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Lecture 2

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Biological Sciences
Ian Brown

Lecture 2: Interactions of the Nucleus and Cytoplasm Two Major Cellular Compartments in eukaryotic cells: ▯ 1. Nucleus: what activities are occurring in the nucleus? ▯ ▯ - We have the genetic information, the genes encoded in DNA, associated with ▯ ▯ chromosomal proteins ▯ ▯ - Activities: DNA replication, Gene expression: certain genes are turned on and ▯ ▯ others are turned off, certain are activated for RNA synthesis (different RNA made: ▯ ▯ rRNA, mRNA, tRNA), RNA slicing (introns are sliced and exons are linked together), ▯ ▯ addition of poly-A tail (multiple adenosine residues in the nucleus giving poly-A for ▯ ▯ message) which is needed for the process message to exit into the cytoplasm and ▯ ▯ also the half-life of the message, how long it lasts for before it breaks down ▯ 2. Cytoplasm: what activities are occurring in the cytoplasm? ▯ ▯ - Activities: site of protein synthesis (mRNA exits into the cytoplasm and are ▯ ▯ translated into cell specific proteins, which requires ribosomes, mRNA, tRNA, these ▯ ▯ proteins can be structural proteins that are need for cellular processes, and enzymes ▯ ▯ that control metabolic pathways), site of certain cellular organelles (e.g. ▯ ▯ ▯ ▯ mitochondria, chloroplast, Golgi apparatus, ribosomes), gives structure and shape of ▯ ▯ a cell, makes and organize proteins for structural shape Early embryogenesis: ▯ - Begins with the zygote (fertilized egg) which is a large cell, single-cell made up of nucleus ▯ and cytoplasmic mass, first undergoes division to create a 2-cell, than 4, than 8, than 16. ▯ When this reaches the 16-cell stage, it is called a morula. With further division, we move ▯ from the morula to the blastula (a hollow ball of cell) with a center called blastocoel (filled ▯ with liquid. Than the process of gastrulation which is caused by an invagination which forms ▯ a double layered Gastrula (inner layer endoderm and outer layer ectoderm) ▯ - Blastomere: a cell resulting from the cleavage of a fertilized egg during early embryonic ▯ development before blastula formation ▯ - Morula: spherical, solid (i.e. no blastocoel) embryonic mass of blastomeres formed before ▯ the blastula and resulting from cleavage of the fertilized ovum Two key questions in Developmental Biology: ▯ 1. Does the nucleus affect the cytoplasm? ▯ ▯ - Yes, it determines what proteins are gonna be made in the cytoplasm ▯ 2. Does the cytoplasm affect the nucleus? ▯ ▯ - Yes, because different factors in the cytoplasm (cytoplasmic factors) which are ▯ ▯ made in the cytoplasm and move into the nucleus and determine which genes are ▯ ▯ gonna be expressed in the nucleus Two Basic Laws of Molecular Biology of Development: ▯ 1. All cells of organism contain the same set of genes (nuclear equivalence) ▯ 2. Different cell types arise in development by a different set of genes turned on (and off) in ▯ different cell types Testing the 1st Law: Nuclear Equivalence: ▯ - Early idea: theory of unequal nuclear division: different genes in the zygote are parceled ▯ out to different blastomeres ▯ - This theory has been proven false ▯ - Three experiments showing nuclear equivalence ▯ Exp. 1 Testing Nuclear Equivalence: ▯ ▯ - By Hans Spemann ▯ ▯ - Model organism: fertilized egg of a newt ▯ ▯ - Experiment: uses delayed nucleation to study nuclear equivalence. He tied a hair ▯ ▯ loop around the large fertilized egg of the newt, divided the fertilized egg into two, in ▯ ▯ a way that one contained the nucleus and the other no nucleus. The one with the ▯ ▯ nucleus went on to divide. He tied in a way that there was still a little bridge between ▯ ▯ the two halves, at about the 16-cell stage a nucleus from the nucleated blob provided ▯ ▯ a nucleus to the other side. When this happened, that blob began to divide and ▯ ▯ turned into an embryo. ▯ ▯ - Result: delayed nucleation causes twinning ▯ ▯ - Conclusion: A nucleus from a 16-celled embryo contains all the genes necessary to ▯ ▯ make a complete embryo ▯ Exp. 2a Testing Nuclear Equivalence by nuclear transplantation in frog: ▯ ▯ - By Briggs and King ▯ ▯ - Model organism: Rana pipiens (frog) ▯ ▯ - Experiment: 1) enucleated activated egg: takes an egg that is on the way to ▯ ▯ development and pokes out the nucleus 2) isolate nucleus from blastula stage cell ▯ ▯ and transfer into enucleated egg ▯ ▯ - Result: 75% of the tadpoles reached metamorphosis ▯ ▯ - Conclusion: At this later stage of embryonic developed, the blastula stage, all of the ▯ ▯ nuclei have all the genes necessary to make a complete tadpole ▯ Exp. 2b Test Nuclear Equivalence by nuclear transplantation in toad: ▯ ▯ - By Gurdon ▯ ▯ - Model organism: Xenopus laevis (toad) ▯ ▯ - Experiment: 1) destroy egg nucleus by UV radiation 2) transplant nucleus of tadpole ▯ ▯ intestinal cell (a differentiated cell) ▯ ▯ - Result: Some of the resultant eggs developed into tadpoles ▯ ▯ - Conclusion: Showed that a differentiated cell nucleus contains all of the genes to ▯ ▯ make a complete toad ▯ What is the proof that nuclei in embryo are derived from the transplanted nucleus? ▯ ▯ - Design an experiment to ‘tag’ transplanted nuclei ▯ ▯ - Toad renal tumour cells are triploid (39 chromosomes) ▯ ▯ - Toad egg is diploid (26 chromosomes) ▯ ▯ - Experiment: Transplant triploid nucleus from renal tumour cell into enucleated egg ▯ ▯ - Result: Tadpole formed and every body cell is triploid in 10 chromosome number ▯ ▯ Important observation from nuclear transplant experiments: ▯ ▯ - Percentage of transplants showing normal development declined as developmental ▯ ▯ stage of the donor nuclei increased. Why? ▯ ▯ - This was due to the fact that the nucleus becomes less able to be re-programmed, ▯ ▯ not due to loss of gene, early nuclei is more amendable to a rapid cell division ▯ ▯ environment ▯ Nuclear Potency varies among species: ▯ ▯ - Developmental restrictions begin at different stages in different species ▯ ▯ - Nuclei are NO LONGER totipotent after: ▯ ▯ ▯ - 2-cell stage: mice ▯ ▯ ▯ - 16-cell stage: sheep ▯ ▯ ▯ - 64-cell stage: cattle ▯ Cellular Determination: ▯ ▯ - Cell determination: a gradual process in which a cell’s potency is progressively ▯ ▯ restricted during development ▯ ▯ - Cell potency: ability of a cell to differentiate into different cell types ▯ ▯ - Totipoten“ tt:otal potency”; cell is able to differentiate into any cell type. Only zygote ▯ ▯ and cells from early cleavage-stage embryos are totipotent ▯ ▯ - Pluripotent: cell is able to differentiate into many cell types. Later stages of ▯ ▯ embryonic development (blastocyst stage) ▯ ▯ - Multipotent: cell is able to differentiate into a limited in number of cell types. E.g. ▯ ▯ adult stem cells ▯ ▯ - Unipotent: cell is able to differentiate into one cell type (but may not have begun ▯ ▯ differentiation yet - it is a progenitor cell found in adults) E.g. spermatogonia ▯ ▯ - Terminall
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