RNA molecules have many uses in the cell besides carrying the information needed to specify the order of amino acids during protein synthesis. Although we have encountered noncoding RNAs in other chapters (tRNAs, rRNAs, snoRNAs, for example), the sheer number of noncoding RNAs produced by cells has surprised scientists. One well understood use of noncoding RNAs occurs in RNA interference, where guide RNAs (miRNAs, siRNAs, piRNAs) base-pair with mRNAs. RNA interference can cause mRNAs to be either destroyed or translationally repressed. It can also cause specific genes to be packaged into heterochromatin suppressing their transcription. In bacteria and archaebacteria, RNA interference is used as an adaptive immune response to destroy viruses that infect them. A large family of large noncoding RNAs (lncRNAs) has recently been discovered. Although the function of most of these RNAs is unknown, some serve as RNA scaffolds to bring specific proteins and how is the final rate of transcription of a gene specified by the hundreds of proteins that assemble on its control regions? will we ever be able to predict this rate from inspection of the DnA sequences of control regions?
how does the collection of cisregulatory sequences embedded in a genome orchestrate the developmental program of a multicellular organism?
how much of the human genome sequence is functional, and why is the remainder retained?
which of the thousands of unstudied noncoding rnAs have functions in the cell, and what are these functions?
were introns present in early cells (and subsequently lost in some organisms), or did they arise at later times?
RNA molecules together to speed up needed reactions.
Which statements are true? Explain why or why not.
7–1 In terms of the way it interacts with DNA, the helix–loop–helix motif is more closely related to the leucine zipper motif than it is to the helix–turn–helix motif.
7–2 Once cells have differentiated to their final specialized forms, they never again alter expression of their genes.
7–3 CG islands are thought to have arisen during evolution because they were associated with portions of the genome that remained unmethylated in the germ line.
7–4 In most differentiated tissues, daughter cells retain a memory of gene expression patterns that were present in the parent cell through mechanisms that do not involve changes in the sequence of their genomic DNA.
Discuss the following problems.
7–5 A small portion of a two-dimensional display of proteins from human brain is shown in Figure Q7–1. These proteins were separated on the basis of size in one dimension and electrical charge (isoelectric point) in the other. Not all protein spots on such displays are products
Figure Q7–1 two-dimensional separation of proteins from the human brain (problem 7–5). the proteins were displayed using two-dimensional gel electrophoresis. Only a small portion of the protein spectrum is shown. (Courtesy of tim Myers and leigh Anderson, large Scale Biology Corporation.) of different genes; some represent modified forms of a protein that migrate to different positions. Pick out a couple of sets of spots that could represent proteins that differ by the number of phosphates they carry. Explain the basis for your selection.
7–6 Comparisons of the patterns of mRNA levels across different human cell types show that the level of expression of almost every active gene is different. The patterns of mRNA abundance are so characteristic of cell type that they can be used to determine the tissue of origin of cancer cells, even though the cells may have metastasized to different parts of the body. By definition, however, cancer cells are different from their noncancerous precursor cells. How do you suppose then that patterns of mRNA expression might be used to determine the tissue source of a human cancer?
7–7 What are the two fundamental components of a genetic switch?