M03.03.06 Production of Eukaryotic Messenger RNA

In eukaryotes, genes consist of coding segments known as exons and noncoding segments called introns. Both exons and introns are transcribed in the nucleus, but introns are removed during the RNA processing phase. Eukaryotic mRNA is predominantly monocistronic, meaning it encodes a single protein. The mature mRNA is then translated into the cytoplasm.

Structure and Transcription of Eukaryotic Genes

The transcription process of a typical eukaryotic gene involves several steps:

Steps of Transcription

1. Initiation
   • Transcription factors assist RNA polymerase II in recognizing and binding to the promoter region of the gene.
   • The basal promoter region typically contains two consensus sequences: the TATA box (also known as the Hogness box) and the CAAT box.
2. Elongation
   • RNA polymerase II unwinds the DNA strands to initiate transcription, reading the template strand in the 3′ to 5′ direction while synthesizing the RNA primary transcript in the 5′ to 3′ direction. Both exons and introns are transcribed.
3. Termination
   • Transcription concludes when RNA polymerase II encounters a termination signal, although the specifics of these signals are not fully understood in eukaryotes.

Figure 1: Eukaryotic Transcription Unit

Processing of Eukaryotic Pre-Messenger RNA

The primary transcript, known as hnRNA (heterogeneous nuclear RNA), undergoes several posttranscriptional modifications to produce mature mRNA:

Steps of Processing

1. Capping
   • A 7-methylguanosine cap is added to the 5′ end of the RNA while it is still being synthesized. This cap serves as a binding site for ribosomes and protects the mRNA from degradation.
2. Poly-A Tail Addition
   • A poly-A tail is added to the 3′ end. An endonuclease cuts the RNA just downstream of the AAUAAA sequence (the poly-A addition signal), and poly-A polymerase adds approximately 200 adenine nucleotides to the new 3′ ends. This tail also protects mRNA and aids in its transport to the cytoplasm.
3. Splicing
   • Introns are excised from hnRNA through splicing, performed by spliceosomes (small nuclear ribonucleoproteins or snRNPs), which consist of snRNA and proteins. The intron is removed in a lariat form, and adjacent exons are joined to create the final coding sequence.
4. Transport
   • The mature mRNA is then transported to the cytoplasm for translation into protein.

Figure 2: Processing of Eukaryotic Pre-mRNA

Points to Remember

• Eukaryotic mRNA is monocistronic, with each mRNA molecule coding for a single protein.
• The transcription process involves initiation, elongation, and termination stages.
• Posttranscriptional modifications include capping, poly-A tail addition, and splicing.
• Spliceosomes play a crucial role in intron removal and exon joining.

Bibliography

• Lodish, H., Berk, A., Kaiser, C. A., & Scott, M. P. (2008). Molecular Cell Biology. W.H. Freeman and Company.
• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. Garland Science.
• Brown, T. A. (2016). Gene Cloning and DNA Analysis: An Introduction. Wiley-Blackwell.