ssii_midterm_ii_version_b_2008_key

ssii_midterm_ii_version_b_2008_key - 1) Eukaryotic DNA is...

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1) Eukaryotic DNA is packaged in a complex series of wrapping and coiling that involves both DNA and protein and results in a substance called chromatin. We discussed four levels of DNA packaging that were distinguished by the increasing size of the diameter of the resulting fibers. These four different fiber types are listed below. Describe briefly what occurs at each level of packaging, the names of the structures created, and the names of the proteins involved in the folding process. a) 10nM Fiber: (4 Points) The 10nM fiber consists of the DNA double helix wrapped twice around an octamer of histones consisting of two subunits each of H2A, H2B, H3, and H4. This histone:DNA complex is called a nucleosome and nucleosomes are strung together in a “beads on a string” like structure. b) 30nM Fiber: (4 Points) The 30nM fiber results when histone 1 (H1) associates with the linker DNA that links one nucleosome to the next. The binding of H1 to the linker DNA leads to a coiling of the nucleosomes into a solenoid structure. c) 300nM Fiber: (4 Points) The 300nM fiber results when the solenoids form large looping structures that begin and end their attachment to a core of nonhistone proteins called the scaffold. d) 700nM Fiber: (4 Points) The 700nM fiber (which is about the diameter of a condensed chromatid at mitosis) results from the coiling of the scaffold into a spiral shape resulting in a type of supercoiling of the genome of more complex eukaryotes. e) What is the primary difference between heterochromatic regions of a chromosome and euchromatic regions? What is the difference between facultative Heterochromatin and Constitutive Heterochromatin? (6 Points) Heterochromatic regions of a chromosome are more tightly packed than euchromatic regions. Facultative heterochromatin is chromatin that has been reversibly modified to repress gene transcription in a given area of a chromosome. Constitutive heterochromatin is chromatin that is always transcriptional silent and does not contain genes but instead is usually repetitive DNA sequences. (give 3 points for the first part and 3 points for the second part)
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2) You are studying translation in Drosophila (eukaryote) and so you create a series of temperature sensitive mutants that show defects in protein synthesis. a) In the first mutant strain (mut-1) generated you find that, when the mutant is switched from a permissive temperature to a restrictive temperature, the ribosomes are able to generate polypeptide chains, but the newly formed polypeptides always remain bound to the ribosome. Provide the mostly likely explanation for this observation. (6 Points) The most likely explanation for this observation is that mut-1 has a mutation in one the release factors (rf-1, rf-2 or rf-3) that prevents the ribosome from releasing the polypeptide and disassociating back into the large and small subunits. b)
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ssii_midterm_ii_version_b_2008_key - 1) Eukaryotic DNA is...

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