5) Proflavin treatment is effective for producing null alleles by mutation. a) W

Question

5) Proflavin treatment is effective for producing null alleles by mutation.

a) Why does it produce null alleles?

b) A proflavin like compound generates only single base pair insertions. A mutant obtained with this compound is treated with the same compound, and some revertants are produced. How is this outcome possible?

Question #6

6) The following sequence is found in the short stretch of DNA:

5'-ATG-3'

3'-TAC-5'

a) Give all DNA sequences that can result from spontaneous depuriniation in this stretch of DNA.

b) Give all DNA sequences that can result from spontaneous deamination in this stretch of DNA

Thanks soo much Ahead of time!!

Explanation / Answer

5) (a)

Null allele is a type of mutant allele which results in a non functional gene product. The mutation in this allele changes the reading frame, so that different amino acids are coded, or, the mutation creates a stop codon in the middle of the reading frame. This produces a mutant, nonfunctional peptide. Proflavin is known to induce single base insertion or deletion, which can result in frameshift mutations; and such mutations are very likely to result in a null allele.

5) (b)

A proflavin like compound that is able to cause single base pair insertions, will exert its effect at random loci on the DNA strand. It could be possible that the initial mutational insertion caused a frameshift mutation in the gene, which resulted in mutated phenotype. However, another insertion at the mutated gene loci in the reverstants, altered the sequence which encoded for a functional gene product, resulting in the wild type phenotype. Probably, the initial insertion encoded a stop codon, resulting in a shorter peptide, but with another insertion, the complete peptide could be formed, which exhibited wild type phenotype.

6) The DNA sequence :

5'-ATG-3'

3'-TAC-5'

(a) Depurination - is a process in which the purines, adenine and guanine, are detached from the DNA strand, by hydrolytic cleavage of N-glycosidic bond between the purine base and the ribose sugar. This creates a gap (denoted by _) in the DNA strand. Generally, spontaneous depurination will result in the removal of a single purine, at a particular locus. So, for the given DNA strand, following sequences can result.

5'-_TG-3' 5'-AT_-3'   5'- ATG-3'

3'-TAC-5' 3'-TAC-5 3'-T_C-5'

(b) Deamination is a reaction which results in the removal of an amino group from a molecule. In case of nucleotides, the purines, adenine and guanine, and the pyrimidine, cytosine contains an amino group attached to their cyclic structures. Deamination of these bases results in a different nucleotide, which may or may not result in altered affinity for the opposite base, and can result in mutation if not repaired. Deamination of adenine results in the formation of hypoxanthine, which selectively base pairs with cytosine, instead of thymine. This results in transition from AT to GC base pair. Spontaneous deamination of guanine produces xanthine, which base pairs with cytosine. Thus, there is no transition here. In case of cytosine, its deamination produces uracil, which has affinity for adenine, resulting in transition from CG to TA base pair. Considering single base deamination at a locus, the following transitions in the DNA sequence can be observed.

Deamination of Adenine:

5'-GTG-3' 5'-ACG-3'

3'-CAC-5' 3'-TGC-5'

Deamination of Guanine: No changes

Deamination of Cytosine:

5'-ATA-3'

3'-TAT-5'

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