Three stop or nonsense codons UAA ( ocher ), UAG ( amber ) and UGA ( opal )
Open reading frame starting at the initiation codon (AUG)
Each codon has 5’ base and a 3’ base e.g. 5’CGU3’
Mutations that modify the genetic code are of 3 types: frameshift (include deletions and insertions), missense (lead to an amino acid replacement) and nonsense (mutation that generates any of the three stop codons leading a a premature truncation of the polypeptide.
The immediate RNA transcript is known as primary transcript and is subject to three modifications prior to moving to the cytoplasm:
A capping enzyme adds a G to the first nucleotide in the transcript in the unusual 5’-5’ direction (phosphate to phosphate bond). Then a methyl thransferase adds methyl groups (-CH 3 ) to the G and one or more of the first few bases of the RNA transcript. Capping and methylation is believed to be critical for efficient translation.
Addition of a poly A tail (100-200 As) at the 3’ end of the primary transcript by a poly-A-polymerase. Tailing is believed to stabilize mRNA so it remains for longer time to be translated to many polypeptide molecules before it is degraded and increase the efficiency of the initial steps of translation.
RNA splicing and removal of introns from the primary transcript followed by a very precise joining of the exons.
Within each intron three sequences are present: splice donors (at the 5’ end of the intron), splice acceptors (at the 3’ end of the intron) and the branch sites (sandwiched between the splice donors and acceptors).
Splicesome is needed to identify and catalyze the sequence of events leading to removal of the intron and rejoining of the two successive exons. The splicesome consists of snRNP (snRNA 100-300 nucleotides long + proteins). Each splicesome is composed of four snRNPs together and each snRNP is five snRNA plus about 50 proteins. Some snRNAs base pair with the splice donor and acceptor in the primary transcript and this way can bring them together.
Alternative splicing (as in the gene encoding the antibody heavy chain) and trans-splicing (as in C. elegans ) are means of controlling gene expression in eukaryotes.