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Example Question #61 : Dna, Rna, And Proteins
The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.
The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.
Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.
Histones are proteins that can interact with some sequences of DNA to help it coil into a more manageable arrangement within the nucleus. If the DNA-histone interaction is mediated primarily by intermolecular bonds, which of the following is likely true of histones?
They are basic and depend on covalent interactions
They are neutral and depend on van der Waals interactions
They are basic and depend on dipole interactions
They are acidic and depend on dipole interactions
They are acidic and depend on covalent interactions
They are basic and depend on dipole interactions
DNA is acidic, and thus has a generally negative charge in aqueous conditions (consider what this means for the electrophoresis pattern of DNA in an agarose gel). Because the interaction has to be tight to coil DNA effectively, it must be a dipole interaction. Dipole interactions are relatively strong intermolecular forces. Covalent forces, however, are intramolecular and much more permanent than dipole interactions.
Acidic DNA has a negativecharge, which will be drawn to a basic histone with a positive charge.
Example Question #1 : Regulation Mechanisms And Epigenetics
A competitive inhibitor for RNA polymerase III would have the most significant direct effect on which of the following?
None of these
Replication
Translation
Transcription
Replication, transcription and translation
Translation
RNA polymerase III is used to transcribe tRNA. tRNA is used to carry amino acids to ribosomes where they can be used for translation.
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