Biotechnology and Bioinformatics (1+1)

1.3.2. Gene structure

Transcription proceeds from left to right, regardless of the orientation of the gene in the chromosome. This means that the promoter lies to the left of the coded region. Taking the gene organization first:

Transcription starts at the transcription initiation site, and stops when it encounters the polyA attachment site.

Transcription produces mRNA as a copy of the DNA, from the initiation site to the polyA attachment site.

A set of enzymes then attaches a series of a hundred or more A’s to the mRNA called the polyA tail. This tail appears to protect the mRNA from degradation by enzymes.

So mRNA is simply a single strand of bases, copied from the genomic DNA, from the initiation site and ending with a polyA tail.

The start codon , for translation, is always AUG, which encodes methionine and at the end is a stop codon UGA. These codons define the coding region. The region of about 30 bases between the transcription initiation and start codon is called the upstream untranslated region (UTR). The region between the stop codon and the polyA attachment site is called the downstream, and in some genes contain sequence s which control mRNA stability.