Question

linus pauling (1901-1994) was an american scientist. he studied dna, the material that carries a living organism’s genetic information.
from the shape of dna
1
like most other scientists, pauling was not interested in dna until 1952, even though swiss biologist friedrich miescher had discovered dna in 1869. but what was dna? scientists knew only a little. it came in long strands, and each strand had a phosphorus-sugar backbone. there were also nucleic bases, which stuck out from the backbone like knobs on a spine. but the shape of the strands and how they linked up were mysteries—important mysteries. as pauling showed with hemoglobin, shape can affect how molecules work. soon the shape of dna became the most important question in biology.
2
and pauling, like many others, assumed he was the only one smart enough to answer this question. this wasn’t, or at least wasn’t only, arrogance: pauling had simply never been beaten before. so in 1952, pauling sat down at his desk in california to crack dna. he decided, incorrectly, that the bulky nucleic bases sat on the outside of each strand with the phosphorus-sugar backbone toward the molecule’s core. pauling also reasoned that dna was a triple helix, with three ribbons of the phosphorus-sugar backbones forming a spiral. unfortunately for pauling, his data came from a dried-out dna sample, which made the molecule seem more complicated. but on paper, all this seemed possible.
3
everything was humming along nicely until pauling asked a graduate student to check his calculations. the student did and was soon tying himself in knots trying to see where he was wrong and where pauling was right. eventually, the student pointed out to pauling that something about his theory didn’t seem quite right.
reset
ameïra l johnson, id#******55
growth: reading 6+ ny 2017
questions about this passage.
this question has two parts. answer part a, and then answer part b.
part a
what information did watson and crick learn that was missing from pauling’s approach to dna?

1. that dna consisted of more than one strand
2. that dna strands always included a phosphorus-sugar backbone
3. that wet samples had to be used to get reliable data regarding dna
4. that the four bases in dna consistently matched up to create two strands

part b
which sentence from the passage best supports the correct answer from part a?

1. “it came in long strands, and each strand had a phosphorus-sugar backbone.” (paragraph 1)
2. “he decided, incorrectly, that the bulky nucleic bases sat on the outside of each strand with the phosphorus-sugar backbone toward the molecule’s core.” (paragraph 2)

Explanation:

Part A

To solve Part A, we analyze each option:

• Option 1: The passage says scientists knew DNA came in long strands, so Pauling likely knew it had more than one strand. Eliminate.
• Option 2: The passage states scientists knew each strand had a phosphorus - sugar backbone, so Pauling knew this. Eliminate.
• Option 3: Pauling used dried - out DNA samples, but there's no indication Watson and Crick's key discovery was about wet samples. Eliminate.
• Option 4: Pauling thought DNA was a triple helix with bases on the outside. Watson and Crick likely discovered the base - pairing (the four bases matching up to form two strands), which was missing from Pauling's approach.

We need to find the sentence that supports the answer in Part A (about base - pairing and two strands).

• Option 1: This sentence is about the structure of each strand (phosphorus - sugar backbone), not about base - pairing or two strands. Eliminate.
• Option 2: This sentence is about Pauling's incorrect assumption about the position of nucleic bases (on the outside of each strand) and his triple - helix idea. It shows that Pauling didn't consider the base - pairing that forms two strands, which supports the idea that this was missing from his approach.

Answer:

1. that the four bases in DNA consistently matched up to create two strands

Part B