1. A scientist has identified a new membrane receptor. He wishes to determine the hormone-binding

Transcription

1 1. A scientist has identified a new membrane receptor. He wishes to determine the hormone-binding specificity of the receptor. Three different hormones, X, Y, and Z, were mixed with the receptor in separate experiments, and the percentage of binding capacity of the receptor was determined as function of hormone concentration, as shown in graph A. He also measures adenylate cyclase as a function of hormone concentration as shown in graph B. i. What concentrations of each hormone yield 50% maximal binding? i Which hormone shows the highest binding affinity for the receptor? What can you conclude about the mechanism of action of the hormone-receptor complex? Carefully observe hormone concentration at 50% of binding to receptor. Compare the hormone concentrations of the three hormones to fill half of the sites. Match the binding affinity of the hormonereceptor complex and the ability of the hormone to enhance adenylate cyclase activity. The scientist now wishes to determine the number of receptors specific for a ligand X, which he has in both radioactive and nonradioactive forms. In one experiment, he adds increasing amounts of radioactive X and measures how much of it is bound to the cells. Next, he performs the same experiment, except that he includes a several hundredfold excess of nonradioactive X. 8 MTBIOLOGY TODAY september 14 Page 8

2 The result of the experiments is as shown through the given graph. iv. 2. What is the purpose of performing the above experiments? v. Identify the curve for the two experiments in the above given graph. vi. Receptor has high affinity for the ligand, hence, for nonradioactive ligand. Receptor binding sites in the cell membrane is limited. Draw the curve for specific binding of ligand and the receptor in the given graph. Now, the scientists wants to calculate the number of receptors in the cell membrane. The specific activity of the ligand is cpm per millmole and that the maximal specific binding is 10 4 cpm per milligram of membrane protein. There are cells per milligram of membrane protein. Assume that one ligand binds per receptor. v Calculate the number of receptor molecules present per cell. The given figure shows the arrangement of the lac operon and its regulatory gene. A series of mutations was introduced to the lac operon. i. The operator is deleted. Assuming lactose is present, what is the effect of this on the transcription of lac Z, lac Y and lac A genes? A point mutation was introduced into lac Z, resulting in the production of a truncated non-functional protein. Assuming lactose is present, explain if the proteins encoded by lac Y and lac A gene are produced? i lac operon is an inducible operon. Prokaryotic mrna is polycistronic. An E. coli strain was isolated from the intestines of a mammal. The bacterium was found to have two different copies of the lac operon. Describe a genetic transfer process that resulted in this observation. MTBIOLOGY TODAY september 14 9 Page 9

3 3. The given cross section through the A bands shows the regular arrangement of overlapping thick and thin filaments. Each thick filament is surrounded by a hexagonal array of six thin filaments, and each thin filament is surrounded by a triangular arrangement of three thick filaments. Altogether, there are twice as many thin as thick filaments in the region of filament overlap. Draw a cross section diagram like the one shown through the (i) H-zone, (ii) I-band, (iii) M-line and (iv) Z-line. 4. (i) (ii) (iii) (iv) Chicken have a structure called a comb on their heads. The given diagram shows two types of comb. The shape of the comb is controlled by two alleles of one gene. The allele for pea comb, A, is dominant to the allele for single comb, a. The colour of chicken eggs is controlled by two alleles of a different gene. The allele for blue eggs, B, is dominant to the allele for white eggs, b. The genes for comb shape and egg colour are situated on the same chromosome. i. A farmer crossed a chicken that had pea comb and produced blue eggs with a chicken that had a single comb and produced white eggs. The chicken that had a pea comb and produced blue eggs are the offsprings of the cross between pure bred pea comb, blue eggs and pure bred single comb, white eggs. The farmer could identify which of the female offspring from this cross would eventually produce blue eggs. Explain how. i However, in certain crosses, the farmer was not able to identify the female offspring which will produce blue eggs. Explain why. In chickens the males are the homogametic sex while the Cross Genotype of females are the heterogametic sex. male parent A gene on the X chromosome controls the rate of feather The allele for slow feather production, F, is dominant to the allele for rapid feather production, f. A farmer made a cross between two chickens with known genotypes. He chose these chicken so that he could tell the sex of the offspring soon after they hatched by looking at their feathers. Which of the crosses shown in the table did he make? Explain your answer. Genotype of female parent A X F X F X f Y B X F X f X f Y C X f X f X F Y D X F X f X F Y 10 MT BIOLOGY TODAY september 14 Page 10

