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Genes and Chromosomes S-274 1. Packaging of DNA in a Virus Bacteriophage T2 has a DNA of molecular weight 120 10 6 con- tained in a head about 210 nm long. Calculate the length of the DNA (assume the molecular weight of a nucleotide pair is 650) and compare it with the length of the T2 head. Answer The T2 DNA molecule has (120 10 6 )/650 1.8 10 5 nucleotide pairs. Recall from Chapter 8 that a base pair occupies a length of 3.4 Å, so the length of the T2 DNA molecule is (1.8 10 5 bp)(3.4 Å/bp) 6.1 10 5 Å 6.1 10 4 nm Thus, the DNA molecule is 6.1 10 4 nm/210 nm 290 times the length of the viral head. This nicely illustrates the necessity of compact packaging of DNA in viruses (see Fig. 24–1 and Table 24–1 for the relative lengths of DNA and viral head for other bacteriophages). 2. The DNA of Phage M13 The base composition of phage M13 DNA is A, 23%; T, 36%; G, 21%; C, 20%. What does this tell you about the DNA of phage M13? Answer The complementarity between A and T, and between G and C, in the two strands of a duplex DNA underlies Chargaff’s rule that the sum of pyrimidine nucleotides equals that of purine nucleotides in DNAs from (virtually) all species: that is, A T, G C, and A G C T for duplex DNA. In M13 DNA, the percentage of A (23%) does not equal that of T (36%), nor does that of G (21%) equal that of C (20%); and A G 44%, whereas C T 56%. This lack of equality between purine and pyrimidine nucleotides shows that M13 DNA is single-stranded, not double-stranded; the relationships expected from complementarity between two strands of a duplex DNA are not seen. The M13 DNA is double-stranded only when replicating in the host cell. 3. The Mycoplasma Genome The complete genome of the simplest bacterium known, Mycoplasma genitalium, is a circular DNA molecule with 580,070 bp. Calculate the molecular weight and contour length (when relaxed) of this molecule. What is Lk 0 for the Mycoplasma chromosome? If j 0.06, what is Lk ? Answer The molecular weight of a single nucleotide pair is 650 (see Problem 1), so the approximate molecular weight of the Mycoplasma DNA molecule is (580,070 bp)(650/bp) 3.8 10 8 Given a length of 3.4 Å per base pair, the contour length is (580,070 bp)(3.4 Å/bp) 2.0 10 6 Å 200 m m For relaxed circular DNA, the linking number is the number of base pairs divided by 10.5. For M. genitalium, Lk 0 (580,070 bp)/(10.5 bp) 55,200 chapter 24 2608T_ch24sm_S274-S282 02/26/2008 12:23 am Page S-274 pinnacle OS9:Desktop Folder:WHQY028:
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If j 0.06, Lk j ( Lk 0 ) ( 0.06)(55,200) 3,310 Thus, Lk Lk 0 3,310 55,200 3,310 51,900. 4. Size of Eukaryotic Genes An enzyme isolated from rat liver has 192 amino acid residues and is coded for by a gene with 1,440 bp. Explain the relationship between the number of amino acid residues in the enzyme and the number of nucleotide pairs in its gene. Answer Each amino acid is encoded by a triplet of 3 bp, so the 192 amino acids are encoded by 576 bp. The gene is in fact longer (1,440 bp). The additional 864 bp could be in introns (noncoding DNA, interrupting a coding segment) or could code for a signal sequence (or leader peptide). In addition, as discussed in Chapter 26, eukaryotic mRNAs have untranslated segments before and after the region coding for the polypeptide chain, which also contribute to the “extra” size of genes.
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