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Biology 180 (Molecular Biology) Lecture
Exam 4
10Dec2003
Hao Nguyen
Name: ____________________
NOTE: No abbreviation. Be thorough.
1. 3’ – poly(A)-UAAUUUAGAUCUUGUGGUACCCGGAGACUCUUAGAGGACCAUUGGUGUACCUUCGGCUACUGA-Cap – 5’
NOTE: Start codon = AUG ; Stop codon = UAG
Please, write out the amino acid sequence (polypeptide) that can be synthesized using the above mRNA sequence as
the template. Be sure to indicate direction of the polypeptide. (20 points)
N-terminus – Methionine-Tryptophane-Leucine-Proline-Glycine-Aspartate-Serine-Glutamine-Arginine-ProlineMethionine-Valine-Phenylalanine – C-terminus
2. The above mRNA sequence (in question #1) contains 2 out of the three major post-transcriptional modifications.
Please, describe all the functions of these two post-transcriptional modifications. (20 points)
NOTE: a) Only describe the functions of the 2 modifications that are shown. DO NOT describe the function of
the third.
b) Only describe the functions, NOT the mechanics of the modifications themselves.
c) Clearly indicate which function is related to which modification.
I.
The Cap is placed at the 5’-end of the mature mRNA whose functions are:
a. to aid in the transport of the mature mRNA from the nucleoplasm into the cytoplasm. The Cap is
recognized and bound by certain nuclear proteins that catalyze the transport of the mRNA.
b. to protect to the mRNA from being digested at the 5’-end once in the cytoplasm.
c. to be recognized and bound by the ribosome to initiate translation.
II. The poly(A) tail is placed at the 3’-end of the mature mRNA [catalyzed by poly(A) polymerase through a
process called polyadenylation] whose function is to slow down the degradation of the mRNA at the 3’-end
once in the cytoplasm.
3. What is so unique about the function of small nuclear RNA’s (snRNA)? (5 points)
Small nuclear RNA is a very important component of a complex called small nuclear ribonuclear protein (or
snRNP). It [snRNA] possesses enzymatic activities that are essential during mRNA splicing.
4. What is produced immediately at the end of translation? Also, include the direction of synthesis. Please, be very
specific. (5 points)
A polypeptide; the direction of synthesis is from the amino- (or N-) teminus to the carboxy- (or C-) terminus
5. What is produced immediately at the end of transcription (discussed in class) in a eukaryotic cell? Also, include
the direction of synthesis. Please, be very specific. (5 points)
A primary mRNA transcript; the direction of synthesis is from the 5’-end to the 3’-end
6. What is the function of RNA splicing? (5 points)
Noncoding regions (introns or intervening sequences) are removed during RNA splicing leaving the mature
transcript with only coding regions (exons). RNA splicing also allows the cell to synthesize multiple forms (versions) of
homologous proteins from the same gene.
7. Please, explain the Wobble theory (hypothesis). Include the following facts: a) tell me what it is; b) what are the
non-Watson-Crick basepairs; c) location; and d) why is this necessary (that is, what is the function). (20 points)
The Wobble hypothesis (or theory) stated that non-Watson-Crick basepairing occur between the codon and
anticodon to explain the fact that fewer (32) tRNA’s (containing anticodon) can recognize 61 different codons.
During the synthesis of the tRNA, an adenine (A) at the 5’ position of the anticodon is usually converted into an
inosine (I). I may basepair with adenine, uracil, and cytosine found in the 3’ position of the codon. A guanine in the 5’
position of the anticodon may also basepair with a uracil in the 3’ position of the codon.
The following are the proposed non-Watson-Crick (or Wobble) basepairs: I/A , I/U, I/C, and G/U.
Wobble basepairing is necessary to allow (and account for) 32 tRN’s to recognize and basepair with 61 codons to
code for 20 major amino acids during the synthesis of a polypeptide. This process saves the cell a lot of energy since
the transcription of tRNA’s requires an enormous amount of energy. Once made, the tRNA’s are recyclable (may be
used again and again like an enzyme).
8. Given a hypothetical cell where each codon on an mRNA consists of four bases. There are only three different
nucleotides available in this hypothetical cell. A primary mRNA transcript that is 1200-bases long and contains 4
exons and 3 introns is synthesized. Exon 1 is 260-bases long, Exon 2 is 200-bases long, Exon 3 is 220-bases
long, and Exon 4 is 180-bases long. The last codon on Exon 4 (at the 3’-end) is the stop codon. Intron 1 is 120bases long, Intron 2 is 140-bases long, and Intron 3 is 80-bases long. The primary transcript undergoes posttranscriptional modification to produce a mature mRNA, which contains all four exons.
a. How many possible codons could there be in this hypothetical cell? Please, show all work.
NO CALCULATOR NEEDED, only need to set up the calculation. Need to write all symbols clearly. (5 points)
4
3 = 81 possible codons
b. How long (that is, how many amino acids) will the polypeptide be after translation using the above mature
mRNA as the template? Please, show all work. NO CALCULATOR. Do need an actual number as an
answer for this question. (15 points)
Exon 1 + Exon 2 + Exon 3 + Exon 4 = 260 bases + 200 bases + 220 bases + 180 bases = 860 bases
Total bases – stop codon = 860 bases – 4 bases = 856 bases
856 bases  4 [4 bases per codon] = 214 amino acids in the synthesized polypeptide