TBR 6 Biology section

[Joker88]

hey guys i was hoping i could get some help on this question. I was confused by the explanation:

129. Which of the following statements supports the idea that DNA is made up of nucleic acids?

A. When the pKa < pH, the molecule is neutral
B. When the pKa < pH, the molecule is negative
C. When the pKa > pH, the molecule is positive.
D. When the pKa > pH, the molecule is neutral

B is the best answer. An acid generally carries a negative charge when its pKa is less than the pH. When the pKa = pH, an acid is 50% neutral and 50% negatively charged. When the pKa is less than the pH, the molecule is now in a more acidic environment, and more of it carries a negative charge. Choice A is a false statement. Choices C and D apply to a base and not an acid. For a base, when the pH is less than the pKa, the molecule retains an extra proton and is positively charged. When the pKa is less than the pH, the base (i.e., an amino group) donates its proton and becomes neutral. The best answer is B.


140. A man afflicted with xeroderma pigmentosum is married to a woman who is heterozygous for the trait. What is the probability that their second child will be a heterozygote?

A. 25%
B. 50%
C. 75%
D. 100%

The answer is B. When I did the monohybrid square I got 25% for some reason. They said the disease is autosomal recessive. I maybe doing the square wrong but I keep getting 25%-hetero, 50%- homo recessive, 25%- normal.

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[whiteshadodw]

hey guys i was hoping i could get some help on this question. I was confused by the explanation:

129. Which of the following statements supports the idea that DNA is made up of nucleic acids?

A. When the pKa < pH, the molecule is neutral
B. When the pKa < pH, the molecule is negative
C. When the pKa > pH, the molecule is positive.
D. When the pKa > pH, the molecule is neutral

B is the best answer. An acid generally carries a negative charge when its pKa is less than the pH. When the pKa = pH, an acid is 50% neutral and 50% negatively charged. When the pKa is less than the pH, the molecule is now in a more acidic environment, and more of it carries a negative charge. Choice A is a false statement. Choices C and D apply to a base and not an acid. For a base, when the pH is less than the pKa, the molecule retains an extra proton and is positively charged. When the pKa is less than the pH, the base (i.e., an amino group) donates its proton and becomes neutral. The best answer is B.


140. A man afflicted with xeroderma pigmentosum is married to a woman who is heterozygous for the trait. What is the probability that their second child will be a heterozygote?

A. 25%
B. 50%
C. 75%
D. 100%

The answer is B. When I did the monohybrid square I got 25% for some reason. They said the disease is autosomal recessive. I maybe doing the square wrong but I keep getting 25%-hetero, 50%- homo recessive, 25%- normal.

for 129, almost ALL acids carry a negative charge when deprotonated. i can't think of many that don't. using henderson hasselbach:

pH = pKa + log (base/acid)

when ph > pKa the base form of an acid is preferred. for carboxylic acid groups (common in amino acids as well), the base form is the deprotonated hydroxyl. when the hydroxyl loses a proton (just a +1 hydrogen atom), the oxygen keeps the electron and thus has a -1 charge.

as for the punnett square, you should assume that the disease is autosomal recessive. a few things give this away. first the mother is a heterozygote, but it doesn't say she's afflicted therefore she has to have the following genotype Aa, where A is the dominant allele ensuring that she isn't afflicted. she COULD be be X^A X^a, but there is no information in the question stem suggesting this is a sex-linked disease. knowing all this, the man must be homozygous recessive, otherwise he wouldn't be afflicted by the disease. so now we have the following genotype for the male: aa

punnett square:
A a
a |Aa | aa
a |Aa | aa

the second child has the same probability of being a heterozygote as the first and as the third, fourth, fifth and so on. everytime you have a child, its like your picking one of the four possibilities out of a hat, except once you take out one of those "sheets of paper" you have to put them back in. next time you go to pick out of the hat, all four options are still in the hat.

1:1 ratio of Aa:aa so 1:1 ratio of not afflicted-carrier to afflicted.



edit: i don't know if i'm overthinking, but for the nucleid acids question, if i think of carboxylic acid pka 2-5, and the pH is 1 then:

pH = pKa + log (base/acid) ; 1 = 2 + log (base/acid); the acid form is preferred and the molecule is neutral. but again, i think we're overthinking if we say that. but i think based on the answer given, you made a typo in typing up the question and answer D should look like:

D. When the pKa < pH, the molecule is neutral

 

[Joker88]

great! thanks a lot! I think for the first question i typed it wrong so you're right in your explanation. thanks again