Developmental Biology

Common to all living systems, from the primitive single cell to human, life is a matter of a continuing regeneration of kind, dynamically influenced through time by mutation and natural selection. Studies of the different parts of a life cycle involve virtually all of the major biological disciplines: molecular biology, genetics, and cell physiology. These disciplines all deal with information transfer in fertilization and reproduction through physical and chemical separations and bondings.

Embryology begins with development from the fertilized egg through growth and morphogenesis to differentiation and specialization of tissues and organs. As living organisms mature, reproduce, age, and die, their physiological and behavioral response and adaptation to the environment change. Success or failure through natural selection of the organisms' adaptation influences the genetic and social inheritance of each succeeding generation. Consequently, the organisms' ability to deal with the environment is also affected. Through the life cycle, an organism is variously vulnerable to its ambient conditions, being perhaps most sensitive in early development. The spaceflight environment includes several potential hazards such as radiation, alterations in atmospheric pressure, prolonged toxic exposure and weightlessness, which may affect developmental processes.

Adenine

Thymine Guanine

Cytosine

Adenine

Thymine Guanine

Cytosine

Figure 1-14. Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life. Pieces of DNA are pairs of molecules, which entwine like vines to form a double helix. Each vine-like molecule is a strand of DNA, i.e., a chemically linked chain of nucleotides (adenine, thymine, guanine, cytosine). The sequence of nucleotides along a DNA strand defines a messenger RNA sequence, which then defines a protein that an organism is liable to manufacture or "express " at one or several points in its li fe using the information of the sequence.

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