15.1: The Genetic Code
The AUC and AUA codons in mRNA both specify isoleucine. What feature of the genetic code explains this?
- nonsense codons
How many nucleotides are in 12 mRNA codons?
Imagine if there were 200 commonly occurring amino acids instead of 20. Given what you know about the genetic code, what would be the shortest possible codon length? Explain.
For 200 commonly occurring amino acids, codons consisting of four types of nucleotides would have to be at least four nucleotides long, because 44 = 256. There would be much less degeneracy in this case.
Discuss how degeneracy of the genetic code makes cells more robust to mutations.
Codons that specify the same amino acid typically only differ by one nucleotide. In addition, amino acids with chemically similar side chains are encoded by similar codons. This nuance of the genetic code ensures that a single-nucleotide substitution mutation might either specify the same amino acid and have no effect, or may specify a similar amino acid, preventing the protein from being rendered completely nonfunctional.
15.2: Prokaryotic Transcription
Which subunit of the E. coli polymerase confers specificity to transcription?
The -10 and -35 regions of prokaryotic promoters are called consensus sequences because ________.
- they are identical in all bacterial species
- they are similar in all bacterial species
- they exist in all organisms
- they have the same function in all organisms
If mRNA is complementary to the DNA template strand and the DNA template strand is complementary to the DNA nontemplate strand, then why are base sequences of mRNA and the DNA nontemplate strand not identical? Could they ever be?
DNA is different from RNA in that T nucleotides in DNA are replaced with U nucleotides in RNA. Therefore, they could never be identical in base sequence.
In your own words, describe the difference between rho-dependent and rho-independent termination of transcription in prokaryotes.
Rho-dependent termination is controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the end of the gene, the polymerase stalls at a run of G nucleotides on the DNA template. The rho protein collides with the polymerase and releases mRNA from the transcription bubble. Rho-independent termination is controlled by specific sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides. This creates an mRNA hairpin that causes the polymerase to stall right as it begins to transcribe a region rich in A–T nucleotides. Because A–U bonds are less thermostable, the core enzyme falls away.
15.3: Eukaryotic Transcription
Which feature of promoters can be found in both prokaryotes and eukaryotes?
- GC box
- TATA box
- octamer box
- -10 and -35 sequences
What transcripts will be most affected by low levels of α-amanitin?
- 18S and 28S rRNAs
- 5S rRNAs and tRNAs
- other small nuclear RNAs
15.4: RNA Processing in Eukaryotes
Which pre-mRNA processing step is important for initiating translation?
- poly-A tail
- RNA editing
- 7-methylguanosine cap
What processing step enhances the stability of pre-tRNAs and pre-rRNAs?
- nucleotide modification
15.5: Ribosomes and Protein Synthesis
The RNA components of ribosomes are synthesized in the ________.
- endoplasmic reticulum
In any given species, there are at least how many types of aminoacyl tRNA synthetases?
Transcribe and translate the following DNA sequence (nontemplate strand): 5'-ATGGCCGGTTATTAAGCA-3'
The mRNA would be: 5'-AUGGCCGGUUAUUAAGCA-3'. The protein would be: MAGY. Even though there are six codons, the fifth codon corresponds to a stop, so the sixth codon would not be translated.
Explain how single nucleotide changes can have vastly different effects on protein function.
Nucleotide changes in the third position of codons may not change the amino acid and would have no effect on the protein. Other nucleotide changes that change important amino acids or create or delete start or stop codons would have severe effects on the amino acid sequence of the protein.