Page 148 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

Problems 307

these mutant proteins are of normal size (but have a deletion that altered or removed the termination co­
amino acid substitutions) while others are short, due don. The information in the mRNA beyond the nor­
to deletions or nonsense mutations. The first extra­ mal stop codon would be translated until another stop
long example was named Hb Constant Spring, in codon in the mRNA was reached.
which the β­globin has all of its normal amino acids b. A splicing defect could explain Hb Constant Spring
plus several extra amino acids attached after the only if an intron existed at a location just before the
normal C­terminal end of the protein. stop codon and the mutation prevented removal of the
a. What is the most plausible explanation for intron from the mature mRNA. In this case, the nucle­
its origin? otides in the intron would be read in frame as triplets
b. Is it possible that Hb Constant Spring arose from until a stop codon was reached. The necessity for the
failure to splice out an intron? presence of an intron in a specific location makes this
c. Estimate how many extra amino acids might scenario much less likely than that in part (a).
be added to the C terminal end of the mutant c. Regardless of whether the explanation for the
protein. Hb Constant Spring protein is a change to the
termination codon or a change in splicing, you can es­
Answer timate the number of amino acids added to the end of
An understanding of the principles of translation and RNA the protein by assuming they are encoded
by a random DNA sequence. In the genetic code,
splicing are needed to answer this question.
3 out of the 64 triplets (~1 in 21) are stop codons. So
a. Because an extension on the C­terminal end you would roughly estimate that on average you
of the protein is present, the mutation probably would add about 21 amino acids to the end of the mu­
affected the termination (nonsense) codon tant protein until the reading frame encountered a stop
rather than affecting splicing of the RNA. This altera­ codon.
tion could have been a base change or a frameshift or

PROBLEMS

Vocabulary i. RNA­like strand 9. AUG in a particular context
1. For each of the terms in the left column, choose the j. intron 10. the linear sequence of amino acids in
the polypeptide corresponds to the
best matching phrase in the right column. linear sequence of nucleotide pairs in
a. codon 1. removing base sequences the gene
corresponding to introns from the k. RNA splicing 11. produces different mature mRNAs
primary transcript from the same primary transcript
b. colinearity 2. UAA, UGA, or UAG l. transcription 12. addition or deletion of a number of
c. reading frame 3. the strand of DNA that has the base pairs other than three into the
same base sequence as the primary coding sequence
transcript m. translation 13. a sequence of base pairs within a gene
d. frameshift mutation 4. a transfer RNA molecule to which the that is not represented by any bases in
appropriate amino acid has been the mature mRNA
attached n. alternative splicing 14. the strand of DNA having the base
e. degeneracy of the 5. a group of three mRNA bases sequence complementary to that of the
genetic code signifying one amino acid primary transcript
f. nonsense codon 6. most amino acids are not specified by o. charged tRNA 15. using the information encoded
a single codon in the nucleotide sequence of an
g. initiation codon 7. using the information in the nucleotide mRNA molecule to specify the
sequence of a strand of DNA to amino acid sequence of a polypeptide
specify the nucleotide sequence of a molecule
strand of RNA p. reverse 16. copying RNA into DNA
h. template strand 8. the grouping of mRNA bases in threes transcription
to be read as codons