Conditional specification is the process by which cells achieve their respective fates by interacting with other cells.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Molecules are groups of atoms that behave as a single unit.

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#Unit 1. Molecules and their Interaction Relevant to Biology

A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom. A molecule, however, is composed of more than one atom.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Some elements exist naturally as molecules. For example, hydrogen and oxygen exist as two-atom molecules.

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#Unit 1. Molecules and their Interaction Relevant to Biology

Sulfur normally exists as an eight-atom molecule, S8, while phosphorus exists as a four-atom molecule, P4

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#Unit 1. Molecules and their Interaction Relevant to Biology

A covalent bond is a chemical bond that occurs when two or more atoms share electrons. The sharing of electrons helps complete the outer shell, or valence shell, of both atoms. The sharing of electrons creates a stable balance of attractive and repulsive forces between atoms.

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#Unit 1. Molecules and their Interaction Relevant to Biology

In a covalent bond, electrons are shared between atoms. In an ionic bond, electrons are transferred between atoms.