The mRNA molecule is completed by the formation of a 5' cap and a 3' poly(A) tail. The 5' cap is a modified guanine nucleotide that is added to the 5' end of the mRNA molecule. The 3' poly(A) tail is a series of adenine nucleotides that are added to the 3' end of the mRNA molecule. These modifications help to protect the mRNA molecule from degradation and enhance its translation.