• Question: how big on average is a single gene?

    Asked by planetdren to Amy, Andy, Georgia, Ollie on 22 Jun 2011.
    • Photo: Ollie Russell

      Ollie Russell answered on 22 Jun 2011:

      That all depends on the gene. One of the biggest genes is called dystrophin that has 2.4 million DNA bases in its code. The genes I work with can be about 1000 bases.

    • Photo: Andy MacLeod

      Andy MacLeod answered on 22 Jun 2011:

      Hi planetdren. The average is usually quoted at about 10-15kb. kb is “kilobases”, which is 1000 bases, the four chemical letters that make up the DNA code.

      It should be relatively easy to calculate. All you need to do is divide [total number of DNA bases that are in genes] by [total number of genes there are]. The tricky thing is, no-one seems to know exactly what those two numbers are.

      Not all of our DNA is in genes. In fact, most of it isn’t. Only about 2% of our the 3-billion or so bases in our chromosomes is what we call “coding DNA”. That is, it contains the instructions on how to make proteins, which are the molecules that cells and tissues need to grow and function properly. But even then, the length of each particular gene isn’t clear. Genes are split into chunks called “exons”, that contain the coding information, that are split up by other chunks called “introns”, which don’t code for protein. Each “gene” can have lots of different exons, that can be put together in different ways to make slightly different versions of the same gene. So what is the length of the gene? The length from the start of the first exon to the end of the last? The length of just the exons? The length of just the exons that go together to make up one version of the gene? It’s still a matter for some debate.

      And then we come to the total number of genes. Back in the last century, when I first went to Uni to learn about this sort of stuff, we were told that humans had about 100,000 genes “give or take 50,000”. The more we know the map of the human chromosomes, the smaller that number gets. Right now, the best guess is “about 20,000” genes. Of course this all depends on what you mean by a “gene” anyway, which again, has a different defintions depending on who you talk to.

      More information on this from the people running the Human Genome Project. You’d think if anyone had some idea, it would be these guys. 🙂