Thanks to Watson, Crick and thousands of others, we know a lot about the chemical basis of heredity. In case you've forgotten or blocked it from your memory, recall that the nucleus of a human cell contains chromosomes -- the threadlike structures carrying all of our genetic information. Human cells have 23 pairs of chromosomes, for a total of 46. Each chromosome consists of a DNA double helix bearing a linear sequence of genes, coiled around proteins known as histones. A single gene is a distinct sequence of nucleotides, the building blocks of DNA, that codes for a corresponding protein.
As scientists hunkered down over the human genome, they noticed that some genes shared certain characteristics. They either carried a similar sequence of nucleotides, or they were dissimilar genes that produced proteins capable of participating in the same cellular process. They grouped these genes into families and then used the classification system to predict the function of newly identified genes based on their similarities to known genes.
There are two BRCA genes -- BRCA1 and BRCA2 -- and each belongs to a different family. BRCA1 belongs to the RNF family of genes, which code for proteins known as RING-type zinc finger proteins. These proteins are so named because the protein molecule has regions that fold around a zinc ion and because the resulting shape of such a region resembles a finger. The unique shape of RING-type zinc finger proteins enables them to bind readily with other molecules, especially proteins and nucleic acids. Once they're bound to another molecule, they perform some enzymatic action that helps a cell maintain a stable environment. Some of these activities include cell growth and division, signal transduction, protein degradation and, as we'll see in the next section, tumor suppression.
BRCA2 genes belong to the FANC gene family. Genes in this group produce a complex of proteins that participate in a process known as the Fanconi anemia (FA) pathway. This pathway primarily works on locating and repairing DNA damage. In particular, the proteins target sections of DNA where the opposite strands of the double helix are not properly linked. When they find such an area, the FANC proteins bind to the DNA and rebuild the cross-links, allowing the DNA to replicate and function normally.
Clearly, the RNF and FANC gene families play important roles in keeping us healthy. If something interferes with the function of these genes, it can lead to a number of diseases. For example, disruption of RNF genes can lead to myotonic dystrophy, which is characterized by progressive muscle wasting and loss. Disruption of FANC genes can result in, you guessed it, Fanconi anemia, which can cause bone marrow failure, physical abnormalities and organ defects. And, of course, both gene families play a role in certain cancers, including breast cancer.
Up next, we'll look very specifically at BRCA1 and BRCA2 to understand how they function normally and how mutations to the genes lead to breast cancer.