establishment of identity of each segment.o The egg can’t know up from down, but the mother left instruction in the egg: left an asymmetric distribution of pipe sulfotransferase inside the egg shell (on the ventral sides) but outside the plasma membrane. This works with a number of proteases out there, to cleave and produce something called a Spaetzle fragment. This fragment binds to the Toll receptor, which induces a cascade in the cell which then degrades the cactus protein, thereby releasing the dorsal protein (throughout the cytoplasm, migrating into the nuclei in only the ventral regions), which then enters the nuclei and acts as a transcription factor, altering the transcription of the nucleus. How the mother, when making the egg, distributes that pipe sulfotransferase, is what determines top from bottom.  Called dorsal because if it’s mutated, the dorsal protein will work oppositely, through the dorsal region.o Front to back: also instructions from the mom. In the ovary, when the egg is beingformed, bicoid mRNA is being deposited in the anterior end of the embryo, anchored by proteins and not translated. Once fertilization happens, that mRNA isreleased and starts being translated, which makes bicoid protein. The bicoid protein then diffuses throughout the embryo (not restricted the way the protein was, diffused throughout entire syncytium, but concentrated at the anterior end). At the posterior end of the egg, mom left nanos mRNA, same way; when translated, the nanos protein becomes concentrated at the posterior end of the egg and inhibits formation of anterior structures. Both proteins have gradients across the embryo, in opposite directions. Essentially have 3 regions: high in bicoid, highin nanos, and low in both. The hunchback gene, which is upregulated by bicoid and downregulated by nanos, has expression high when there is bicoid and lowwhen it encounters nanos. Individual cells across this axis, with their concentration factors of both of the proteins, causes a hunchback gradient, which makes more regions: high B high H, high H low B, low H low N, low H high N. The promoters of genes are like a digital calculator; have a series of binding sites that bind these, some acting as transcriptional activators and some as repressors. Within some cell, in high region of bicoid and hunchback are activators; tendencyto turn on this gene in this region. If in a region of high bicoid and high hunchback, but also high Giant (repressor), gene is not expressed here. Kruppel is established via nanos much like hunchback is from bicoid. Kruppel is also a repressor. Getting a digital readout of the program in the enhancer.  Even-skipped (eve) expression is limited by Giant and Kruppel, turning it off on either side, resulting in a very narrow band of even-skipped expression in the middle of