Question 1
A researcher analyses the base composition of a double-stranded DNA molecule and records the following partial data:
Base Percentage (%) Adenine 17 Thymine ? Guanine ? Cytosine ? What is the expected percentage of cytosine in this molecule?
No clue? Show me the answer
Correct answerCorrect!
IncorrectStep-by-step walkthrough
Choose a solution method
Method #1Approach 1Step 1: State Chargaff's Rules
Chargaff's rules state that in double-stranded DNA, A = T and G = C. This means the percentage of adenine equals thymine, and the percentage of guanine equals cytosine.
Step 2: Determine thymine percentage
Since A = T, the percentage of thymine is also 17%.
Step 3: Calculate the remaining percentage
Total percentages must sum to 100%. So G + C = 100% − 17% − 17% = 66%.
Step 4: Calculate cytosine
Since G = C, cytosine = 66% ÷ 2 = 33%.
Step 5: Confirm the answer
The expected percentage of cytosine is 33%, which matches the third option.
Method #2Approach 2Step 1: What is being asked
We need the percentage of cytosine in a double-stranded DNA molecule where adenine is 17%.
Step 2: Eliminate 17%
17% would only be correct if C = A, but Chargaff's rules state C = G, not C = A. So this option is incorrect.
Step 3: Eliminate 34%
34% = 2 × 17%, suggesting C = A + T. This is not a relationship supported by Chargaff's rules.
Step 4: Eliminate 66%
66% represents the combined total of G + C, not cytosine alone. Since G = C, this value must be halved.
Step 5: Select the correct answer
With A = T = 17%, the remaining 66% is shared equally between G and C. Therefore cytosine = 33%.
Question 2
Which of the following best explains how a DNA molecule composed of only four different nitrogenous bases can encode the vast diversity of genetic information found across all living organisms?No clue? Show me the answer
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Method #1Approach 1Step 1: Identify the key principle
The capacity of DNA to store genetic information lies in the sequence of its bases, not their chemical structure or the backbone.
Step 2: Apply the sequence diversity principle
With four possible bases at each position, the number of possible sequences for a molecule of base pairs is . Because DNA molecules are often millions of base pairs long, the number of possible sequences is astronomically large.
Step 3: Distinguish sequence from structure
The backbone (sugar and phosphate) is identical in all DNA molecules and contributes nothing to informational diversity. It is the variable base sequence that encodes genetic information.
Step 4: Select the correct answer
The correct option states that the four bases can be arranged in enormous numbers of combinations over many base pairs — this directly explains informational diversity.
Method #2Approach 2Step 1: What is being asked
We need to identify how four bases can encode vast genetic diversity.
Step 2: Eliminate the hydrogen bond option
"Different numbers of hydrogen bonds" describes only A–T (2) and G–C (3) pairing — a fixed structural feature, not a source of informational diversity.
Step 3: Eliminate the backbone option
The sugar–phosphate backbone is uniform throughout the DNA molecule and does not vary — it cannot encode information.
Step 4: Eliminate the ribosome interaction option
Bases do not interact directly with ribosomes during translation; codons in mRNA are read by tRNA anticodons. This option is biologically inaccurate.
Step 5: Select the correct answer
The remaining option — variable sequence over many base pairs — correctly explains how four bases generate nearly unlimited genetic diversity.