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Viruses, Transposable Elements, and Cancer Practice Test (page 4)

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By — McGraw-Hill Professional
Updated on Aug 23, 2011

Transposable Elements

  1. (a)  Most of the DNA in bacteria, unlike the DNA in eukaryotic cells, is coding information. There is relatively little DNA that is not serving some function. Thus, the movement of most transposons to a new location would inactivate one or more vital genes, causing cell death or weakening the cell so that it cannot compete with normal cells.     (b)  Plasmids rarely are essential to their host cells, and therefore could tolerate the integration of transposable elements without interfering with vital gene functions.
  2. The transposable element must first be copied into RNA. This RNA is then copied by reverse transcriptase into cDNA. This double-stranded cDNA is then capable of integrating into the genome at a different site.
  3. Transposition may result in several types of insertional and recombinational mutation events: insertion in a coding region of a protooncogene, resulting in loss of function; insertion into the promoter of a protooncogene, resulting in loss of function; insertion near the controlling regions of a gene and acting as a fortuitous promoter, resulting in altered gene regulation; various types of recombination events that would result in translocations or deletions, upon "jumping out" a transposon may take nearby gene sequences with it, creating a deletion.

Eukaryotic Viruses

  1. (1)  Minus-strand RNA viruses transport into the cell a replicase enzyme that synthesizes mRNA from the (–) strand template.     (2)  Plus-strand RNA viruses (other than retroviruses) use their infective strand as a template for synthesizing mRNA using host RNA polymerase.     (3)  Retroviruses use their (+) strand as a template for DNA synthesis, which is then transcribed into mRNA.     (4)  Double-stranded RNA viruses bring a replicase into the host cell that copies double-stranded RNA and synthesizes a (+) strand that functions as mRNA.
  2. (a)  They replicate from a DNA intermediate.     (b)  RNA-dependent DNA polymerase (reverse transcriptase). Enzyme functions: (1) converting the single-stranded viral RNA to a DNA-RNA hybrid, (2) digesting RNA from a DNA-RNA hybrid, and (3) copying a primed single-stranded DNA to form a double-stranded DNA.     (c)  The double-stranded DNA that is formed by reverse transcription from retroviral RNA.     (d)  The DNA-RNA hybrid is made in the cytoplasm. The hybrid is converted to double-stranded DNA and becomes inserted into a host chromosome. Messenger RNA is made from the proviral DNA in the nucleus by host RNA polymerase.     (e)  Their integrated proviral DNA is terminated at each end by a long terminal repeat and a short inverted repeat (like a composite transposon), which in turn is flanked by a short, direct repeat (like a target sequence).
  3. (1) Viral proteins usually enter the infected cell along with the viral genome.     (2)  The RNA of some viruses is converted to DNA.     (3)  The mRNA of viruses is processed just like the cellular mRNA of their eukaryotic hosts.     (4)  Polyproteins are produced by some viruses.

Cancer

  1. An established cell line has already experienced one or more early steps (e.g., immortalization) in the induction of neoplastic transformation before exposure to the effects of the oncogene.
  2. These viruses might become integrated into the host DNA near a cellular protooncogene and activate it, via a viral enhancer sequence, to become an oncogene.
  3. (a)  A retrovirus becomes integrated as a provirus adjacent to a cellular protooncogene. The provirus and the adjacent protooncogene are transcribed into a single transcript. The RNA transcript is processed to remove introns and becomes packaged into a viral capsid. The infective virus is released from the host cell and infects another cell.     (b)  Each of the steps in part (a) involves known genetic mechanisms. There is no known mechanism by which introns from viruses can be inserted into cellular protooncogenes.
  4. (a) (b)  None of the progeny is expected to develop breast cancer because non-infected females have nursed them.     (c)  None of the progeny is expected to develop breast cancer because, in isolation, the infected females could never produce a litter and subsequently enter a lactation period, a prerequisite for expression of the milk factor.     (d)  50% females × 90%of females develop breast cancer = approximately 45%.
  5. (a)  This disease is similar to the Rh blood group system incompatibility between a human mother and her baby. In this case, antibodies are transferred to the offspring through the milk rather than across the placenta.     (b)  The particular stallion that is used has an immediate effect on the character. This is not true in the acquisition of breast cancer in mice. The incompatibility disease in horses cannot be transmitted to later generations, so there is no evidence of a specifically self-duplicating particle like the infective agent that causes cancer in mice.
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