Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 17
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In addition, the base uracil (U) in RNAreplaces the base thymine (T) in DNA. Therefore, the RNA transcript (shown paired with the DNA template toillustrate the polarity relation) isRNARNADNA5'-AUGCUUCAUUUCAGCUCGAAUUUUGCC-3'3'-TACGAAGTAAAGTCGAGCTTAAAACGG-5'Problem 4: RNA transcripts can be translated in vitro using ribosomes, transfer RNAs, and other necessaryconstituents extracted from cells, but the first codon can be any sequence of three nucleotides (instead of AUG,which is used in vivo). A synthentic mRNA consists of the repeating tetranucleotide 5'-AUGC-3' and hence has thesequence5'-AUGCAUGCAUGCAUGCAUGCAUGC.
. . -3'.When this molecule is translated in vitro, the resulting polypeptide has the repeating sequenceMet—His—Ala—Cys—Met—His—Ala—Cys . . .What does this result tell you about the number of nucleotides in a codon? Using the fact that the only methioninecodon is 5'-AUG-3', deduce a codon for histidine (His), alanine (Ala), and cysteine (Cys). Would the result differ ifPage 28the mRNA were translated from the 3' end to the 5' end instead of in the actual direction from the 5' and to the 3'end?Answer: The result means that each codon consists of three nucleotides and that they are translated innonoverlapping groups of three.
A repeating sequence of four nucleotides repeats four codons; in this case, thesequence5'-AUGCAUGCAUGCAUGCAUGCAUGCAUGCAUGC-3'is translated by grouping into the codons5'-AUG CAU GCA UGC AUG CAU GCA UGC AUG CAU GC-3'(You should verify that it does not matter at which nucleotide in the mRNA the translation begins, because all threepossible reading frames yield the same set of repeating codons.) Because 5'-AUG-3' codes for Met, it follows that5'-CAU-3' codes for His (the next amino acid after Met), 5'-GCA-3' codes for Ala (the next in line), and 5'-UGC-3'codes for Cys. Translation of the RNA in the 3'5' direction is precluded, because in this direction, there is noAUG codon, so the resulting polypeptide could not contain methionine.Analysis and Applications1.1 Considering that favism is brought on by eating broad beans, would you consider this a "genetic" trait or a traitcaused by the environment? Why?1.2 What is the end result of replication of a duplex DNA molecule?1.3 What is the role of the messenger RNA in translation? What is the role of the ribosome? What is the role oftransfer RNA? Is there more than one type of ribosome? Is there more than one type of transfer RNA?1.4 What important observation about S and R strains of Streptococcus pneumoniae prompted Avery, MacLeod,and McCarty to study this organism?1.5 In the transformation experiments of Avery, MacLeod, and McCarty, what was the strongest evidence that thesubstance responsible for the transformation was DNA rather than protein?1.6 What feature of the physical organization of bacteriophage T2 made it suitable for use in the Hershey-Chaseexperiments?1.7 Although the Hershey-Chase experiments were widely accepted as proof that DNA is the genetic material, theresults were not completely conclusive.
Why not?1.8 The DNA extracted from a bacteriophage contains 28 percent A, 28 percent T, 22 percent G, and 22 percent C.What can you conclude about the structure of this DNA molecule?1.9 The DNA extracted from a bacteriophage consists of 24 percent A, 30 percent T, 20 percent G, and 26 percentC. What is unusual about this DNA? What can you conclude about its structure?1.10 A double-stranded DNA molecule is separated into its constituent strands, and the strands are separated in anultracentrifuge. In one of the strands the base composition is 24 percent A, 28 percent T, 22 percent G, and 26percent C. What is the base composition of the other strand?1.11 While studying sewage, you discover a new type of bacteriophage that infects E.
coli. Chemical analysisreveals protein and RNA but no DNA. Is this possible?1.12 One strand of a DNA duplex has the base sequence 5'-ATCGTATGCACTTTACCCGG-3'. What is the basesequence of the complementary strand?1.13 A region along one strand of a double-stranded DNA molecule consists of tandem repeats of the trinucleotide5'-TCG-3', so the sequence in this strand is5'-TCGTCGTCGTCGTCG. . . -3'What is the sequence in the other strand?1.14 A duplex DNA molecule contains a random sequence of the four nucleotides with equal proportions of each.What is the average spacing between consecutive occurrences of the sequence 5'-GGCC-3'? Between consecutiveoccurrences of the sequence 5'-GAATTC-3'?1.15 A region along a DNA strand that is transcribed contains no A.
