Why are some untranslated sequences of bases are seen in mRNA coding for a polypeptide ? Where exactly are they present on mRNA?

By translation process mRNA codes for protein but the region on mRNA which are not translated are called untranslated regions. These regions are before start codon and after stop codon. They are present on the either of the two sections on each side of a coding sequence on a strand of mRNA. 
They are involved in post transcriptional regulation of gene expression.