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Ideally, DNA itself is the same in DNA-containing cell types. Cell type depends on expression, which depends on epigenetics. I.e. histone modification, DNA methylation, and few other mechanisms. Also, epigenetics is the mechanism for gene expression regulation. For example, it can silence (usually, methylate promoter regions) an expression of genes on one ...


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Most cells in a human body contain a complete set of the genome, which is two sets of 23 chromosomes. Having two of each chromosome is called diploidy. Within an individual human the DNA is approximately identical in every cell. Different cells are produced by differential use of that DNA: certain genes are more (or less) highly expressed etc. You can read ...


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In oviparous animals (those that lay eggs which hatch outside the body), the eggs need to be provided nutrition for embryonic growth, which is the main function of yolk. In placental mammals, the embryo is provided nutrition directly from the mother via the placenta. Therefore, most mammalian oocytes do not need to develop large quantities of yolk to ...


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The 2nd polar body contains almost no cytoplasm, so if only this body is fertilized it contains no maternal genes to guide the development and moreover not enough nutrients. So this wouldn't work. "Source" Moreover, there wasn't reported even any case of "twin" resulting from the fertilization of 2nd polar body according to this article There was reported ...


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Although not a biologist, Steven Pinker notes that bilateral symmetry evolved in organisms to allow them to move in a straight line. For a bilaterally symmetrical animal to move, they simply alternate movements between one half of their body and the other. This is true for fish, snakes, insects, mammals, etc. There are exceptions, for instance, flying and ...


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It is, of course, not just mammals, but nearly all animal life is symmetric. Even plants are usually symmetric in some degree. There are exceptions here and there. For example, one interesting exception is that of the fiddler crab which has one claw larger than another. In general, when a single appendage is present on an animal it is nearly always on the ...


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There's really two answers to the question. The first is overall symmetry: mammals, like all tetrapods, are bilaterally symmetric. This comes from a distant common origin with other bilaterally symmetric organisms. Organisms which evolved from this common ancestor often have organs in pairs, probably evolving as a re-use of regulatory genes. The other ...



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