Molecular biology is the study of life at the level of atoms and molecules. Suppose, for example, that one wishes to understand as much as possible about an earthworm. At one level, it is possible to describe the obvious characteristics of the worm, including its size, shape, color, weight, the foods it eats, and the way it reproduces.
Long ago, however, biologists discovered that a more basic understanding of any organism could be obtained by studying the cells of which that organism is made. They could identify the structures of which cells are made, the way cells change, the substances needed by the cell to survive, products made by the cell, and other cellular characteristics.
Molecular biology takes this analysis of life one step further. It attempts to study the molecules of which living organisms are made in much the same way that chemists study any other kind of molecule. For example, they try to find out the chemical structure of these molecules and the way this structure changes during various life processes, such as reproduction and growth. In their research, molecular biologists make use of ideas and tools from many different sciences, including chemistry, biology, and physics.
The key principle that dominates molecular biology is known as the Central Dogma. (A dogma is an established belief.) The Central Dogma is based on two facts. The first fact is that the key players in the way any cell operates are proteins. Proteins are very large, complex molecules made of smaller units known as amino acids. A typical protein might consist, as an example, of a few thousand amino acid molecules joined to each other end-to-end. Proteins play a host of roles in cells. They are the building blocks from which cell structures are made; they act as hormones (chemical messengers) that deliver messages from one part of a cell to another or from one cell to another cell; and they act as enzymes, compounds that speed up the rate at which chemical reactions take place in cells.
The second basic fact is that proteins are constructed in cells based on master plans stored in molecules known as deoxyribonucleic acids (DNA) present in the nuclei of cells. DNA molecules consist of very long chains of units known as nucleotides joined to each other end-to-end. The sequence in which nucleotides are arranged act as a kind of code that tells a cell what proteins to make and how to make them.
Amino acid: An organic compound from which proteins are made.
Cell: The basic unit of a living organism; cells are structured to perform highly specialized functions.
Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.
DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.
Enzyme: Any of numerous complex proteins that are produced by living cells and spark specific biochemical reactions.
Hormone: A chemical produced in living cells that is carried by the blood to organs and tissues in distant parts of the body, where it regulates cellular activity.
Nucleotide: A unit from which DNA molecules are made.
Protein: A complex chemical compound that consists of many amino acids attached to each other that are essential to the structure and functioning of all living cells.
Ribosome: Small structures in cells where proteins are produced.
The Central Dogma, then, is very simple and can be expressed as follows:
DNA → mRNA → proteins
What this equation says in words is that the code stored in DNA molecules in the nucleus of a cell is first written in another kind of molecule known as messenger ribonucleic acid (mRNA). Once they are constructed, mRNA molecules leave the nucleus and travel out of the nucleus into the cytoplasm of the cell. They attach themselves to ribosomes, structures inside the cytoplasm where protein production takes place. Amino acids that exist abundantly in the cytoplasm are then brought to the ribosomes by another kind of RNA, transfer RNA (tRNA), where they are used to construct new protein molecules. These molecules have their structure dictated by mRNA molecules which, in turn, have structures originally dictated by DNA molecules.
The development of molecular biology has provided a new and completely different way of understanding living organisms. We now know, for example, that the functions a cell performs can be described in chemical terms. Suppose that we know that a cell makes red hair. What we have learned is that the reason the cell makes red hair is that DNA molecules in its nucleus carry a coded message for red-hair-making. That coded message passes from the cell's DNA to its mRNA. The mRNA then directs the production of red-hair proteins.
The same can be said for any cell function. Perhaps a cell is responsible for producing antibodies against infection, or for making the hormone insulin, or assembling a sex hormone. All of these cell functions can be specified as a set of chemical reactions.
But once that fact has been realized, then humans have exciting new ways of dealing with living organisms. If the master architect of cell functions is a chemical molecule (DNA), then that molecule can be changed, like any other chemical molecule. If and when that happens, the functions performed by the cell are also changed. For these reasons, the development of molecular biology is regarded by many people as one of the greatest revolutions in all of scientific history.