Đề TBO lần 1

Đề thi TBO ngày 2 lần I
Thời gian làm bài: 120 phút.
Đề thi gồm có 10 câu hỏi.

I. SINH HỌC PHÂN TỬ

1. You are investigating a newly discovered operon in bacteria. This operon has two genes that are regulated together under the control of a single promoter and a single transcription factor. There are five regions:
- A regulatory gene for the transcription factor protein
- A promoter for the structural genes in the operon
- A binding site for the transcription factor
- A structural gene for an enzyme (Gene 1)
- A structural gene for a channel protein (Gene 2)
Expression of the wild-type operon is dependent on an external signal, namely the presence or absence of an amino acid. To determine the function of different regions of this segment of DNA, you make mutations and determine the effect on protein production in the presence and absence of the signal. The results are shown in the table. A plus sign (+) indicates that a functional protein is made; a minus sign (-) indicates that no functional protein is made.

	Signal		No signal
Mutation	Enzyme	Channel	Enzyme	Channel
None	-	-	+	+
A	-	-	-	-
B	-	-	+	-
C	+	+	+	+
D	+	+	+	+
E	-	-	-	+

Based on the effect of mutating the transcription factor gene and the binding site for the transcription factor, what do you now know about how this operon is regulated? Select all that apply. 

A. The transcription factor is a repressor. 

B. The transcription factor is an activator. 

C. The operon is under negative inducible regulation. 

D. The operon is under negative repressive regulation. 

E. The operon is under positive inducible regulation. 

F. The operon is under positive repressible regulation.

2. We know that TFIID is a big protein complex that comprises the important Tata Binding Protein (TBP) and several other TBP associated factors of TAFs. We also know that TBP is involved in recognizing the TATA box but the entire complex occupies a region of the promoter much larger than just the TATA box. In a quest to elucidate what the roles ofthe different TAF proteins are an in-vitro transcription assay was set up using the following different DNAs bearing four different promoters as templates.

The scientists measured transcription by a special assay that essentially looked at RNA made after transcription (which was labeled) and running a gel to visualize the presence and absence of RNA or the intensity of the band, which would demonstrate the efficiency of transcription They could also control which proteins they were adding to their assay.

The results of their in vitro assay are indicated below. The labels indicate the proteins they included in each reaction.

a. Based on your analysis of the data above answer True or False for the following statements

A. The TATA box is recognized by TBP but only as part of TFIID complex.

B. The TBP cannot initiate transcription of genes containing the TATA box in the absence of other factors.

C. TBP and TFIID allow for transcription equally well from the promoters that contained only the TATA box.

D. TFIID worked much better than TBP in supporting transcription from promoters that contain TATA box plus Inr plus DPE.

b. Pick any one statement that you indicate as TRUE' and explain the data in the experiment that indicates that it is true.

i.e which lanes are you comparing and what are you comparing

3. RT-PCR is a commonly used diagnostic method used for the detection of HIV infections, caused by an RNA virus. Briefly, following infection, the viral RNA genome is converted to a double stranded DNA which integrates into the genome. Following integration the virus may remain dormant for several years. Following activation, the viral DNA is transcribed and initiates its replicative cycle; spreading from cell to cell. The following figure illustrates RT-PCR reactions done on the blood from different individuals. Reactions were carried out in the presence and absence of reverse transcriptase in the reaction mixture (+RT and –RT respectively). Simultaneous RT-reactions were performed on the house keeping gene; GAPDH.

a) Which individual (s) is (are) HIV positive?
b) Which individual (s) is (are) HIV Negative?
c) Which individual (s) is (are) actively replicating HIV?
d) Which individual (s) has (have) more copies of the HIV genome integrated in their genome?
e) The above experiment was repeated as previously, but on DNAse treated samples. One of the patterns observed is shown below. To which individual (s) could this pattern correspond to?

II. DI TRUYỀN

4. There are two alleles (A and a) for gene A in bread mold (Neurospora crassa). With A, the ascospores could be dyed to red. With a, the ascospores cannot be dyed to red and remain as colorless. Two haploid cells mate with each other and their progeny asci are dyed with the following result:

	Pattern 1	Pattern 2	Pattern 3	Pattern 4	Pattern 5	Pattern 6
1st (top) ascospores	Red	Red	Red	Colorless	Colorless	Colorless
2nd ascospores	Red	Red	Red	Colorless	Colorless	Colorless
3rd ascospores	Colorless	Red	Colorless	Red	Colorless	Red
4th ascospores	Colorless	Red	Colorless	Red	Colorless	Red
5th ascospores	Red	Colorless	Colorless	Red	Red	Colorless
6th ascospores	Red	Colorless	Colorless	Red	Red	Colorless
7th ascospores	Colorless	Colorless	Red	Colorless	Red	Red
8th (bottom) ascospores	Colorless	Colorless	Red	Colorless	Red	Red
Observed # of asci	30	82	34	29	77	27

a. What are the genotypes of the two parental cells?

b. Calculate the distance between gene A and the centromere.

5. Five SSRs designated 1, 2, 3, 4, and 5, are known to map along chromosome 4 of corn. There is an A and B allele for each SSR. A plant breeder has obtained a strain of corn that carries a pesticide-resistance gene that (from previous experiments) is known to map somewhere along chromosome 4. The plant breeder crosses this pesticide-resistance strain that is homozygous for SSRs 1A, 2B, 3A, 4B, and 5A to a pesticide-sensitive strain that is homozygous for 1B, 2A, 3B, 4A, and 5B. The F1 generation plants were allowed to self-hybridize to produce the following F2 plants: 

Based on these results, which SSR does the pesticide-resistance gene map closest to? 

All of the corn plants in the F2 generation carry the SSR marker 4B. Thus the results suggests that the SSR marker 4B is mapped closest to the pesticide-resistance gene.

III. TIẾN HÓA

6. Computer simulations of the combined effect of natural selection and genetic drift were run in Populus. In all simulations, the effective population size, N_e = 50 and the initial allele frequency, p₀  = 0.5. Two different fitness sets were simulated 5 times and the number of generations until loss or fixation is tabulated below.

Fitness Set	Number of Generations Until Loss or Fixation
	Run1	Run2	Run3	Run4	Run5	Mean
W11 = 1, W12 = 1, W22 = 1	120	63	222	46	173	124
W11 = 0.1, W12 = 1, W22 = 0.9	27	21	45	64	30	37

a. What mode of selection is operating for each of these fitness sets?

b. What should happen to allele frequencies with each of these fitness sets in the absence of genetic drift? Why does loss or fixation happen more quickly with the second fitness set than the first?

7.  In humans, two genes encoding the opsin visual pigments that are sensitive to green and red wavelengths of light are found adjacent to one another on the X chromosome. They encode proteins that are 96 percent identical. Nonprimate mammals possess just one gene encoding an opsin sensitive to the red/green wavelength.
a. Offer one explanation for the presence of the two opsin genes on the human X chromosome.
b. How would you test your explanation further and pinpoint when in evolutionary history the second gene arose?