The coding gene is larger than 900 nucleotides.
A gene consists of two regions:
- a transcriptional region that contains the 900 nucleotides that code for the 300 amino acids
- a regulatory region that contains additional nucleotides where for examples enzymes bind that have to carry out the transcription.
Then we have the start and stop codons, these are the signals that tell where the transcriptional region starts and where it ends:
- the stop codon doesn't code for an amino acid and is therefore not included in the mRNA.
- the start codon codes for methionine and is included in the mRNA. Methionine will often be removed from the protein.
So the number of nucleotides that are included in the mRNA is 900 + 3 (for the start codon if methionine is removed afterwards). The gene itself is even larger.