Expert: Dana Krempels, Ph.D. - 12/1/2010

Question
How do some cells become brain cells and others become skin cells, when the DNA in ALL the cells is exactly the same.  In other words, if the instructions are exactly the same, how does one cell become a brain cell and another a skin cell?

Answer
Dear Spartacus,

This is one of the great miracles of life, and it it only relatively recently that the mechanism has been discovered.  The entire process is incredibly complicated, but the short answer is this:  As a cell begins to differentiate into one with a specific job, some of the genes in the cell turn "off" (either permanently or temporarily), and others turn "on", meaning that they are *expressed*.  

Remember that the chromosomes are essentially long strands of DNA (wound on "spools" of protein), and that each chromosome is "home" to many, many different genes.  Each gene is a unit of inheritance, and each gene encodes the production of either a protein or a working nucleic acid "cousin" of DNA, known as RNA.  (There are many different types of RNA, and they are the machinery that guides the production of protein from DNA.)

Both proteins and RNA (depending on their identity) can affect the activity of other genes, either turning them on or off.  As cells divide and mature, different genes are turned on and off in each cell lineage, and end product is a complex organism with many different types of cells engaging in a division of labor to run that organism.

It's all about differential gene expression.  Though every diploid cell carries two copies of the genome, NOT all the same genes are active during the development of each cell, nor during its normal function.

Hope that helps!

Dana