Last time we left off with Mendel’s experiments on inherited traits (or phenotypes) of peas. We mentioned these traits are passed down from their DNA sequence and genes.

But what do we mean by DNA sequence?

The structure of DNA was elucidated in the early 1950’s by Francis Crick and James Watson using data that were generated by Rosalind Franklin. Franklin used a difficult technique to work on the structure called X-Ray crystallography.

If a protein or other material can be induced to form a crystal (think of making rock candy!) the structure of the material will affect the way…


Previously, we formed a basic outline of what genes, RNA, and DNA are. Now we’ll take a closer look at how these impact our development and how their malfunction can lead to disease.

The DNA in all of our cells is the same. For the most part. It turns out that the DNA is different in different cells because although the linear sequence of ATCGs is the same there are chemical “marks” on the base pair nucleotides that vary in different cells. This forms the basis of an amazing story of complexity and faithfulness that is emerging right now and…


Previously, we mentioned there are sections of the DNA (called non-coding DNA) that do not lead to the production of a protein because they get cut out of the RNA before it leaves the nucleus of the cell. These sections of non-coding DNA are called introns.

But what is in the intron sequences and why are they there?

There is not a complete answer to that question but we know that they contain special sequences called enhancers that also bind regulatory proteins. Enhancer sequences are not just in intron regions though, these sequences can be almost anywhere in the DNA…


In part 1 we learned that even though two cells may have the same DNA they can behave differently because of how and when they are expressing proteins. Much like a fork in a path of dominoes, cascading events can cause these cells to go down very different paths.

Gene expression is the mechanism by which the many forks in the developmental paths are taken. Gene expression is regulated by a number of factors, principally by transcription factors that act like little molecular switches on a gene. …


Where do babies come from?

Developmental biology is a field of study that attempts to answer this question with modern genetics. How is it that starting with one cell (the egg) following fertilization we become trillions of cells in hundreds of tissues that is us? Cells divide to form two cells (called daughter cells) and those two cells can be different from each other even though they both have exactly the same DNA. But the genes encoded by the DNA are not all similarly expressed. …


Genes can be turned on and off just like switches. The switches are complex little machines with many pieces that we are only now just beginning to understand. Imagine confronting a light switch with no knowledge of how it works only that it makes lights go on and off. If you were a curious person you might try to figure out how it worked.

But what would you do?

More than likely you would start by taking it apart and see the internal parts and how they fit together. Then you might start removing parts to determine their effect on…


Francis Crick (right) and James Watson (middle) with Maclyn McCarty (left). Image from Lederberg J, Gotschlich EC (2005) A Path to Discovery: The Career of Maclyn McCarty. PLoS Biol 3(10): e341 doi:10.1371/journal.pbio.0030341

DNA has four “letters” (a,c,t,g) with which to define proteins that are long chains of chemicals called amino acids (there are 20 amino acids). The proteins are what make a cell what it is, and the job of DNA is to “code” that information. We have 30,000 genes embedded in three billion “letters” and these are what cause us to look and be as we are. …

Philip Iannaccone

Phil Iannaccone is a Professor of Pediatrics and Pathology at Northwestern University Feinberg School of Medicine.

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