There are evidences in support of the idea of the role of chromosomes in protein synthesis. These evidences come from observations on the giant chromosomes of dipterans. Occasionally the DNA strands which make up these chromosomes uncoil in a given region and form loops. These loops make the chromosome appear puffed up locally. Such regions are described as chromosome puffs. Experiments with special stains and radioactive tracers have shown that these chromosome puffs are the site of messenger RNA synthesis. Moreover, puffing occurs in different parts of the chromosome at different times as the insect develops, suggesting that different parts of the chromosome (i.e the DNA) are being used at different times. For example, puffing has been observed to of cure in a particular part of the chromosome just before the larva moults. If moulting hormone is injected into an immature larva, puffing occurs prematurely in this very part of the chromosome.
If different parts of the genetic code come into operation at different time as differentiation takes place, there must be a mechanism which ensures that the right parts of the code operate at the correct time: a mechanism which, in other words, switches the appropriate genes on or off, as and when they are required.
Before considering the evidence of this, let us look at an example to illustrate the basic idea. During human development, two types of hemoglobin are formed. In the adult, the red blood corpuscles contain the type of hemoglobin consisting of a haem-group surrounded by two α and two β polypeptide chains. In the foetus, however, the central haem-group is surrounded by two α polypeptide chains and two þ chains. The differences between the β and þ chains, though slight, are sufficient to confer on these two forms of hemoglobin the markedly different oxygen-carrying powers. At birth the production of foetal hemoglobin is apt (with the β chain). Now their polypeptide chains are controlled by a single gene. This evidence comes mainly from studying abnormal hemoglobin, such as that which occurs in sickle-cell anemia. Sickle-cell anemia is caused by a single gene, have been traced to defects in one or other of the polypeptide chains. From these studies a hypothesis can be put forward to explain in genetic terms the replacement of foetal with adult hemoglobin at birth. Briefly the idea is this: the gene which specified the α chain works all the time that is during adult as well as embryonic life. The gene which specifies the þ chain is inactive during embryonic life, and is switched on at birth. We can also predict the existence of two other genes, one for switching off the þ gene and one for switching on the β gene at birth.