4–2 The four core histones are relatively small proteins 4–5 Gene duplication and divergence is thought to with a very high proportion of positively charged amino have played a critical role in the evolution of increased bioacids; the positive charge helps the histones bind tightly to logical complexity. DNA, regardless of its nucleotide sequence.
Discuss the following problems. 4–3 Nucleosomes bind DNA so tightly that they cannot 4–6 DNA isolated from the bacterial virus M13 con-move from the positions where they are first assembled.
tains 25% A, 33% T, 22% C, and 20% G. Do these results 4–4 In a comparison between the DNAs of related strike you as peculiar? Why or why not? How might you organisms such as humans and mice, identifying the con-explain these values?
Figure Q4–1 Three nucleotides from the interior of a single strand of DNA (Problem 4–7). Arrows O at the ends of the DNA strand indicate that the structure continues in both directions.
4–7 A segment of DNA from the interior of a single strand is shown in O Figure Q4–1. What is the polarity of this –O P O DNA from top to bottom? O on a molar basis. What are the mole percents of A, G, and T?
in the human lineage (Figure Q4–2). Draw the intermediate chromosome that resulted from the first inversion and explicitly indicate the segments O included in each inversion.
(Problem 4–9). Differently colored blocks indicate segments of the chromosomes that are homologous in DNA sequence.
4–10 Assuming that the 30-nm chromatin fiber contains about 20 nucleosomes (200 bp/nucleosome) per 50 nm of length, calculate the degree of compaction of DNA associated with this level of chromatin structure. What fraction of the 10,000-fold condensation that occurs at mitosis does this level of DNA packing represent?
4–11 In contrast to histone acetylation, which always correlates with gene activation, histone methylation can lead to either transcriptional activation or repression. How do you suppose that the same modification—methylation—can mediate different biological outcomes?
4–12 Why is a chromosome with two centromeres (a dicentric chromosome) unstable? Would a backup centromere not be a good thing for a chromosome, giving it two chances to form a kinetochore and attach to microtubules during mitosis? Would that not help to ensure that the chromosome did not get left behind at mitosis?
4–13 Look at the two yeast colonies in Figure Q4–3. Each of these colonies contains about 100,000 cells descended from a single yeast cell, originally somewhere in the middle of the clump. A white colony arises when the Ade2 gene is expressed from its normal chromosomal location. When the Ade2 gene is moved to a location near a telomere, it is packed into heterochromatin and inactivated in most cells, giving rise to colonies that are mostly red. In these largely red colonies, white sectors fan out from the middle of the colony. In both the red and white sectors, the Ade2 white colony of yeast cells red colony of yeast cells with white sectors
Figure Q4–3 Position effect on expression of the yeast Ade2 gene (Problem 4–13). The Ade2 gene codes for one of the enzymes of adenosine biosynthesis, and the absence of the Ade2 gene product leads to the accumulation of a red pigment. Therefore a colony of cells that express Ade2 is white, and one composed of cells in which the Ade2 gene is not expressed is red.
gene is still located near telomeres. Explain why white sectors have formed near the rim of the red colony. Based on the patterns observed, what can you conclude about the propagation of the transcriptional state of the Ade2 gene from mother to daughter cells in this experiment?