[Alvin Raj]

Alvin Raj  asked the Naked Scientists:

Hi everyone,

Love the podcast! I've been listening to all the old archives at about 2 to 5 podcasts a day, and will hopefully get up to speed soon. I've finished all the old shows up to August 2006.

My question is, how does DNA guide the first embryonic cell and all that later divide from it, what to grow, and where to grow it? For instance, why doesn't a finger or an eye grow out of my back or thigh?

Many thanks!

Alvin Raj
Cambridge, Massachusetts
USA

What do you think?

[blakestyger]

This is a very complex process but the key concept is pattern formation that creates a spatially ordered and differentiated embryo from the seemingly homogeneous egg cell.

Some texts use a jigsaw puzzle as an analogy where pieces are fitted together. This doesn't work perfectly though, as, in a jigsaw the pattern exists in its own right and the shapes of the pieces are independent of the pattern from which they are cut i.e the cuts in the pattern are superimposed on a pre-existing picture that is then reassembled.

In biological development, however, the emergence of the picture on each piece is caused by the shape and position of each piece. On top of this, the picture changes throughout the organism's life through growth and aging.

This is studied by utilising mutations so that genes can be identified that control development and see how they interact.

If you can get it, there is a very good introductory text "Genetics, a Beginner's Guide" by Guttman et al (2004), One World, Oxford. Chapter 11 is all about gene regulation and development.

[RD]

Homeotic (HOX) genes define the body plan: what grows where...

Homeotic genes set up the basic regional layout of an organism, so that eyes form on the head and not on the abdomen, and limbs form at the sides and not on the head. Even a single mutation in the DNA of these genes can have drastic effects for the organism (see Homeotic Mutants, below), and so these genes have changed relatively little over time.

http://en.wikipedia.org/wiki/Homeotic_gene

Antennapedia is a hox gene first discovered in Drosophila which controls the placement of legs. A loss-of-function mutation in the regulatory region of this gene can result in the development of the second leg pair into ectopic antennae [pair of legs growing from head].

http://en.wikipedia.org/wiki/Antennapedia