Following fertilization, the single-celled zygote undergoes cleavage, where it is then divided into numerous tiny cells. After 3 to 4 days of fertilization, the cleavage produces a 16 celled embryonic stage known as morula, which is contained within the zona pellucida.
After about 4-5 days of fertilization, cleavage produces a sphere mass of cells known as blastomeres, which is covered by a layer known as trophoblast and has a fluid-filled cavity in the center, known as blastocoel. The blastomeres then migrate inward, changing position and undergoing rearrangements. This is known as gastrulation.
Gastrulation then causes the development of three germ layers: ectoderma, endoderma, and mesoderma. The interaction of cells and the rearrangement of tissues and organs occurs only after the formation of germ layers.
Atoms sharing valance electrons
"Mathew Meselson and Franklin Stahl performed classic experiments that supported semiconservative nature of DNA replication in 1985."
A band of hybrid 14N and 15N DNA was produced as a result of the DNA's initial duplication in a 14N medium. The conservative replication mode was so abandoned. "Based on their observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively."
One original DNA serves as a template for the production of two DNA copies during conservative replication. One of these two is made entirely of fresh DNA, while the other is made of strands from old DNA. In semiconservative replication, the parent strand is used to make two copies of the DNA, each of which contains one original strand and the other new strand.
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