The start codon marks the site at which translation into protein sequence begins, and the stop codon marks the site at which translation ends.

Why do we need a start codon?

The start codon sets the reading frame: instead of continuing to move down the mRNA transcript one base at a time, the ribosome now reads the mRNA codons consecutively, three bases at a time (Fig. 3.18). The sequence of the triplet codon determines which amino acid is added next to the growing protein.

What would happen without start and stop codons?

Without stop codons, an organism is unable to produce specific proteins. The new polypeptide (protein) chain will just grow and grow until the cell bursts or there are no more available amino acids to add to it.