Biotechnology and Bioinformatics (1+1)

1.4.9. Processing the mRNA Transcript

In eukaryotic cells, the newly-formed mRNA transcript (also called heterogenous nuclear RNA or hnRNA) must be further modified before it can be used. The eukaryotic gene s consist of blocks of coding sequence s separated from each other by blocks of non-

coding sequences. The coding sequences are called exon s and the intervening sequences are called intron s. As a consequence of this alternating pattern of exons and introns, genes bearing non-coding interruptions are often said to be “split”. Exons are too short whereas introns are too long. A cap is added to the 5’ end and a poly-A tail (150 to 200 Adenines) is added to the 3’end of the molecule. The newly-formed mRNA has regions that do not contain a genetic message.

Like the uninterrupted genes of prokaryotes, the split genes of eukaryotes are transcribed into a single RNA copy of the entire gene. Thus, the primary transcript for a typical eukaryotic gene contains introns as well as exons. The primary transcripts of split genes must have their introns removed before they can be translated into protein . Introns are removed for the pre-mRNA by a process called RNA splicing . This process converts the pre-mRNA into mature messenger RNA and must occur with great precision to avoid the loss, or addition, of even a single nucleotide at the sites at which the exons are joined.

DNA is located in an organelle called the nucleus . Transcription and mRNA processing occur in the nucleus. The nucleus is surrounded by a double membrane. After the mature mRNA transcript is produced, it moves out of the nucleus and into the cytoplasm through pores in the nuclear membrane.