The nucleotide shown here is called cordycepin triphosphate. It is a natural product of a fungus that is used in traditional medicines. If cordycepin triphosphate is added to a cell-free transcription reaction, the nucleotide is added onto the growing RNA chain but then no more nucleotides can be added. Examine the structure of cordycepin and explain why it ends transcription.

In a particular bacterial species, temperature-sensitive conditional mutations cause expression of a wild-type phenotype at one growth temperature and a mutant phenotype at another—typically higher—temperature. Imagine that when a bacterial cell carrying such a mutation is shifted from low to high growth temperatures, RNA polymerases in the process of elongation complete transcription normally, but no new transcripts can be started. The mutation in this strain most likely affects: a. the terminator sequence b. the start codon c. sigma d. one of the polypeptides of the core RNA polymerase

Explain what's wrong with this statement: All point mutations change the genotype and the phenotype.

In what ways are a promoter and a start codon similar? In what ways are they different?

Draw a hypothetical metabolic pathway in Neurospora crassa composed of five substrates, five enzymes, and a product called nirvana. Number the substrates 1–5, and label the enzymes A–E, in order. (For instance, enzyme A catalyzes the reaction between substrates 1 and 2.) (a) Suppose a mutation made the gene for enzyme C nonfunctional. What molecule would accumulate in the affected cells?

Draw a hypothetical metabolic pathway in Neurospora crassa composed of five substrates, five enzymes, and a product called nirvana. Number the substrates 1–5, and label the enzymes A–E, in order. (For instance, enzyme A catalyzes the reaction between substrates 1 and 2.) (b) Suppose a mutant strain can survive if substrate 5 is added to the growth medium, but it cannot grow if substrates 1, 2, 3, or 4 are added. Which enzyme in the pathway is affected in this mutant?

One of the possibilities considered about the genetic code was that the code was overlapping, meaning that a single base could be part of up to three codons. How many amino acids would be encoded in the sequence 5′-AUGUUACGGAAU-3′ by a non-overlapping and a maximally overlapping triplet code? a. 4 (non-overlapping) and 16 (overlapping) b. 4 and 12 c. 4 and 10 d. 12 and 4