4 5. Plants have developed defense mechanisms against pathogens such as bacteria, fungi and viruses. Chemicals released by these pathogens can trigger a defense response in infected plant cells. For example, the production of hydrogen peroxide (H 2 ) which reacts with pathogen membranes and cellular chemicals, eventually kills both the cell and the pathogen. The OSRac1 gene was isolated and introduced into a number of rice plant (Oryza spp.) lines to study its role in disease resistance of plants to Blast fungus. Experiments are carried out to see if the OSRac1 gene is part of the signalling pathway for hydrogen peroxide i. How would you design an experiment to test the role of OSRac1 gene in the production of hydrogen peroxide? What conclusion can be best drawn from the given data? 1.(i) X 10 7 M; Y ; Z 10 3 M (ii) Because much less X is required to fill half of the sites, X displays the highest affinity. (iii) The binding affinity almost perfectly matches the ability to stimulate adenylate cyclase, suggesting that the hormone-receptor complex leads to the stimulation of adenylate cyclase. (iv) The purpose of adding radioactive ligand 'X' is to determine total binding of 'X' to the membrane as radioactive ligand binds to different receptors on the membrane. The rationale to perform second experiment is that receptor will have a high affinity for the ligand. Thus, in the presence of excess nonradioactive ligand, the receptor will bind to nonradioactive ligand. Therefore, any binding of the radioactive ligand must be nonspecific. (v) P is total binding and Q is nonspecific binding. solutions (vi) The difference between the two curves is specific binding. (vii) Number of receptors per cell = 4 10 cpm mg of membrane protein mg of membrane protein cells mmol molecules = cpm mmol MTBIOLOGY TODAY september Page 11

5 2.(i) There is no operator for the repressor to bind to thus; RNA polymerase can bind to promoter to start transcription of the genes. (ii) Yes, they will be produced the enzymes are translated separately because each has its own start and stop codon on the polycistronic mrna, so an unexpected stop codon in one does not affect the others. (iii) Conjugation F plasmid carrying a second copy of lac operon is transferred to this bacterial cells; Transduction Bacteriophage added a second copy of lac operon from a previous host; Transformation DNA fragment containing a second copy of lac operon taken up by cell (i) The cross can be illustrated as following: separate alleles; if crossing over occurred some gametes will contain alleles A and b (pea comb, white eggs). (iii) Cross C/X f X f and X F Y where all male offsprings produced are of one phenotype and all females are of contrasting phenotype. Thus there is no confusing situation as all male offsprings are of slow feather production and all female offsprings are of fast feather 5.(i) An experiment can be designed by having five plants i.e., a control and four genetically modified rice plants, two of which have OSRac1 gene turned 'ON' and rest two have OSRac1 gene turned 'OFF' or suppressed. Then, the five plants are exposed to chemicals known to initiate defence response and the production of hydrogen peroxide. Make three sets for comparison and to find result: of control with transgenic plants having OSRac1 gene. of control with transgenic plants having suppressed OSRac1 gene. of two types of transgenic plants. Infer and draw conclusion. (ii) Transgenic plants with functioning gene showed more H 2 OR Transgenic plants with suppressed gene showed less H 2 Hence, pea comb offspring will produce blue eggs as alleles A and B are inherited together/are on the same chromosome. (ii) As can be seen in the cross, the crossing over will 12 MT BIOLOGY TODAY september 14 Page 12