Collisions between molecules are random and the results of the collisions in terms of new bond formation are based on how the electrons of the molecules respond to the collision. In most cases it is useful to avoid thinking that there is an attraction between molecules. Molecules usually have to randomly come in contact, so that their electrons can interact to form bonds.
Random Atomic Interactions and Bonding
Atoms collide like billiard balls. Each atom is composed of a halo of electrons and a nucleus of protons and neutrons. The outer, valence electrons, of two colliding atoms sometimes rearrange into lower energy, bonding orbitals and the energy released as a result of the new molecular arrangement is converted into vibrations and movement of the new molecule. This lower energy arrangement is what holds the atoms together and is called a bond. Collisions with even more energy than the original are required to separate the atoms, i.e. to break the bond.
Limits of Random Motion in Cells
Reactions can be very rapid, because molecules are moving very fast and making millions of collisions per second. This rapid, short distance, random movement of molecules, diffusion, is sufficient for the thousands of enzyme-catalyzed reactions within a bacterial cell, but is inadequate to cross the huge nucleated cells of plants and animals. These cells have dimensions 10 to 100 times larger than bacteria. In these cells, molecules, such as proteins, are usually synthesized where they will be used or are moved from place to place on an intracellular protein network (cytoskeleton of microtubules).
Random Mutations with Every Replication of DNA
The importance of randomness in biology extends from molecular motion to reactions that synthesize all of the biological metabolites and macromolecules. Random collisions ultimately result in the self-replication of cells and organisms. Each of the reactions is accompanied by a certain rate of errors, so that each new bacterium is only an approximation of its mother or twin.
Accumulation of Genetic Changes Leads to Cancer or Evolution
The same is true of each human cell. With each division there are changes in a few of the billions of nucleic acid bases in the thousand genes of the DNA molecule in each chromosome of the human genome. Each of these changes is a random mutation. It is these naturally occurring, spontaneous mutations that can eventually lead to cancer. More replications means more accumulated mutations. In a few cases, e.g. tobacco smoking or sunbathing, behaviors can increase the rate of cell divisions to replace damaged cells and increase the mutation rate leading to cancer. The accumulation of random genetic changes is also the foundation for evolution.
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