Question 14.19

Duchenne muscular dystrophy is caused by a mutation in a gene that comprises 2.5 million nucleotides and specifies a protein called dystrophin. However, less than 1% of the gene actually encodes the amino acids in the dystrophin protein. On the basis of what you now know about gene structure and RNA processing in eukaryotic cells, provide a possible explanation for the large size of the dystrophin gene.

Question 14.20

What would happen in the experiment illustrated in Figure 14.2 if the DNA and RNA that are mixed together came from very different organisms, for example a worm and a pig?

Question 14.21

For the ovalbumin gene shown in Figure 14.3, where would the 5′ untranslated region and 3′ untranslated regions be located in the DNA and in the RNA?

Question 14.22

How do the mRNAs of bacterial cells and the pre-mRNAs of eukaryotic cells differ? How do the mature mRNAs of bacterial and eukaryotic cells differ?

Question 14.23

Are the 5′ untranslated regions (5′ UTR) of eukaryotic mRNAs encoded by sequences in the promoter, exon, or intron of the gene? Explain your answer.

Question 14.24

Draw a typical eukaryotic gene and the pre-mRNA and mRNA derived from it. Assume that the gene contains three exons. Identify the following items and, for each item, give a brief description of its function:

• a. 5′ untranslated region
• b. Promoter
• c. AAUAAA consensus sequence
• d. Transcription start site
• e. 3′ untranslated region
• f. Introns
• g. Exons
• h. Poly(A) tail
• i. 5′ cap

Question 14.25

How would the deletion of the Shine–Dalgarno sequence affect a bacterial mRNA?

Question 14.26

What would be the most likely effect of moving the AAUAAA consensus sequence shown in Figure 14.7 ten nucleotides upstream?

Question 14.27

How would the deletion of the following sequences or features most likely affect a eukaryotic pre-mRNA?

• a. AAUAAA consensus sequence
• b. 5′ cap
• c. Poly(A) tail

Question 14.28

Suppose that a mutation occurs in the middle of a large intron of a gene encoding a protein. What will the most likely effect of the mutation be on the amino acid sequence of that protein? Explain your answer.

Question 14.29

A geneticist induces a mutation in a line of cells growing in the laboratory. The mutation occurs in one of the genes that encodes proteins that participate in the cleavage and polyadenylation of eukaryotic mRNA. What will the immediate effect of this mutation be on RNA molecules in the cultured cells?

Question 14.30

A geneticist mutates the gene for proteins that bind to the poly(A) tail in a line of cells growing in the laboratory. What will the immediate effect of this mutation be in the cultured cells?

Question 14.31

A geneticist isolates a gene that contains eight exons. He then isolates the mature mRNA produced by this gene. After making the DNA single stranded, he mixes the single-stranded DNA and RNA. Some of the single-stranded DNA hybridizes (pairs) with the complementary mRNA. Draw a picture of what the DNA-RNA hybrids will look like under the electron microscope.

Question 14.32

A geneticist discovers that two different proteins are encoded by the same gene. One protein has 56 amino acids, and the other has 82 amino acids. Provide a possible explanation for how the same gene can encode both of these proteins.

Question 14.33

What conclusion can you make about the relative sizes of the two proteins produced by alternative splicing in Figure 14.12?

Question 14.34

Explain how each of the following processes complicates the concept of colinearity.

• a. Trans-splicing
• b. Alternative splicing
• c. RNA editing

Question 14.35

RNA interference may be triggered when inverted repeats are transcribed into an RNA molecule that then folds to form double-stranded RNA. Write out a sequence of inverted repeats within an RNA molecule. Using a diagram, show how the RNA with the inverted repeats can fold to form double-stranded RNA.

Question 14.36

In the early 1990s, Carolyn Napoli and her colleagues were working on petunias, attempting to genetically engineer a variety with dark purple petals by introducing numerous copies of a gene that codes for purple petals (C. Napoli, C. Lemieux, and R. Jorgensen. 1990. Plant Cell 2:279-289). Their thinking was that extra copies of the gene would cause more purple pigment to be produced and would result in a petunia with an even darker hue of purple. However, much to their surprise, many of the plants carrying extra copies of the purple gene were completely white or had only patches of color. Molecular analysis revealed that the level of the mRNA produced by the purple gene was reduced 50-fold in the engineered plants compared with levels of mRNA in wild-type plants. Somehow, the introduction of extra copies of the purple gene silenced both the introduced copies and the plant’s own purple genes. Provide a possible explanation for how the introduction of numerous copies of the purple gene silenced all copies of the purple gene.

White petunia.

[roger ashford/Alamy.]