Chapter 15 Quiz
The AUC and AUA codons in mRNA both specify isoleucine. What feature of the genetic code explains this?
Degeneracy
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?
4
Where are the RNA components of ribosomes synthesized?
Nucleolus
What process transfers heritable material to the next generation?
Replication
You sequence a gene of interest and isolate the matching mRNA. You find that the mRNA is considerably shorter than the DNA sequence. Why is that?
The processed mRNA is shorter because introns were removed.
Which molecule in the central dogma can be compared to a disposable photocopy of a book kept on reserve in the library?
mRNA
A poly-A sequence is added at the —
3' end of a transcript in the nucleus
Which cell structure does NOT contain heritable information?
Cytoplasmic membrane
How many nucleotides are in 12 mRNA codons?
36
How would you describe the difference between rho-dependent and -independent termination of transcription in prokaryotes?
Rho-dependent termination is controlled by the rho protein, and the polymerase stalls near the end of the gene at a run of G nucleotides on the DNA template. In rho-independent termination, when the polymerase encounters a region rich in C-G nucleotides, the mRNA folds into a hairpin loop that causes the polymerase to stall.
What is the main structure that differentiates rho-dependent and -independent termination in prokaryotes?
Rho-independent termination involves the formation of a hairpin.
What is often the first amino acid added to a polypeptide chain?
Methionine
In any given species, there are at least how many types of aminoacyl tRNA synthetases?
20
Which pre-mRNA processing step is important for initiating translation?
Adding the 7-methylguanosine cap
What processing step enhances the stability of pre-tRNAs and pre-rRNAs?
Methylation
What is the major challenge in the production of RNA in eukaryotes compared to prokaryotes?
Exporting the mRNA across the nuclear membrane
A mutation in the promoter region of the gene for beta-globin can cause beta-thalassemia, a hereditary condition that causes anemia. Why would mutations in the promoter region lead to low levels of hemoglobin?
Globin chains do not fold properly and are nonfunctional.
Explain the initiation of transcription in prokaryotes. Include all proteins involved.
In prokaryotes, the polymerase comprises five polypeptide subunits, two of which are identical. Four of these subunits, denoted and compose the polymerase core enzyme. The fifth subunit, is involved only in the initiation of transcription. The polymerase, which comprises all five subunits, is called the holoenzyme.
If the protein is missing, will a prokaryotic gene be terminated?
It depends on the gene.
Suppose a gene has the sequence ATGCGTTATCGGGAGTAG. A point mutation changes the gene to read ATGCGTTATGGGGAGTAG. How would the polypeptide product of this gene change?
It would change from MRYRE to MRYGE.
If mRNA is complementary to the DNA template strand and the DNA template stand is complementary to the DNA non-template strand, why are the base sequences of mRNA and the DNA non-template strand not identical? Could they ever be?
No, they cannot be identical because the T nucleotide in DNA is replaced by the U nucleotide in RNA, and AUG is the start codon.
Which polymerase is responsible for the synthesis of 5S rRNA?
Polymerase III
What transcripts will be most affected by low levels of -amanitin?
Pre-mRNAs
Which molecule does NOT contain genetic information?
Protein
Which step in the transcription of eukaryotic RNA differs the most from its prokaryotic counterpart?
The initiation step in eukaryotes requires an initiation complex with enhancers and transcription factors. Also, the separation of the DNA strand is different, as histones are involved.
A tRNA is chemically modified so that the bound amino acid is different than the one specified by its anticodon. Which codon in the mRNA would the tRNA recognize: the one specified by its anticodon or the one that matches the modified amino acid it carries?
The anticodon will match the codon in mRNA.
How does the enzyme reverse transcriptase violate the central dogma of molecular biology in HIV?
The enzyme reverse transcriptase reverse-transcribes the RNA in the genome of HIV into DNA.
What part of the central dogma is NOT always followed in viruses?
The flow of information in HIV is from RNA to DNA, then back to RNA to protein. Influenza viruses never go through DNA.
What characteristic of the genetic code points to a common ancestry for all organisms?
The genetic code is almost universal.
How should the following DNA sequence (non-template strand) be transcribed and translated?5'-ATGGCCGGTTATTAAGCA-3'
The mRNA would be 5'-AUGGCCGGUUAUUAAGCA-3' and the protein will be MAGY.
What would happen if the 5' methyl guanosine was not added to an mRNA?
The transcript would degrade when the mRNA moves out of the nucleus to the cytoplasm.
When comparing transcription of heritable information in prokaryotes and eukaryotes, which events are the same?
The transcription by polymerase, the recognition of a consensus sequence in the promoter, and the termination by a hairpin loop are conserved.
Why are the -10 and -35 regions of prokaryotic promoters called consensus sequences?
They are similar in all bacterial species.
What are introns?
Untranslated DNA sequences in a gene
Would you be able to determine which RNA polymerase you isolated from a eukaryotic cell without analyzing its products?
Yes, they can be determined as they differ in -amanitin sensitivity.
In prokaryotic cells, ribosomes are found in/on the —
cytoplasm
Which subunit of the E. coli polymerase confers specificity to transcription?
o