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Primary Transcript and RNA processing
Transcription of the DNA, which is highly regulated in gene expression, takes place to produce primary transcripts. However, this step involved in the production of primary transcripts is only the first step that should be followed by many modifications and processing to yield functional forms of RNAs. Otherwise stated, the newly synthesized primary transcripts are modified in many ways to be converted to their functional forms to produce different RNAs such as mRNA, tRNA, and rRNA so that they can be translated to make various kind of protein products from the messages embedded.

 Processing of Primary Transcripts 

The basic primary transcript processing process is similar for tRNA and rRNA in both eukaryotic and prokaryotic cells. On the other hand, primary transcript processing varies in mRNAs of prokaryotic and eukaryotic cells. For example, in bacteria, mRNAs serve as templates for synthesis of protein as they are being transcribed. On the other hand, precursor mRNAs (mRNA primary transcripts) of eukaryotic cells undergo wide-range of modifications prior to their transport from nucleus to cytoplasm of the cell. The modifications that mRNAs undergo are responsible for the different types of messages encoded to synthesize various types of mRNA products. Furthermore, primary transcript processing provides a control of gene expression, as well as regulation of degradation rates of different mRNAs. The modifications and primary transcript processing in eukaryotic cells include 5' capping, 3' Polyadenylation, and Alternative splicing (see Figure).

 5' Capping 

After precursor mRNA is synthesized from transcription in eukaryotes, its 5' end is modified with the addition of 7-methylguanosine cap which is also known as 5' cap. The 5' capping modification is initiated by adding a GTP to the 5' terminal nucleotide end of the precursor mRNA in reverse orientation, which is then progressed by adding methyl groups to the G residue. This process of 5' capping is essential to the production of functional mRNA due to the fact that 5' cap is responsible for aligning the mRNA to the ribosome during the process of translation.

 Polyadenylation 

In eukaryotes, the reaction called polyadenylation further modifies the pre-mRNA, in which a structure called poly-A tail is added. Signals for polyadenylation, which include several sequence elements, are detected by a group of proteins to add poly-A tail of approximately 200 nucleotides. Polyadenylation reaction provides a call for the end of transcription, usually few hundreds of nucleotides downstream of the poly-A tail site.

 Alternative Splicing 

In complex eukaryotic cells, one precursor mRNA is able to prepare large amount of mature mRNAs by the process of alternative splicing. Alternate splicing is regulated in gene expression so that each mature mRNA codes for multiple number of proteins. The effect of alternative splicing in gene expression is seen in complex eukaryotes that have fixed amount of genes in their genome but produce extremely large amount of gene products. Most eukaryotic pre-mRNAs (precursor mRNA) contain multiple introns and exons in their transcript. By joining together different combinations of 5' and 3' splice sites in a pre-mRNA, different exons are combined together and and introns are eliminated. Thus, various kinds of mature mRNAs are generated. Alternative splicing takes place in a large protein complex of Spliceosome. Such process of alternative splicing is the key to tissue-specific and developmental regulation in gene expression. It is important to note that alternative splicing can be affected by various factors, which includes mutations like chromosomal translocation.