- Specification route
- 3.1.5
- Question bank
- 21 questions
- Course stage
- AS / Year 12
Sample questions
A DNA strand contains $20\%$ adenine. According to specific complementary base pairing, what is the percentage of cytosine in this DNA molecule?
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Adenine pairs with Thymine, and Cytosine pairs with Guanine. Total bases = $100\%$.
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Answer: $30\%$
If adenine is $20\%$, then thymine is also $20\%$ (total $40\%$). The remaining $60\%$ must be divided equally between cytosine and guanine, meaning cytosine is $30\%$.
During the validation of the Watson-Crick model, Meselson and Stahl grew bacteria in heavy nitrogen ($^{15}N$) and then transferred them to light nitrogen ($^{14}N$). What did the presence of a single intermediate-density DNA band after one generation prove?
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An intermediate band means every molecule consists of one old (heavy) strand and one new (light) strand.
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Answer: It proved that DNA replication is semi-conservative.
The intermediate band indicated that all DNA molecules contained one strand of $^{15}N$ and one strand of $^{14}N$. This directly supports the semi-conservative model, where each parental strand acts as a template for a new strand.
Which statement correctly describes the structural differences between DNA and RNA?
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Consider the stability and size required for their respective functions in the cell.
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Answer: DNA is a double helix with long polynucleotide chains, whereas RNA is a relatively short, single polynucleotide chain.
A DNA molecule is a double helix with two long polynucleotide chains. An RNA molecule is a relatively short polynucleotide chain. DNA has deoxyribose; RNA has ribose.
Why did many scientists initially doubt that DNA carried the genetic code?
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Proteins are made of 20 different amino acids, making them seem like much better candidates for a complex code.
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Answer: The relative simplicity of DNA, with only four different bases, seemed insufficient to carry complex genetic instructions.
Students should appreciate that the relative simplicity of DNA (being a polymer of only four different nucleotides) led many scientists to doubt that it carried the genetic code, favoring proteins instead.
If a chemical inhibitor specifically prevents the action of DNA polymerase, what would be the immediate effect on a dividing cell?
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Identify what specific part of replication DNA polymerase is responsible for.
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Answer: Phosphodiester bonds would not form between adjacent nucleotides, preventing the synthesis of a new DNA strand.
The role of DNA polymerase is in the condensation reaction that joins adjacent nucleotides (forming phosphodiester bonds). Helicase unwinds and breaks hydrogen bonds. Attraction of free nucleotides happens spontaneously via hydrogen bonding.
What are the three components of a DNA nucleotide?
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DNA contains a specific sugar and does not contain uracil.
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Answer: Deoxyribose, a phosphate group, and one of the organic bases adenine, cytosine, guanine or thymine
The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.
Which organic base is found in RNA nucleotides but never in DNA nucleotides?
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This base pairs with adenine in RNA molecules.
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Answer: Uracil
The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.
What type of bond forms between two nucleotides in a condensation reaction?
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This bond forms the strong sugar-phosphate backbone of nucleic acids.
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Answer: Phosphodiester bond
A condensation reaction between two nucleotides forms a phosphodiester bond.
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