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mutations

before A, B, and D; and group II mutations affect the conversion of compound C into one of the other compounds:

Group I

mutations

Group II

mutations compounds A, B, D

Group III mutants allow growth if compound B or D is added but not if compound A or C is added. Thus group III mutations affect steps that follow the production of A and C; we have already determined that compound C precedes A in the pathway; so A must be the next compound in the pathway:

Group I

mutations compound C

Group II

mutations compound A

Group III

mutations compounds B, D

Finally, mutants in group IV will grow if compound D is added, but not if compound A, B, or C are added. Thus compound D is the fourth compound in the pathway, and mutations in group IV block the conversion of B into D:

Group I

mutations compound C

Group II

mutations compound A

Group III

mutations compound B

Group IV

mutations compound D

The underlying principle used to determine the order of the compounds in the pathway is as follows: If a compound is added after the block, it will allow the mutant to grow, whereas, if a compound is added before the block, it will have no effect. Applying this principle to the data in the table, we see that mutants in group I will grow if compound A, B, C, or D is added to the medium; so these mutations must affect a step before the production of all four compounds:

Group II mutants will grow if compound A, B, or D is added but not if compound C is added. Thus compound C comes

2. If there were five different types of bases in mRNA instead of four, what would be the minimum codon size (number of nucleotides) required to specify the following numbers of different amino acid types: (a) 4, (b) 20, (c) 30?

To answer this question, we must determine the number of combinations (codons) possible when there are different numbers of bases and different codon lengths. In general, the number of different codons possible will be equal to:

blg = number of codons where, b equals the number of different types of bases and lg equals the number of nucleotides in each codon (codon length). If there are five different types of bases, then:

51 = 5 possible codons

52 = 25 possible codons

53 = 125 possible codons

The number of possible codons must be greater than or equal to the number of amino acids specified. Therefore, a codon length of one nucleotide could specify 4 different amino acids, a codon length of 2 nucleotides could specify 20 different amino acids, and a codon length of 3 nucleotides could specify 30 different amino acids: (a) 1, (b) 2, (c) 3.

3. A template strand in bacterial DNA has the following base sequence:

5' - AGGTTTAACGTGCAT - 3' What amino acids would be encoded by this sequence?

To answer this question, we must first work out the mRNA sequence that will be transcribed from this DNA sequence. The mRNA must be antiparallel and complementary to the DNA template strand:

DNA template strand: 5' - AGGTTTAACGTGCAT - 3' mRNA copied from DNA: 3'-UCCAAAUUGCACGUA-5'

An mRNA is translated 5':3'; so it will be helpful if we turn the RNA molecule around with the 5' end on the left:

mRNA copied from DNA: 5'-AUGCACGUUAAACCU-3'

The codons consist of groups of three nucleotides that are read successively after the first AUG codon; using Figure 15.14, we can determine that the amino acids are:

fMet His Val Lys Pro

4. The following triplets constitute anticodons found on a series of tRNAs. Give the amino acid carried by each of these tRNAs.

To solve this problem, we first determine the codons with which these anticodons pair and then look up the amino acid specified by the codon in Figure 15.14. The codons are antiparallel and complementary to the anticodons. For part a, the anticodon is 5' - UUU - 3'. According to the wobble rules in Table 15.2, U in the first position of the anticodon can pair with either A or G in the third position of the anticodon, so there are two codons that can pair with this anticodon:

Anticodon: 5' - UUU - 3' Codon: 3'-AAA-5' Codon: 3' - GAA - 5'

Listing these codons in the conventional manner, with the 5' end on the right, we have:

According to Figure 15.14, both codons specify the amino acid lysine (Lys). Recall that the wobble in the third position allows more than one codon to specify the same amino acid; so any wobble that exists should produce the same amino acid as the standard base pairings would, and we do not need to figure the wobble to answer this question. The answers for parts b, c, and d are:

The New Genetics

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