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FCH 532 Homework 8
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1. Write out the different mechanisms for the GU-AG intron, Group I intron, and
spliceosome intron splicing.
GU-AG intron
Group I intron
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Spliceosome intron
2. Why can’t eukaryotic transcription be regulated by attenuation?
In eukaryotes, transcription occurs in the nucleus and translation occurs in the
cytoplasm. Because ribosomes are an essential aspect of the attenuation mechanism,
this will not happen in the nucleus or eukaryotes.
3. Draw out the steps for the initiation, elongation, and release of the polypeptide
during translation of an mRNA sequence. Use pictures and diagrams to help
4. Define the following terms:
a. Initiation factors- Proteins that are necessary for the initiation of translation.
Include IF-1, IF-2, and IF-3.
IF-3-binds to inactive ribosome 30S subunit and displaces the 50S subunit
IF-2 binds to GTP and fMet-tRNA
IF-1 binds to mRNA and 30S subunit to make initiation complex.
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b. Elongation factors-EF-Tu, EF-Ts, EF-G
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c. Release factors-RF 1, 2, 3, RRF
RF-1 = UAA
RF-2 = UAA and UGA
Cannot bind if EF-G is present.
RF-3-GTP binds to RF1 after the release of the polypeptide.
Hydrolysis of GTP on RF-3 facilitates the release of RF-1 (or RF-2).
EF-G-GTP and ribosomal recycling factor (RRF)-bind to A site. Release
of GDP-RF-3
EF-G hydrolyzes GTP -RRF moves to the P site to displace the tRNA.
RRF and EF-G-GDP are released yielding inactive 70S
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d. Post-translational modification-Covalent modifications, Primary structure is
altered, Derivates are introduced into amino acid side chains, Reaction is
catalyzed by a specific enzyme.
•Proteolytic cleavage
•Formation of S-S bonds
•Protein splicing
•Acylation
•Glycosylation
•Hydroxylation
•Methylation
•Phosphorylation
•ADP-ribosylation
e. Synonyms- describes different codons that encode the same amino acid
f. Nonsense mutation– a point mutation that results in converting a codon that
normally encodes an amino acid in a protein into a premature stop codon.
g. Nonsense suppressor- a mutation that counters a nonsense mutation by
inserting an amino acid instead of a stop codon.
h. Missense suppressor-act similarly to nonsense suppressors but substitute one
amino acid for another
i. Frameshift suppressor -have 8 nucleotides in their anticodon loops rather than
the normal 7 so they are able to read 4 base codons beyond a base insertion in
order to restore reading frame mutations.
j. Amber codon- UAG stop codon
k. Transitions-one purine or pyrimidine is replaced by another.
l. Transversions-purine is replaced by a pyrimidine or vice versa.
m. 5-bromouracil-a base analog that resembles T but because of the
electronegative Br, tautomerizes to to pair with G instead of A. If 5 BU is
incorporated into DNA in place of T, can induce A-T to G-C transitions. Can
also incorporate in place of C to generate G-C to A-T transitions.
n. 2-aminopurine-adenine analog that normally base pairs with thymine but can
base pair with cytosine.
o. essential amino acids-must be taken in from diet.
p. nonessential amino acids-mammals can synthesizee nonessential amino acids
from metabolic precursors.
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5. What is the product of the reaction of guanine with nitrous acid? Is this
mutagenic? Explain.
xanthine can base pair with C has 2 H bonds instead of 3. Therefore it is not mutagenic.
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6. The fingerprint of a protein from a phenotypically revertant mutant of
bacteriophage T4 indicates the presence of an altered tryptic peptide with respect
to the wild type. The wild-type and mutant peptides have the following sequences:
Wild type
Cys-Glu-Asp-His-Val-Pro-Gln-Tyr-Arg
Mutant
Cys-Glu-Thr-Met-Ser-His-Ser-Tyr-Arg
Indicate how the mutant could have arisen and give the base sequences, as far as possible,
of the mRNAs specifying the two peptides. Comment on the function of the peptide in
the protein.