mRNA in Prokaryotic and Eukaryotic cell

RIBONUCLEIC ACID - mRNA (Prokaryotic and Eukaryotic)

• mRNA accounts for 5% of the total RNA. A ribose nucleotide in the chain of RNA consists of a ribose sugar, phosphate group, and a base. 
• In each ribose sugar, one of the four bases is added: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
• The base is attached to a ribose sugar with the help of a phosphodiester bond. As RNA comprises many ribose nucleotides, the length of the chains of nucleotides can vary according to their types or their functions.
• RNA thus differs from DNA, on the type of sugar used to make the molecule and replacement of base Thymine in DNA with Uracil in RNA. Additionally, DNA is a double-stranded molecule whereas RNA is a single-stranded molecule.
• ·   It carries genetic information in the form of triplet codon.
• START CODON - AUG  
• STOP CODON   - UAA, UGA, UAG.

 

 

 

PROKARYOTIC mRNA

 

·   mRNA is a single-stranded RNA molecule running from 5’ to 3’ direction.

·   It has sort lifespan and less stable.

·   In a prokaryotic cell, there is a lack of a distinct nucleus, thus the mRNA synthesized contained a copy of DNA sequences with a terminal 5’- triphosphate group and 3’ hydroxyl group. Prokaryotic mRNA at its 5’ end has a Shine Dalgarno sequence which is rich in purine nucleotide and is essential for facilitating the binding of mRNA to ribosome.

·   Prokaryotic mRNA starts at 5’ with a triphosphate group, followed by Shine Dalgarno sequences (untranslated region) and as we move along to the 3’ direction, the coding regions begin. The coding regions begin with a start codon and end when it reaches a stop codon, after encountering a stop codon, protein-coding regions come to an end, after which there is another untranslated region that terminates at the end of 3’.

·   This prokaryotic mRNA is polycistronic in nature.

·   It has Operon (Cluster of genes).

·   Translation and transcription take place in cytoplasm.

·   mRNA processing is not seen in prokaryotes.

 

EUKARYOTIC mRNA

• However, in a eukaryotic cell, the nucleus is distinct and has numerous enzymes in it that make the synthesized mRNA molecule go through modification at their terminals 5’ and 3’ to maintain the integrity and stability of the mRNA molecule (post-transcriptional modification).
• The 5’ triphosphate group of eukaryotic mRNAs is esterified forming a cap structure. This process is referred to as 5’ capping, which happens when a 7-methylguanosine cap is added to a 5’ free triphosphate group via 5’-5’ phosphate linkage with the help of an enzyme called guanyl transferase.
• The 5’ capping is important for the recognition of mRNA by the Ribosomes during protein synthesis. 
• And likewise, the 3’ hydroxyl group is cleaved to give a free hydroxyl group to which numerous adenine monophosphates are added to make the mRNA molecule more stable and prevent it from degradation. This tail of numerous adenine nucleotides is referred to as Poly-A-tail.
• The Poly-A-tail added to the 3’ end of eukaryotic mRNA are usually 100-200 bases long; the addition is catalyzed by an enzyme poly(A) polymerase that recognizes the sequence AAUAAA as a single for addition.
• The poly-A-tail is vital during transporting mRNA from the nucleus to the cytoplasm for proteins synthesis.
• Eukaryotic mRNA starts at 5’ with a cap followed by untranslated regions (5’ UTR), and as we move along 3’ direction, the coding regions begin with the start codon and continue till it reaches the stop codon, which marks the termination of coding regions, after which there is another untranslated region (3’ UTR) that terminate when Poly-A-tail begins, which further terminates at the end of 3’.

• UTR – Untranslated region sis mRNA domain, it contains regulatory elements which have two types, Cis regulatory elements, Trans – regulatory elements. The CRE elements present I the nearby location of the genes that are going to regulate the genes by acting as a Promotor (initiating transcript), Enhancer (Enhance the transcriptase), Silencer (Inhibit the Transcript).   &   The Trans Elements located at different location of the genome to the gene that they regulate. They are regulating by the repressor molecule.

• The 3′-untranslated region plays a crucial role in gene expression by influencing the localization, stability, export, and translation efficiency of an mRNA. It contains various sequences that are involved in gene expression, including microRNA response elements (MREs), AU-rich elements (AREs), and the poly(A) tail. In addition, the structural characteristics of the 3′-UTR as well as its use of alternative polyadenylation play a role in gene expression.