AS Physics Handwritten Notes 2015


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#140 Artificial selection

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Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock. - selective pressure: humans - individuals with desirable features are chosen to interbreed = selective breeding - some of these desirable alleles are passed onto offspring - offspring with the most desirable features are chosen to interbreed - this is repeated over many generations Over many generations, alleles deemed desirable by the breeder increase in frequency, while the 'disadvantageous' ones may completely disappear over time. Dairy cattle - desirable features: docility, fast growth rates, high milk yield - cows with desirable features are chosen to interbreed, and so are their offspring. This is repeated over many generations. Bulls cannot be assessed for milk production as this is a sex-limited trait. Therefore, progeny testing is used: the performance of the bull's femail offpsirng is looked at to see whether or not t...
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# 29 DNA structure

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In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each  chromosome  is made up of  DNA  tightly coiled many times around proteins called  histones  that support its structure. Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during  cell division  and is then visible under a microscope.  Each chromosome has a constriction point called the  centromere,  which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. A  chromatid  is 1 of the 2 identical strands of DNA that make...
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#101 Photosynthetic Pigments

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Photosynthetic pigments are pigments presented in chloroplasts or photosynthetic bacteria. They capture light energy necessary for photosynthesis and convert it to chemical energy .  Pigments A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light that are reflected (not absorbed).  If pigments absorb all wavelengths they will appear black. If pigments reflect most of the wavelengths they will appear white. The light absorption pattern of a pigment is called the absorption spectrum. When pigments absorb light, electrons are temporarily boosted to a higher energy level. Energized electrons move further from the nucleus of the atom. When the e- returns to a lower energy level the energy may be: dissipated as heat re-emitted as a longer wavelength of light - fluorescence captured in a chemical bond (carbon gain!) Photosynthetic pigments in chloroplast Choloroplats contain several different pigments, which absorb different wavele...
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