Figure 29.3(a):group I splicing                       Figure 29.3(b):group II splicing

Alternative splicing: Alternative splicing is a method substantially used for many mammalian genes can result in multiple products that vary structurally and functionally from the same primary transcript.Sometimes, alternate splicing is unregulated phenomenon while in some it is strictly regulated. One of the best examples of regulated alternative splicing occurs in sex determination in drosophila. In drosophila three genes are involved in sex determination sex lethal gene (sxl), transformer gene (tra), doublesex gene (dsx). Due to alternative splicing, functional genes are produced in females and non- functional genes are produced in males. Alternative splicing occurs using two mechanisms - one when two poly (A) or cleavage sites are available in primary transcript. Cleavage occurs at either site resulting in two different products. Such mechanism is followed by variable domains of immunoglobulins heavy chains and their diversity is due to this mechanism of alternative splicing. Similarly alternative splicing with such mechanism results in production of two different hormones- calcium regulating hormone in rat thyroid and calcitonin-gene related peptide in rat brain. Other mechanism involves more than one 3’ splice site for one 5’ splice site. Hence splicing occurs by taking either of those 3’ splice site resulting in different products. Here in Figure  29.4 we are describing the steps of alternate splicing in which protein A formed by exon 1, 2 ,3 and protein B formed by exon1,3,4