1.In mammalian cells, DNA supercoiling : (A) results in compaction of the DNA structure. (B) makes the DNA molecdule relaxed. (C) occurs only in viruses. (D) occurs in only one direction (i.e. is only positive) (E) usually results from overwinding. 2.DNA之双股若 overwound（缠绕更紧）或 underwound （解开螺旋）时，均会造成DNA supercoiling，能使ＤＮＡ形成浓缩紧密构造（ DNA compaction ）。既然如此，为何 “Most cellular DNA is underwound”? What is a significant consequence of underwound DNA in vivo? 3.What would be the specific linking difference for a DNA molecule of 420 base pairs from which eight turns were removed? 4.Human chromosomes are extremely large, complex structures. Which of the following statements correctly describes the organization ofhuman chromosomes? (A) all the genetic information of the cell is encoded in the nuclear, chromosomal DNA. (B) Genes for histones and ribosomal RNAs are structural genes. (C) Most of the chromosomal DNA codes for proteins. (D) A and B (E) A, B, and C 5.There are several levels of protein architecture, and the same is true for chromosomal DNA. Indicate whether each of the following statements about the structure and packaging of chromosomal DNA is true or false. (1).Histones are basic proteins that make up about half the mass of chromatin. (2).Histones grip the DNA double helix like a fist, with the DNA in the center. (3).The nucleosome structure acts to condense DNA by decreasing its length. (4).Formation of the nucleosome structure is the only condensing step in chromosomal DNA. (5).DNA constitutes approximately 90% of the total mass of chromatin; the rest is protein and s small amount of RNA. (6).The amino acid sequences of histones vary widely from species to species. (7).Between the nucleosmes are protease-sensitive regions of DNA. 6.In a certain organism, the gene for the glycolytic enzyme hexokinase has 21,000 bases. The molecular weight of hexokinase is approximately 110,000. Is this organism a prokaryote or eukaryote? How do you know? (average molecular weight of an amino acid =110; of a base =450) 7.Why does the human genome contain thousands of origins of replication but the E. coli genome contain only one? 作业缴交 时间 和 规定 都同第一次 谢谢 同学的配合 --

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