Which of the following best explains why the ratios of A/T and G/C are 0.77 and

Question

Which of the following best explains why the ratios of A/T and G/C are 0.77 and 1.08, respectively in bacteriophage phi X174? A) the base pairing in bacteriophages is not always A-T and C-G B) studies done with most bacteriophages do not use complete genomic DNA due to the inherent instability of phage DNA C) bacteriophage DNA often uses bases other than A, T, C and G D) many viruses often use single stranded DNA E) none of the above An observable parameter used in evaluation of oxygen affinity for heme proteins is the term Yo, which represents the fraction of sites occupied by oxygen divided by the number of free (unoccupied) sites. A) True B) False What effect would you expect each of the following have on the affinity of oxygen binding to hemoglobin? decrease of the pH in solution from 7.8 to 7.0; A) increase; B) decrease; C) stay the same increase the amount of CO_2 available to the hemoglobin solution: A) increase; B) decrease; C) stay the same for whole red cells, starve them of glucose just long enough to deplete them of BPG: A) increase; B) decrease; C) stay the same The binding of BPG to human hemoglobin stabilizes the T-state of the tetramer (deoxy form). A) True B) False Changes in hemoglobin's oxygen affinity are primarily the result of changes in the _____ structure of the protein. A) primary B) secondary C) tertiary D) quaternary E) all of the above

Explanation / Answer

35) According to chargaff rule, the DNA from any cell should have 1:1 ration of purine and pyrimidine bases. However, there are few exceptions to this rule.
In bacteriophage, phix174, the ratios of purine and pyrimidine bases is 0.77:1.08. This is because these phages have single stranded genomes.
In double stranded genome, the secondary structure requires that every guanine pairs with a cytosine and every thymine pairs with an adenine, resulting in 1:1 ratios between the nucleotides. But, in single stranded genome, the pairing is not consistent throughout molecules.

Hence, the correct opton is (D).

36) The parameter, used in the evaluation of oxygen affinity for heme proteins is y0 , it represents the fraction of sites occupied by oxygen divided by the number of free(unoccupied sites).Y0 or fractional saturation is the fraction of oxygen binding sites occupied by oxygen.
YO2 = [PO2]/ [K] + [PO2] where PO2 is the partial pressure of oxygen.

Hence the correct option is (A).

37)Bohr effect- when hemoglobin binds to oxygen it releases protons. This indicates that lower pH induces hemoglobin to release oxygen.

Hence, the correct option is (B).

38) Oxyform of Hb has more oxygen binding capacity than the deoxy form of Hb. Increase in the amount of carbondioxide decreases oxygen affinity to hemoglobin by preferentially binding to deoxy form .

Hence, the correct option is (B).

39) BPG- Biphosphoglycerate binds to hemoglobin present in the blood and decreases its oxygen affinity.Therefore, if the whole red cells are starved of glucose to make them deplete BPG , it increases oxygen affinity allowing it to release less oxygen at the capillaries.

Hence, the correct option is (A).

40) The binding of BPG to human hemoglobin stabilizes the T-state of the tetramer. T-state is a low affinity state, BPG preferntially binds to the molecules in this state, thereby stabilizing its conformation.

Hence, the correct option is (A).

41) Changes in hemoglobin's oxygen affinity, are primarily the result of changes in the quarternary structure of protein. This is beacuse , when oxygenbinds to hemoglobin, alpha and beta subunits contacts shift and changes the quaternary structure.

Hence, the correct option is (D).

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