You are working with a newly discovered mutagen and wish to determine the base c

Question

You are working with a newly discovered mutagen and wish to determine the base change it introduces into DNA. Thus far, you have determined that the mutagen chemically alters a single base in such a way that its base-pairing properties are altered permanently, thereby causing substitution mutations. To determine the specificity of the alteration, you examine the amino acid changes that take place after exposing bacteria to the mutagen. A sample of what you find is shown below.

Original sequence #1: Gln – His – Ile – Glu – Lys

Mutant sequence #1: Gln – His – Met – Glu – Lys

Original sequence #2: Ala – Val – Asn – Arg

Mutant sequence #2: Ala – Val – Ser – Arg

Original sequence #3: Arg – Ser – Leu

Mutant sequence #3: Arg – Ser – Leu – Trp – Lys – Thr – Phe

(a) What possible substitution mutation(s) could convert original sequence #1 to mutant sequence #1? Briefly explain.

(b) What possible substitution mutation(s) could convert original sequence #2 to mutant sequence #2? Briefly explain.

(c) What possible substitution mutation(s) could convert original sequence #3 to mutant sequence #3? Briefly explain.

(d) Presuming that the newly discovered mutagen causes the same type of base change in each of the three cases above, what do you conclude is the base-change specificity of the newly discovered mutagen? Briefly explain how you have reached this conclusion.

Second base UAU TyuGc UAC UAA Stop UGA Stop A UAG Stop UGG Trp G UGU cysc UCU UCC UUC UUA UUG CUU CUC UCG CCU CUA Leu/CCC CCG CAC CAA CAG AAU AAC CGU CGC CGA CGG CUG AUU ACU AUA AUG Met ACG GUU GUC GUA Val GCC AAG Lys AGA GAU GAC GAA GAG Glu GGA Gly /C AGG Arg GGU GGC GCU GUG GCG

Explanation / Answer

a) Isoleucine mutates to methionine. Ile is coded by AUU, AUC, AUA. Whereas Met is coded by AUG. A point mutation in the third N-Base of three letter code can lead to substitution of Ile by Met. The mutation can be either of two types, transition(U->G or C->G) OR transversion (A->G).

b) Asn is coded by AAU, AAC and Ser is coded by AGU,AGC. This replacement of amino acid occurs due to mutation of the middle N base of the three letter sequence. It is a transversion type of mutation where, a purine( A) is substitued by pyrimidine(G)

C) In this mutant sequence extra piece of DNA has been inserted. Hence, this is an Insertion mutation

D)

We have noted that the original sequence is a lot shorter than mutant sequence. Given that the mutation is a base change and not a chromosomal mutation then the change has to be in an amino acid that codes translation to stop. Hence the mutation has to be in a part of the mRNA that lies beyond the last original codon. (By the way when I say codon I mean a combination of 3 letter code that codes for a single amino acid). Hence it is safe to assume that after the last codon of the amino acid there lies a stop codon, either UAA, UAG or UGA. Looking at the mutant amino acid sequence the following amino acid is Tryptophan (Try) that is UGG. Only one stop codon out of the three shares two of the base codons with that of Tryptophan, that would be UG_. Thus the single base change is from A to G. This makes sense because where once a stop codon lied there is now an amino acid. The absence of the stop codon allows for more amino acids to be coded all the way until the translation units read a stop codon and stop the amino acid chain build

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