Base stacking Although the base pairing brought about by hydrogen bonding is responsible for the specificity of the base interactions, much of the stability of a duplex nucleic acid is due to interactions that result from base stacking. If you look back at Figure 3a, you will notice that, when seen from a side view, our schematic r
2.2 General features of higher-order nucleic acid structure Polynucleotide chains are intrinsically flexible molecules and have the potential to form many different higher-order structures. Their flexibility derives from rotation around bonds in the sugar-phosphate backbone (Figure 3b). In vivo, the structures that form are obviously determined by both the proteins that synthesise the nucleic acid chains (polymerases) and the ancillary proteins that bind to and modify them. We will discuss these aspects of structure later in this unit. What dri
1.2 Nucleic acids: genetic, functional and structural roles in the cell The first role that one immediately thinks about for nucleic acids is that of an inherited genetic material, principally in the form of DNA. In some cases, the inherited genetic material is RNA instead of DNA. For example, almost 60% of all characterised viruses have RNA genomes and these are more common in plant viruses than in animal viruses. There is considerable variation in the amount of genetic material present within organisms (Author(s):
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2.6 Obesity and ageing So, it seems that a part of the explanation for Ron's obesity, and the health problems that have led him to seek medical advice, may relate to the environment in which he is living, and more specifically the diet that he has chosen. It may also be a diet that is especially effective in activating the reward circuits in Ron's brain. However, Ron is also in his later middle age. A longitudinal study of people of this age in the USA suggests that average body weight increases by 1–2 kg per dec
1.5 Obesity At the time of writing (2004) 20% of the adult population of the UK is classified as obese. The number of obese children has doubled since 1982, 10% of six year olds and 17% of fifteen year olds are now classified as obese. As shown in Table 4, obesity is recognized when the BMI exceeds 30 and occurs quite simply wh
1.1 The components of a balanced diet A balanced diet contains six key nutrient groups that are required in appropriate amounts for health. These groups are outlined below. Proteins are involved in growth, repair and general maintenance of the body. Carbohydrates are usually the main energy source for the body. Lipids or fats are a rich source of energy, key components of cell membranes and signalling molecules, and as myelin they insulate neurons (nerve cells
4.2 The terrestrial carbon cycle Figure 1.10 shows the rates of natural carbon exchange between the terrestrial system and the atmosphere. 2.1 Some basic concepts Energy is defined as the capacity to do work, and Author(s): 7.5 Outstanding issues Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes. Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years. It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat 7.3 The central engine An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt. The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System. The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius Radio-loud AGNs The second model (Figure 36b) is similar to the first, but now the engine is producing a pair of jets that will eventually end in a pair of lobes, as seen in radio galaxies and some quasars. Looking at the model from the side, one expects to see narrow lines in the spectrum (but not broad lines) and two 5.3 The broad- and narrow-line regions In our model, the engine is surrounded by gas clouds (Figure 32). You have already seen how common these are in our own and other galaxies, so it is reasonable to expect them to be present in at least the spiral galaxies that contain AGNs. If these gas clouds are illuminated by ultraviolet or X-rays from the e 4.2 The size of AGNs AGNs appear point-like on optical images. It is instructive to work out how small a region these imaging observations indicate. Optical observations from the Earth suffer from ‘seeing’, the blurring of the image by atmospheric turbulence. The result is that star-like images are generally smeared by about 0.5 arcsec or more. One can do much better with the Hubble Space Telescope where, thanks to the lack of atmosphere, resolved images can be as small as 0.05 arcsec. What does this mean in 1 How did the notion of public dialogue arise? There is a good case to be made that the emphasis on ‘dialogue’ in relation to science and the public in the UK coincided with the publication in 2000 of the House of Lords report on Science and Society. But the impact of that report has to be seen in the context of what was happening under the ‘public understanding of science’ (PUS) banner in the years between the publication of the Bodmer report (1985) and the House of Lords report 15 years later. In the UK, this period 7.3 Disposal requirements When carrying out a risk assessment, you must consider disposal requirements. For example, any chemical d 6.1 Control of Substances Hazardous to Health (COSHH) Regulations For those companies involved in the handling of chemicals and/or biological material there are a number of additional regulations. These are known as the Control of Substances Hazardous to Health (COSHH) Regulations. These were introduced in 1988, and the last update was in 1999. These regulations apply to chemicals, biological hazards and dusts. The essential requirements are that the employer must: make an assessment of the health risk to empl References A strategy for ridding the world of VAD? In July 2000, Time magazine announced that a potential solution to VAD had been found – ‘Golden Rice’ (Figure 8). This was a variety of rice that had been genetically modified to introduce β-carotene into the endosperm (part of the grain of the rice). The name arises from the fact that the otherwise white grains of rice are given a golden colour by the presence of carotenoid compounds. The announcement came at the height of the global controversy over genetically modified 2.2 Using A. tumefaciens to genetically modify plant cells Genetic engineers have capitalised on the fact that part of the DNA from the Ti plasmid of A. tumefaciens is integrated into the plant genome during the infection process. Ti plasmids can be isolated and a foreign gene spliced in at an appropriate point, making it possible to transfer the novel gene into the plant.
