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Which statement best describes the function of the enzyme DNA helicase?
DNA helicases are enzymes that separate the two DNA strands, and unwinds them as it progresses along the helix. It functions much like a zipper unwinding the DNA.
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Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear.
Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:
PrPC + PrPRes → PrPRes + PrPRes
The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.
This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:
Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:
Rabbit 1 and 2: injected with normal saline on days 1 and 2
The above trials serve as controls.
Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2
The above trials use unmodified brain matter.
Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2
The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.
Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2
The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.
Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2
The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.
In the material used with Rabbits 5 and 6, irradiation was used to destroy DNA. In functioning, normal cells, what types of genes typically code for DNA repair proteins?
I. Tumor suppresor genes
II. Proto-onco genes
III. Pro-apoptotic genes
Tumor suppresor genes, like p53 and Rb, usually code for DNA repair enzymes. Proto-oncogenes typically code for cell growth factors or receptors, and pro-apoptotic proteins would not lead to DNA repair, but would prevent tumor development via cell death pathways.
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A culture of human tissue is being grown in a lab to study mitosis. A solution containing radioactively labelled cytosines was added to the culture in the middle of prophase, and then growth was halted at the end of telophase. Where would the scientists see radioactively labelled DNA?
DNA is replicated in S phase. Prophase is a part of mitosis, or M phase. Since all of the DNA that would be present at the end of telophase had already been synthesized in S phase, none of the radioactively labelled cytosines would be incorporated into the DNA of any cells in the culture.
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Which answer choice correctly matches the molecule with its function in DNA replication?
Topoisomerase functions to untangle the supercoiling of DNA, which is when DNA overwinds into itself. This mechanism facilitates the unwinding action of helicase during replication. Single-stranded binding proteins bind to the two unzipped DNA strands to prevent them from prematurely coming back together into a whole molecule; otherwise replication would be interrupted.
The other proteins discussed serve the following functions.
Polymerase—adds nucleotides to new strands
Primase—adds RNA primers prior to replication
DNase—cleaves and degrades DNA molecules
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Which of the following is the first to act during DNA replication?
Helicase is the first component of the DNA replication machinery to act during replication. It works by "unzipping" the double-stranded DNA so that replication can subsequently occur. Following the work of helicase, primase creates a primer to which the DNA polymerase will subsequently add deoxynucleotides and elongate the strand. DNA ligase acts at the end of replication by joining together the Okazaki fragments of the lagging strand.
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Several enzymes are required for DNA replication. What is the class of enzymes that is required for unwinding the DNA at the replication fork?
DNA helicases use ATP to break the hydrogen bonds that separate complementary strands of DNA. During DNA replication, DNA helicases move along the DNA backbone with the replication fork and are responsble for unwinding the DNA at the fork.
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The Meselson-Stahl experiment provided the necessary evidence to discover the mechanism by which DNA replicates. They accomplished this discovery by first culturing DNA with the heavy 15N nitrogen isotope. They then allowed the "heavy" DNA to replicate with DNA grown in normal 14N nitrogen. The density of each generation of replicated DNA was recorded by marking its position in a test tube after centrifugation. The position of each generation was then compared to the positions of pure 15N DNA and pure 14N DNA.
Suppose that the first generation after replication revealed two bands after being centrifuged: one at the pure 14N mark, and one at the pure 15N mark. Which method of replication would this observation support?
Conservative replication proposes that both strands of DNA act as the template, but do not separate during replication. If the heavy strands were to stay together, we would expect to see a "heavy" set of DNA at the 15N mark as well as a "normal" set of DNA at the 14N mark.
Both semiconservative and dispersive replication would predict a singular band of DNA in between the two marks.
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DNA replication is much more accurate than RNA transcription. In replication, only one base in every ten billion, on average, is inaccurately placed.
What is the primary reason that transcription results in more errors than DNA replication?
In addition to creating a new DNA strand, DNA polymerase can function as an exonuclease. DNA polymerase I has the ability to remove mismatched nucleotides from the new strand and correct them. As a result, DNA replication is very accurate, because DNA polymerase has a proofreading mechanism.
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Compared to RNA polymerase, DNA polymerase has a much lower error rate for nucleotide incorporation. What structural difference between the two polymerases accounts for this?
RNA polymerase does not contain a proof reading domain, making it much more error prone than DNA polymerase. This domain in DNA polymerase prevents incorrect nucleotide insertion, reducing the errors made in DNA replication.
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Which base pair requires the most amount of energy to break?
The guanine and cytosine base pairing forms 3 hydrogen bonds. Both adenine and thymine form only 2 hydrogen bonds. Thus the G-C base pair has the strongest interactions, and requires the most amount of energy to break.
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Which base pair requires the least amount of energy to break?
The adenine and thymine base pairing forms 2 hydrogen bonds. Both cytosine and guanine form three hydrogen bonds. Thus the A-T base pair has the weakest interaction, and requires the least amount of energy to break.
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Which of the following choices represents the structure of DNA from least organized to most organized?
Deoxyribose is the sugar in the backbone of DNA. Next, a nitrogenous base is attached to the deoxyribose, which together, is called a nucleoside. A nucleoside with one or more phosphate groups attached is called a nucleotide. The double stranded DNA coils around histone proteins; this complex is called a nucleosome. Lastly the DNA gets further packaged and condensed into chromatin.
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The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.
The dorsal column pathway is a fast and advanced signaling system. Some researchers believe that this pathway evolved not to long ago. If this transformation required the change in the DNA, how might this have occurred?
I. Point mutation
II. Frame shift
III. Nonsense mutation
All of these choices may cause a change in the DNA. A point mutation involves a single base pair change in the DNA or the RNA. A frameshift mutation involves a deletion or insertion, in which any number of base pairs are inserted or deleted, which causes a shift in the reading frame of the DNA (not RNA). A nonsense mutation involves the change in DNA that results in a premature stop codon in the mRNA.
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