Quiz 16: Gene Regulation in Eukaryotes Ii: Epigenetics

Biology

(A). img The original DNA strand is 5'-GGACTAGATAC-3' 5'-GGACTAGATAC-3' 5'-GAACTAGATAC-3'. This is a transition mutation. In this DNA strand there is exchange of purine for a purine. Here guanine (purine) has been exchanged by adenine (purine). It is G to A changE. img (B). 5'-GGACTAGATAC-3' 5'-GGACTAGAGAC-3'. This is a transversion mutation. In this strand there is exchange of pyrimidine for purine, Thymine which is a pyrimidine has been exchanged by guanine which is a purine. This is T to G changE. img (C). 5'-GGACTAGATAC-3' 5'-GGACTAGTAC-3'. This is a deletion. In this strand the nucleotide adenine (A) is deleted or removed from the original strand. In this mutation the reading frame is altered due to the deletion of one nucleotidE. img (D). 5'-GGACTAGATAC-3' 5'-GGAGTAGATAC-3'. This is a transversion mutation. In this stand the nucleotide cytosine; (pyrimidine) has been exchanged by guanine (purine).

The physiological adaptation hypothesis states that the mutation rate must be a constant value and it also depends on their exposure to the bacteriophagE.If this physiological adaptation theory is correct, in the fluctuation test of Luria and Delbruck, the mutations should have taken place in the E. coli cells after their exposure to bactriophage T1. Also the number of resistant bacteria called ton r should be equal in all the plates as the number bacteria were same in each plate when it was exposed to the media with T1 bacteriophage. In this case, the timing of mutation is not taken into account. In the fluctuation test of Luria and Delbruck, the results shown were fluctuating greatly. The results of their tests were consistent with the random mutation theory. In the random mutation theory, the timing of the mutation during the growth of the bacteria in the culture has their effect on the number of mutant cells. • When the mutation happened during the early stage of the growth of bacteria in a test tube, there was large number of ton r colonies in the tubE.• When the mutation occurred in the late stage of the growth of the bacteria, there was less number of colonies on the platE.Hence, in the test results, the random mutation theory was supported rather than the physiological adaptation theory.

The rate of spontaneous mutations would increase if- 1. There is an error in DNA replication. There may be an error in DNA replication due to base pair substitution, addition, and deletion of base pairs. It may be due to tautomeric shift. In tautomerism, incorrect base pairing is found due to alteration in the hydrogen bonding. 2. It may be due to spontaneous lesions like depurination (loss of guanine or adenine from the DNA), deamination (deamination of cytosine gives uracil) and oxidatively damaged bases (which include super oxide radicals or hydroxyl radicals cause oxidative damage to DNA). 3. Due to transposable elements (It is a sequence which alters the size of the genome by changing its position). 4. Due to translation process. 5. Double strand breaks, which occur naturally (in this, both the strands are damaged).

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