B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 90
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Explain why white sec-Figure Q4–2 Chromosometors have formed near the rim of the red colony. Based on3 in orangutans and humansthe patterns observed, what can you conclude about the(Problem 4–9). Differently coloredpropagation of the transcriptional state of the Ade2 geneblocks indicate segments of thechromosomesthat arehomologousfrom mother to daughter cells in this experiment?Problemsp4.03/4.03/Q4.1in DNA sequence.two inversionsorangutanwhite colony ofyeast cellsOhuman4–10 Assuming that the 30-nm chromatin fiber contains about 20 nucleosomes (200 bp/nucleosome) per 50nm of length,calculate the degree of compaction of DNAProblemsp4.06/4.05/Q4.2associated with this level of chromatin structure.
Whatfraction of the 10,000-fold condensation that occurs atmitosis does this level of DNA packing represent?4–11 In contrast to histone acetylation, which alwayscorrelates with gene activation, histone methylation canlead to either transcriptional activation or repression. Howdo you suppose that the same modification—methylation—can mediate different biological outcomes?4–12 Why is a chromosome with two centromeres (adicentric chromosome) unstable? Would a backup centromere not be a good thing for a chromosome, giving ittwo chances to form a kinetochore and attach to microtubules during mitosis? Would that not help to ensure thatthe chromosome did not get left behind at mitosis?4–13 Look at the two yeast colonies in Figure Q4–3.
Eachof these colonies contains about 100,000 cells descendedfrom a single yeast cell, originally somewhere in the middle of the clump. A white colony arises when the Ade2 geneis expressed from its normal chromosomal location. Whenthe Ade2 gene is moved to a location near a telomere, itis packed into heterochromatin and inactivated in mostcells, giving rise to colonies that are mostly red. In theselargely red colonies, white sectors fan out from the middleof the colony.
In both the red and white sectors, the Ade24–14 Mobile pieces of DNA—transposable elements—that insert themselves into chromosomes and accumulateduring evolution make up more than 40% of the humangenome. Transposable elements of four types—long interspersed nuclear elements (LINEs), short interspersednuclear elements (SINEs), long terminal repeat (LTR)retrotransposons, and DNA transposons—are insertedmore-or-less randomly throughout the human genome.These elements are conspicuously rare at the four homeobox gene clusters, HoxA, HoxB, HoxC, and HoxD, as illustrated for HoxD in Figure Q4–4, along with an equivalentregion of chromosome 22, which lacks a Hox cluster.
EachHox cluster is about 100 kb in length and contains 9 to 11genes, whose differential expression along the anteroposterior axis of the developing embryo establishes the basicbody plan for humans (and for other animals). Why do yousuppose that transposable elements are so rare in the Hoxclusters?chromosome 22chromosome 2100 kbHoxD clusterFigure Q4–4 Transposable elements and genes in 1-Mb regions ofchromosomes 2 and 22 (Problem 4–14). Blue lines that project upwardindicate exons of known genes. Red lines that project downwardindicate transposable elements; they are so numerous (constituting morethan 40% of the human genome) that they merge into nearly a solidblock outside the Hox clusters.
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