06413_16 - P P(parental generation The individuals that are...

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P P (parental generation) The individuals that are selected to begin a breeding ex- periment, crosses between which yield the * F 1 generation. Only pure-breeding (homozygous) individuals are selected for the P generation. pacemaker 1. (sinoatrial node) A small mass of specialized muscle cells in the mammalian heart, found in the wall of the right atrium near the opening for the vena cava. The cells initiate and maintain the heart beat: by their rhythmic and spontaneous contractions they stimulate contraction of the atria ( see also atrioven- tricular node ). The cells themselves are controlled by the autonomic nervous sys- tem, which determines the heart rate. Similar pacemakers occur in the hearts of other vertebrates. 2. An electronic or nu- clear battery-charged device that can be implanted surgically into the chest to produce and maintain the heart beat. These devices are used when the heart’s own pacemaker is defective or diseased. pachytene The period in the Ü rst prophase of * meiosis when paired * ho- mologous chromosomes are fully con- tracted and twisted around each other. packing density 1. The number of de- vices (such as * logic circuits) or inte- grated circuits per unit area of a * silicon chip. 2. The quantity of information stored in a speci Ü ed space of a storage system associated with a computer, e.g. * bits per inch of magnetic tape. packing fraction The algebraic differ- ence between the relative atomic mass of an isotope and its mass number, divided by the mass number. paedogenesis Reproduction by an ani- mal that is still in the larval or pre-adult form. Paedogenesis is a form of * neoteny and is particularly marked in the axolotl, a larval form of the salamander, which retains its larval features owing to a thy- roid de Ü ciency but can breed, producing individuals like itself. If the thyroid hor- mone thyroxine is given, metamorphosis occurs. pahoehoe See lava . pairing (synapsis) The close association between * homologous chromosomes that develops during the Ü rst prophase of * meiosis. The two chromosomes move together and an exact pairing of corre- sponding points along their lengths oc- curs as they lie side by side. The resulting structure is called a bivalent . pair production The creation of an electron and a positron from a photon in a strong electric Ü eld, such as that sur- rounding an atomic nucleus. The electron and the positron each have a mass of about 9 × 10 –31 kg, which is equivalent on the basis of the mass–energy equation ( E = mc 2 ) to a total of 16 × 10 –14 J. The fre- quency, ν , associated with a photon of this energy (according to E = h ν ) is 2.5 × 10 20 Hz. Pair production thus requires photons of high quantum energy (Bremsstrahlung or gamma rays). Any ex- cess energy is taken up as kinetic energy of the products.
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06413_16 - P P(parental generation The individuals that are...

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