The double membrane. It's theorized that chloroplasts used to be their own single-celled organism in the environment (and therefore with its own membrane). Then, the theory states, a larger cell took in the chloroplast by endocytosis, which involves surrounding the chloroplast in the large cell's own membrane. The chloroplast would then have two membranes surrounding it. This is similar to the thinking for mitochondria becoming an organelle as well. Hope this helps! :)
Well, when chromosomes cross over during prophase 1 of meiosis, the homologous pair of chromosomes exchange segments of DNA which happen to be the genes. So the significance of crossing-over during prophase 1 is that it adds genetic variation to sexually reproducing organisms.
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Don’t quote me, but A.
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There is a bug, called a Water Strider, that use surface tension to ‘walk on water.’
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The logistic population growth happens as the growth rate declines as the population exceeds the carrying power. The highest number of people in a community that the ecosystem can sustain is carrying capacity.
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