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. The transcription factors in early development often form binary switches (meaning they can only be ‘on’ or ‘off’) with two transcription factors said to be in cross-antagonism with each other.
But what is the point of this and how are they regulated?
Transcription factors are like master switches that turn other genes ‘on’ or ‘off.’ The set of expressed genes defines the differentiated state of the cell. In early development genes act cooperatively to achieve this in network or pathways called signal transduction pathways. The coordinated action of the genes is how patterning the body plan happens. It has been estimated that perhaps as few as 17 of these pathways are all that is required for normal development.
The signal transduction pathway starts with a signal, something outside of the cell. This is usually a protein hormone or a steroid hormone (like estrogen or testosterone). This signal molecule reacts with a receptor either on the surface of the cell (or within the cell as in the case of estrogen and testosterone).
When that interaction occurs a cascade of molecular events follows, the end result of which is the activation of transcription factors or enhancers. The effect of incorrect amounts of estrogen or testosterone or incorrect timing of their presence is that master switches in these pathways are miss-set and the wrong genes are tuned ‘on’ or ‘off’ or the switch fails to function at all.
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