(a) the 30s ribosomal subunit (yellow) and the 50s subunit (blue) can be distinguished from the additional electron density that can be attributed to the release factor rF2 (purple). (B) the known molecular structure of rF2 modeled into the electron density from (a). (From U.B.s. rawat et al., Nature 421:87–90, 2003. With permission from Macmillan publishers Ltd.) macromolecules can often be “fitted” to the images derived by electron microscopy. In this way, the TEM is increasingly able to bridge the gap between structures discovered by x-ray crystallography and those discovered with the light microscope.
Figure Q9–1 paths of light rays through dry and oil-immersion lenses (problem 9–3). the red circle at the origin of the light rays is the specimen.
9–4 Figure Q9–2 shows a diagram of the human eye. The refractive indices of the components in the light path are: cornea 1.38, aqueous humor 1.33, crystalline lens 1.41, and
Which statements are true? Explain why or why not.
9–1 Because the DNA double helix is only 2 nm wide— well below the limit of resolution of the light microscope— it is impossible to see chromosomes in living cells without special stains.
9–2 A fluorescent molecule, having absorbed a single photon of light at one wavelength, always emits it at a longer wavelength.
Discuss the following problems.
9–3 The diagrams in Figure Q9–1 show the paths of light rays passing through a specimen with a dry lens and with an oilimmersion lens. Offer an explanation for why oilimmersion lenses should give better resolution. Air, glass, and oil have refractive indices of 1.00, 1.51, and 1.51, 9–5 Why do humans see so poorly under water? And why do goggles help?
9–6 Explain the difference between resolution and magnification.
9–7 Antibodies that bind to specific proteins are important tools for defining the locations of molecules in cells. The sensitivity of the primary antibody—the antibody that reacts with the target molecule—is often enhanced by using labeled secondary antibodies that bind to it. What are the advantages and disadvantages of using secondary antibodies that carry fluorescent tags versus those that carry bound enzymes?
9–8 Figure Q9–3 shows a series of modified fluorescent proteins that emit light in a range of colors. How do you suppose the exact same chromophore can fluoresce at so many different wavelengths?
Figure Q9–3 a rainbow of colors produced by modified fluorescent proteins (problem 9–8). (Courtesy of nathan shaner, paul steinbach and roger tsien.) 9–9 Consider a fluorescent detector designed to report the cellular location of active protein tyrosine kinases. A blue (cyan) fluorescent protein (CFP) and a yellow fluorescent protein (YFP) were fused to either end of a hybrid protein domain. The hybrid protein segment consisted of a substrate peptide recognized by the Abl protein tyrosine kinase and a phosphotyrosinebinding domain (Figure Q9–4A). Stimulation of the CFP domain does not cause emission by the YFP domain when the domains are 1.3 (problem 9–4). binding protein retina 1.0 vitreous humor 1.38. Where does the main refraction—the main focusing—occur? What role do you suppose the lens 1.2 plays?
1.1 iris of the human eye