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exam 3 key - Name BIOLOGY 325H Genetics Spring 2008 Exam 3...

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Name ___________________ BIOLOGY 325H Genetics Spring 2008 Exam 3 Before you do anything else , please: check to make certain that your copy of the exam is complete, i.e. 6 pages. print your name legibly in the space provided at the top of every page. _______________________________________________________ 1. [10 pts] A human subject is exposed to bright white light followed by complete darkness, causing his retina to undergo dark adaptation. As shown at the right, visual sensitivity - measured as the minimum intensity of light he can detect - improves as a function of time in the dark. A. What is the principal reason that the subject's retina becomes more sensitive to light during this time? The bleached opsin proteins reassociate with molecules of 11-cis retinal. B. After 5 minutes the subject's visual sensitivity has reached a plateau value of 6 units. But his retina then goes through a second phase of dark adaptation, and after another 10-15 minutest reaches a plateau value of 3.5 units. From a biological standpoint, what is different about these two phases of dark adaptation? The first phase of dark adaptation reflects the recovery of light sensitivity by the cones; the second phase reflects the recovery of the rods.
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Name ___________________ 2. [15 pts] The graph at the right shows the absorption spectra of the photopigments found in the red and green cones of the human retina. A. What is different about these two photopigments that causes them to have different absorption spectra? These photopigments contain opsin proteins whose structure is subtly different because they are encoded by different genes. B. A neuroscientist purifies the green photopigment, and places it in a device that measures light absorption. When she shines monochromatic light of 500 nm wavelength onto the green pigment sample, 30% of the light is absorbed and 70% passes through the sample. If she were to illuminate the pigment sample with the same intensity of 450 nm light, what percentage of that light will be absorbed? 15% The curves above show relative absorption . From this, you can see that the green pigment absorbs 0.5X as much light at 450 nm as 500 nm. The question asks about actual absorption , which includes things you aren't told such as how many photons actually hit pigment molecules. But you are told that the pigment sample absorbs 30% of the 500 nm light that shines on it, so you can calculate 30% X 0.5 = 15% absorption for 450 nm light. C. The human retina contains three different kinds of cone. Yet we are able to distinguish millions of different colors. How is this possible? Color is a perception that reflects the ratio of signal strength in three separate channels, i.e. the red, green, and blue cones. Any one of these cones has a graded response to light of varying intensity, and the number of possible ratios between the different color channels vastly exceeds the number of channels.
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