Tài liệu Khoa Học Tự Nhiên

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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The genome of viruses includes other options than the double-stranded DNA that we have studied. Viral genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the specific type of virus. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. ...

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One type of mutant required only the addition of arginine to the minimal growth medium. Beadle and Tatum concluded that this mutant was defective somewhere in the biochemical pathway that normally synthesizes arginine. They identified three classes of arginine-deficient mutants, each apparently lacking a key enzyme at a different step in the synt...

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Further evidence that DNA was the genetic material was derived from studies that tracked the infection of bacteria by viruses. Viruses consist of a DNA (sometimes RNA) enclosed by a protective coat of protein. To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery. Viruses that specifically attack bacteria are ca...

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Morgan observed this linkage and its deviations when he followed the inheritance of characters for body color and wing size. The wild-type body color is gray (b+) and the mutant black (b). The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg). Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous recessive m...

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In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. It was mainly Mendel’s quantitative analysi...

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In asexual reproduction, a single individual passes along copies of all its genes to its offspring. Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells. Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis.

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