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MIT7 01SCF11 exam2 sol.pdf
MIT7_01SCF11_exam2_sol.pdf
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MIT7 01SCF11 exam2 sol.pdf-Solutions to Molecular ...
MIT7_01SCF11_exam2_sol.pdf-Solutions to Molecular Biology Unit Exam
MIT7 01SCF11 exam2 sol.pdf-Solution...
MIT7_01SCF11_exam2_sol.pdf-Solutions to Molecular Biology Unit Exam
Page 5
Question 4
a) Below is a schematic of gene Y, which encodes protein Y. The promoter region is indicated by the dotted
box. Transcription begins immediately following the promoter.
5’
3’
Promoter
Exon 1
Intron 1
Intron 2
1
1100
2100
Exon 2
3100
4100
5100 base pairs
Positions 200-203 = Start codon
100
Positions 4800-4802 = Stop codon
Transcriptional terminator
Exon 3
•
The transcript first produced by this gene would be approximately how many nucleotides long?
The transcript first produced by this gene would be approximately 5000 nucleotides long.
•
Two different transcripts are produced from this gene, one is approximately 2000 nucleotides long,
the other is approximately 3000 nucleotides long. Explain how two different transcripts can be
produced from this gene.
Alternative splicing could produce two different transcripts. One transcript could contain exons 1, 2
and 3, the other could contain only exons 1 and 3.
•
Assume each transcript produces a protein. Given your answer above and the diagram, what is the
approximate size of the two proteins produced from this gene?
The transcript contain all three exons would encode a protein beginning at the ATG at position 200
and end at the stop codon. This is about 2600 nucleotides, which represents about 866 codons.
SO
the two proteins expected would be about 866 amino acids long and about 533 amino acids long.
b) Assume gene Y was mutated such that the base pair found at position 200 was changed from an A/T to a
G/C. Transcription and translation of the altered gene Y still occurs.
•
Would the first transcript be the same length, shorter or longer than the first transcript produced from
the wild type gene Y? Give all possible answers and explain your thinking.
This mutation changes a single base pair well after the promoter, so it is unlikely to affect
transcription. The first transcript is expected to be the same length.
•
Would the protein produced be the same length, shorter or longer than the protein produced from the
wild type gene Y? Give all possible answers and explain your thinking.
This mutation changes a single base pair, eliminating the start codon. It is impossible to tell what
affect this will have on the protein. If there is another start codon that maintains the original reading
frame, then the protein will be shorter. If there is another start codon that changes the reading frame,
then the protein will certainly be different.
Because the location of the stop codon is also altered in
this alternative reading frame, the resulting protein might be longer or shorter or even the same.
c) Now assume gene Y was mutated such that only the base pair found at position 2100 was changed from a
C/G to an A/T. Transcription and translation of gene Y still occur.
•
Would the first transcript be the same length, shorter or longer than the first transcript produced from
the wild type gene Y? Give all possible answers and explain your thinking.
This mutation changes a single base pair well after the promoter, so it is unlikely to affect
transcription. The first transcript is expected to be the same length.
•
Would the protein produced be the same length, shorter or longer than the protein produced from the
wild type gene Y? Give all possible answers and explain your thinking.
This mutation changes a single base pair of a single codon.
There are two possibilities.
If the new
codon still encodes an amino acid, then the resulting protein will be the same length. If the new codon
creates a stop codon, the resulting protein will be the shorter.
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