What base will be missing in thecorresponding region of the RNA?1.16 The duplex nucleic acid molecule shown here consists of a strand of DNA paired with a complementarystrand of RNA. Is the RNA the top or the bottom strand? One of the base pairs is mismatched. Which pairs is it?5'-AUCGGUUACAUUCCGACUGA-3'3'-TAGCCAATGTAAGGGTGACT-5'1.17 The sequence of an RNA transcript that is initially synthesized is 5'-UAGCUAC-3', and successivenucleotides are added to the 3' end.
This transcript is produced from a DNA strand with the sequence3'-AAGTCGCATATCGATGCTAGCGCAACCT-5'What is the sequence of the RNA transcript when synthesis is complete?Page 291.18 An RNA molecule folds back upon itself to form a "hairpin" structure held together by a region of basepairing. One segment of the molecule in the paired region has the base sequence 5'-AUACGAUA-3'. What is thebase sequence with which this segment is paired?1.19 A synthetic mRNA molecule consists of the repeating base sequence5'-UUUUUUUUUUUU. .
. -3'When this molecule is translated in vitro using ribosomes, transfer RNAs, and other necessary constituents from E.coli, the result is a polypeptide chain consisting of the repeating amino acid Phe—Phe—Phe—Phe . . .. If youassume that the genetic code is a triplet code, what does this result imply about the codon for phenylalanine (Phe)?1.20 A synthetic mRNA molecule consisting of the repeating base sequence5'-UUUUUUUUUUUU. . . -3'is terminated by the addition, to the right-hand end, of a single nucleotide bearing A. When translated in vitro, theresulting polypeptide consists of a repeating sequence of phenylalanines terminated by a single leucine. What doesthis result imply about the codon for leucine?1.21 With in vitro translation of an RNA into a polypeptide chain, the translation can begin anywhere along theRNA molecule.
A synthetic RNA molecule has the sequence5'-CGCUUACCACAUGUCGCGAACUCG-3'How many reading frames are possible if this molecule is translated in vitro? How many reading frames arepossible if this molecule is translated in vivo, in which translation starts with the codon AUG?1.22 You have sequenced both strands of a double-stranded DNA molecule. To inspect the potential amino acidcoding content of this molecule, you conceptually transcribe it into RNA and then conceptually translate the RNAinto a polypeptide chain. How many reading frames will you have to examine?1.23 A synthetic mRNA molecule consists of the repeating base sequence 5'-UCUCUCUCUCUCUCUC . .
. -3'.When this molecule is translated in vitro, the result is a polypeptide chain consisting of the alternating amino acidsSer—Leu—Ser—Leu—Ser—Leu . . .. Why do the amino acids alternate? What does this result imply about thecodons for serine (Ser) and leucine (Leu)?1.24 A synthetic mRNA molecule consists of the repeating base sequence 5'-AUCAUCAUCAUCAUCAUC .
. . 3'. When this molecule is translated in vitro, the result is a mixture of three different polypeptide chains. Oneconsists of repeating isoleucines (Ile—Ile—Ile—Ile . . .), another of repeating serines (Ser—Ser—Ser—Ser . . .),and the third of repeating histidines (His—His—His—His . . .).
What does this result imply about the manner inwhich an mRNA is translated?1.25 How is it possible for a gene with a mutation in the coding region to encode a polypeptide with the sameamino acid sequence as the nonmutant gene?Further ReadingBearn, A. G. 1994. Archibald Edward Garrod, the reluctant geneticist. Genetics 137: 1.Birge, R. R. 1995. Protein-based computers. Scientific American, March.Calladine, C. R. 1997. Understanding DNA: The Molecule and How It Works. New York: Academic Press.Erwin, D.
H. 1996. The mother of mass extinctions. Scientific American, July.Gehrig, A., S. R. Schmidt, C. R. Muller, S. Srsen, K. Srsnova, and W. Kress. 1997. Molecular defects inalkaptonuria. Cytogenetics & Cell Genetics 76: 14. Gould, S. J. 1994. The evolution of life on the earth. ScientificAmerican, October.Horgan, J. 1993. Eugenics revisited. Scientific American, June.Horowitz, N. H. 1996.
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