Tài liệu Sinh Học

Reproduction and life cycles are highly varied among protists. Mitosis occurs in almost all protists, but there are many variations in the process. Some protists are exclusively asexual or at least employ meiosis and syngamy (the union of two gametes), thereby shuffling genes between two individuals. Others are primarily asexual but can also rep...

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Molecular evidence accumulated over the last two decades has led to the conclusion that there are two major branches of prokaryote evolution, not a single kingdom as in the five-kingdom system. These two branches are the bacteria and the archaea. The archaea inhabit extreme environments and differ from bacteria in many key structural, biochemical...

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For the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular. The Earth formed about 4.5 billion years ago, but rock bodies left over from the origin of the solar system bombarded the surface for the first few hundred million years, making it unlikely that life could survive. No clear fossi...

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Rarer than mineralized fossils are those that retain organic material. These are sometimes discovered as thin films between layers of sandstone or shale. As an example, plant leaves millions of years old have been discovered that are still green with chlorophyll. The most commonfossilized material ispollen, which has ahard organic casethat r...

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No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers. Typically, these barriers are intrinsic to the organisms, not simple geographic separation. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges ove...

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The Origin of Species convinced most biologists that species are the products of evolution, but acceptance of natural selection as the main mechanism of evolution was more difficult. What was missing in Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for ...

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In contrast to Cuvier’s catastrophism, James Hutton, a Scottish geologist, proposed that the diversity of landforms (e.g., canyons) could be explained by mechanisms currently operating. Hutton proposed a theory of gradualism, that profound change results from slow, continuous processes. Later, Charles Lyell proposed a theory of uniformitarianism,...

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Apical meristems, perpetually embryonic regions in the tips of shoots and roots, are responsible for the plant’s continual growth and formation of new organs, such as leaves and roots. In animals, ongoing development in adults is restricted to the differentiation of cells, such as blood cells, that must be continually replenished. The importance ...

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The potential uses of cloned genes fall into two general categories. First, the goal may be to produce a protein product. For example, bacteria carrying the gene for human growth hormone can produce large quantities of the hormone for treating stunted growth. Alternatively, the goal may be to prepare many copies of the gene itself. This may en...

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As chromosomes enter mitosis the beaded string undergoes higher-order packing. The beaded string coils to form the 30-nm chromatin fiber. This fiber forms looped domains attached to a scaffold of nonhistone proteins.

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