Question 1 · 6.1 DNA and RNA Structure
Which pair of components is joined directly to the deoxyribose sugar within a single nucleotide of a DNA strand?
- A A phosphate group and a nitrogenous base
- B A nitrogenous base and a second nitrogenous base
- C A phosphate group and a second phosphate group
- D A phosphate group and the sugar of the paired strand
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A — A nucleotide has three parts arranged around the sugar, which sits at the center. The nitrogenous base attaches at carbon 1 and the phosphate at carbon 5. The base points inward toward the helix axis and the phosphate outward into the backbone, so the sugar is the hinge that gives the molecule its shape.
Question 2 · 6.2 DNA Replication
In cells lacking telomerase, a linear chromosome loses a short stretch of DNA from each end during every round of replication. Which feature of replication accounts for that loss?
- A The two template strands separate unevenly, so one end is copied twice and the other once
- B Nucleases trim both ends of every chromosome to a fixed length after each round of copying
- C Repeated sequences at the ends fold back on themselves and block the polymerase from binding
- D Removing the final primer leaves a terminal gap with no upstream 3' end available to extend
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D — On the template that is copied in short pieces, the outermost piece begins with an RNA primer sitting at the very tip of the chromosome. Every other primer in the chromosome is replaced with DNA by a polymerase extending the 3' end of the piece behind it. The last primer has nothing behind it, so when it is excised the gap has no 3' end to extend from and stays empty. Each round of replication therefore shortens that new strand by roughly the length of one primer. Germ line and stem cells escape this by expressing telomerase, which extends the template end so the loss falls in expendable repeat sequence.
Question 3 · 6.3 Transcription and RNA Processing
A segment of the template strand of a bacterial gene reads 3'-TACGGATCA-5'. Which sequence represents the RNA product made from this segment?
- A 5'-AUGCCUAGU-3'
- B 5'-UGAUCCGUA-3'
- C 5'-UACGGAUCA-3'
- D 5'-ATGCCTAGT-3'
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A — The product is built antiparallel to the strand that guides it, so a strand read from its 3' end yields a chain written from its 5' end. Pairing base by base along the given order (T with A, A with U, C with G, G with C) gives AUGCCUAGU. Two habits keep this straightforward: every base of the product differs from the base it pairs with, and uracil occupies the position thymine would hold in DNA.