Solution Key - 7.013 Exam 2 (4/3/13) - With Answers (Page 3 of 7) in pdf

Question 3 (20 points)

Shown below is the schematic of Gene A. The numbers within the boxes indicate the length (in base

pairs) of each region. The DNA sequence corresponding to the translational start and the stop codons

and the splice donor and splice acceptor sites are indicated.

Promoter

Intron

Exon

Transcription stop site

Transcription start

site

’

3’

ATG

TAA

228

100

300

200

TAC

ATT

5’

’

Stop

Start

Splice

Splice donor

Splice

Splice donor

codon

acceptor

a) You observe that Gene A is transcribed both in epidermal and muscle cells to produce a nascent /

primary mRNA transcript. This mRNA directs the synthesis of two different proteins in these two

different cell types.

In the muscle cells Gene A encodes a protein (100 amino acids long) that functions as a nuclear protein

•

(TF-1)

In epidermal cells, Gene A encodes a protein (200 amino acids long) that functions as a cell membrane

•

protein.

Could Gene A direct the synthesis of two different proteins due to the…

Difference in….

Explain why you selected this option

Splicing (Yes/ No)?

These proteins can be produced from the same gene due to alternative splicing

of introns i.e. if the splice donor site of Intron1 base pairs with splice acceptor

site of Intron 2 you get a mature mRNA corresponding toTF-1. In

comparison, if both Introns 1 & 2 are spliced out as two separate exons you get

a mature mRNA transcript that encodes the cell membrane protein.

Protein processing

Yes, if you assume that the nascent polypeptide chain in muscle cells is post-

(Yes/ No)?

translationally cleaved to form functional protein of 100KD but it does not get

cleaved in epidermal cells. No, if you say that post translational modifications

such as glycosylation or addition of lipids may alter the molecular weight of

the proteins but will not have any impact on the primary amino acid length of

the proteins.

Promoter sequence

No, both proteins are encoded by the same gene and hence have the same

(Yes/ No)?

promoter. A change in the promoter sequence affects the amount of gene

expression but does not influence the type of gene products encoded by a gene.

b) You want to study another nuclear protein, TF-2 in mouse muscle cells. You identify a mutant cell

line, which shows a cytosolic location of TF-2 in muscle cells.

i. Name a stretch of amino acid sequence that the TF-2 in mutant muscle cell line lacks.

Nuclear localization sequence

ii. In the wild-type muscle cells, if this stretch of amino acid sequence is located at the N-terminus of

TF-2, where in the mature mRNA transcript (choose from the

5’ end

or the 3’ end) would the

corresponding base sequence be?

iii. The proteasome is a multi-protein complex that degrades any misfolded protein in a cell. How

does the proteasome recognize which proteins in the cell are misfolded?

The proteasomes recognize the misfolded proteins once they are ubiquitinylated

Solution Key - 7.013 Exam 2 (4/3/13) - With Answers Page 3

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