Question 1 · 6.1 DNA and RNA Structure
Two different genes are both stretches of DNA. What makes the information in them different?
- A The kind of sugar in each backbone
- B The type of bond linking the nucleotides
- C The shape of each double helix
- D The order of the bases along each strand
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D — DNA stores information in the sequence of its bases, just as the order of letters spells different words. Every gene uses the same sugar, the same backbone bonds and the same double helix 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
Inside a transcription bubble of a moss gene, the DNA strand being read and the transcript built so far are:
DNA strand being read: 3'-TACGGTCAGCTA-5'
Transcript so far: 5'-AUGCCAGU-3'
The transcript lies against the leftmost eight DNA bases as drawn.
Which ribonucleotide does the enzyme add in its next step, and where on the growing chain does that ribonucleotide join?
- A C, joined at the 3' end of the RNA
- B G, joined at the 3' end of the RNA
- C C, joined at the 5' end of the RNA
- D G, joined at the 5' end of the RNA
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A — The ninth template base (reading the template 3' to 5', the direction the enzyme travels) is G, so the incoming ribonucleotide must carry the complementary base, C. RNA polymerase can only attach a new nucleotide to the free 3' hydroxyl of the last nucleotide in the chain, so the RNA grows 5' to 3' and the C joins the 3' end, right after the U. The RNA's 5' end was made first and stays fixed.