How is the Demand for Large Quantities of Gene Product Met ?
Some proteins in the cell are required in small quantities, but others may be required in large quantities, so that the demands can not be easily met. These demands can be met in either of the following two ways : (i) In some cases, a solitary gene is present in the cell but is transcribed repeatedly to produce large number of messenger RNA molecules and these mRNA molecules act as templates for many rounds of protein synthesis. For instance, although the gene coding for silkworm protein fibroin is present in only one copy per haploid complement, but it keeps on making- silk actively. Similarly in a cell engaged in exclusive synthesis of haemoglobin, a solitary copy of the gene produces large quantities of α or β polypeptide chains of haemoglobin. However, although a whole family of related a globin genes are found in each cell, different but solitary a globin genes function at different stages of development (see later), (ii) In other cases, multiple copies of a gene are found in the same cell, all taking part in transcription within a cell, giving rise to large quantities of the gene product as in case of histone genes, tRNA genes, heat shock protein genes, SnRNA (small nuclear RNA) genes, actin genes, etc. It is these multigene families, which will make the subject of discussion in this section.