So what exactly is DNA?

by: Christina Mackey

So there I was, blithely leading a tour of preteens when one of the chaperones asked me for the proper name of the abbreviation DNA.  As I struggled to recall the answer, deoxyribonucleic acid lilted musically from the lips of several students and then, much to the bemusement of myself and several of the other adults, they went on to expound at great length (as well as far beyond my meager knowledge) on the subject.

So what precisely are genes, chromosomes and DNA and, perhaps more pertinently, what is the difference between them?

A DNA molecule consists of two long strands that twist around each other like a spiral staircase which is why the molecule is called a double helix.  This DNA molecule is made up of four similar chemicals (called bases and abbreviated A, T, C and G) that are repeated over and over in pairs (called basepairs) which carry the genetic information in the body’s cells.

Chromosomes are the X-forming structures in which DNA is rolled.  These structures carry the coded instructions for making everything the body needs to become a living organism.

Genes are the carriers of genetic properties passed from the parents of the organism (i.e., eye color, blood type).  Because many genetic properties are determined by a combination of genes, a wide variety can occur as the genes of both parents are combined.

The species of an organism is determined by the genome which is the particular order of the pairs of As, Ts, Cs and Gs.  Sometimes there is a mistake and one of the pairs gets switched, dropped or repeated and this changes the coding for one or more genes.  This is called a genetic mutation.  A mutation may cause a disease or may be harmless.  And, if the mutation results in a superior ability to survive and produce offspring, the species may slowly evolve to incorporate the mutation.

Humans have 6 million DNA basepairs, 20,000 to 25,000 genes and 46 chromosomes (22 twin pairs and 1 pair with two X chromosomes (for women) or an X and Y chromosome (for men)). 

As David Comings put it (1) "Being a little chauvinistic toward our own species, we like to think that man is surely one of the most complicated species on earth and thus needs just about the maximum number of genes. However, the lowly liverwort has 18 times as much DNA as we, and the slimy, dull salamander known as Amphiuma has 26 times our complement of DNA. To further add to the insult, the unicellular Euglena has almost as much DNA as man."

And, although we beat out the fruit fly’s 13,500 genes, we are tied with the 21,000 genes of the pufferfish and rice, with 40,000 to 50,000 genes puts us to shame.

Taking one last chance, surely humans must be a winner with our 46 chromosomes!  But even here we are not so lucky, since chimpanzees come in with 48 chromosomes, carp have 104 and shrimp have 86-92.  Well, perhaps if one considers that dolphins have 44, we ought to consider less as better?  But no, not in light of the fact that starfish have 36 and mosquitoes only 6.

It turns out the human genome is thoroughly average in size for a mammal and is put to shame by many plants, insects and even some single-celled protozoa.  However, before you decide to devolve completely, it is now recognized that that the expectation that genome size is directly proportional to an organism’s complexity due in large part to the realization that genomes contain excess (and largely repetitive) DNA that is not used.  In fact, depending upon the body size, metabolism, organ complexity, geographical distribution and ecological niche, genome size may increase or decrease through evolution.

Whew!  And here I was wondering whether I ought to seriously weigh the merits of wishing upon a star to become an ant with 2 chromosomes (for females – the males get a single chromosome not that I’m commenting on that fact) or ferns with their 1,260 chromosomes.

So the next time you lead a tour past the DNA lab, sagely nod your head and explain that HIMB researchers are currently involved in a study of the eating habits of Hawaiian monk seals by analyzing the DNA sequences in the seals’ fecal matter.

(1) Comings, D.E. Structure and function of chromatin. In Advances in Human Genetics, H. Harris and K. Hirschhorn (eds.), Academic Press, New York, 3: 237-431, 1072.
 Please Note - the original post had pulled this section from a tertiary, or possibly much more remote bit of information on the web - which had slightly altered it and set it as a non-attributed bit of info. Apologies from our side for not checking further and a reminder to all who use the web.  Thanks to the anonymous emailer who reminded, "cite your sources." I still do not have the original article for this, I will have to stop by the campus library later this week to check it out. In the meantime to correct the record, for the quote used above and the  correct attribution, see this 2006 blog article from the American Institute of Biological Sciences' T. Ryan Gregory: Genomic Puzzles Old and New.  Thanks - Mark