Molecular biology is a subdivision of cell biology (sometimes called cellular biology.) While cell biologists study the whole cell, molecular biologists look at what is going on inside a cell, and usually focus on a set of chemical reactions, called a biochemical pathway.

Often the biochemical pathway studied in molecular biology involves proteins. Proteins are the most common biologically important molecules. Proteins function as hormones, as enzymes, and as receptors; different proteins give a cell its structure, regulate its division, and play important roles in immune function. Researchers in molecular biology are able to closely examine the interactions between these proteins, as well as the path by which they are created from DNA and RNA. Some diseases are caused by errors in these pathways; a molecular biologist's job might determine where the error is so that a treatment or drug can be created to cure or treat the disease.

Because we can't see cell biochemistry with our eyes, or even with ordinary microscopes, the techniques in molecular cell biology require another way to visualize biochemical pathways and proteins. The most common is gel electrophoresis, which uses electric current to carry molecules through a gelatin-like matrix and separates them on the basis of size and charge. Molecular biology techniques also involve cloning. Cloning is simply defined as the reproduction of a DNA sequence. Although most people think of Dolly the sheep when they hear “clone,” the most common and useful form of cloning is done by polymerase chain reaction (PCR,) in which a small piece of DNA is amplified (copied) millions of times. Cloning is also accomplished by the use of plasmid vectors, small circular pieces of DNA that are inserted into a bacteria cell. The bacteria cell then divides, and creates copies of the inserted DNA.

Careers that involve molecular biology include medicine, research, and forensics.