Difference Between pre-mRNA and mRNA

The key difference between pre-mRNA and mRNA is that pre-mRNA is the first product of the transcribed gene and contains both non-coding sequences (introns) and coding sequences (exons) while mRNA is the second product of a transcribed gene which contains only coding sequences.

Gene is the functional unit of heredity. It is a specific DNA segment comprising of the genetic code to manufacture a protein. Genes undergo a two-step process called gene expression. Transcription is the first step in which the DNA sequence converts into an mRNA sequence. Translation is the second step in which the mRNA sequence converts into an amino acid sequence. During the transcription, DNA sequence first converts into a pre-mRNA molecule which is the primary transcript containing both coding and non-coding sequences. Pre-mRNA molecule produces the mRNA (messenger RNA) molecule after undergoing RNA splicing or processing. mRNA sequence contains only coding sequences.

1. Overview and Key Difference
2. What is pre-mRNA
3. What is mRNA
4. Similarities Between pre-mRNA and mRNA
5. Side by Side Comparison – pre-mRNA vs mRNA in Tabular Form
6. Summary

What is pre-mRNA?

Pre-mRNA (precursor mRNA) is the primary transcript and the immediate product of transcription. It is a single-stranded RNA sequence that is complementary to the DNA sequence of the gene. It contains both coding and non-coding sequences. The bulk of pre-mRNA consists of heterogeneous nuclear RNA (hnRNA). Hence, it undergoes several processing steps before converting into an mRNA molecule. The most important step is the removal of intervening sequences that do not specify the appropriate amino acids. In other words, it is the step called pre-mRNA splicing, which removes non-coding sequences (introns) from pre-mRNA and joins coding sequences together.

Figure 01: Pre-mRNA

During the processing, the addition of stabilizing and signalling factors at the 5′ and 3′ ends of the molecule takes place. A cap is added to 5’ end while a poly-A tail is added to 3’ end. Both 5’ cap and 3’ poly-A tail protect the mRNA molecule from degradation.

What is mRNA?

mRNA (mature messenger RNA) is a single-stranded RNA sequence that contains the genetic information of a gene to produce a protein. Pre. mRNA becomes mRNA after processing. Hence, it contains only the exons or coding sequences of the gene. Once made, mRNA travels away from the nucleus to the cytoplasm. In the cytoplasm, ribosomes read the base sequence of the mRNA and recruit corresponding amino acids. Once the amino acid sequence is made, it undergoes folding and becomes a functional protein.

Figure 02: mRNA

The processing and transporting of mRNA differ among eukaryotes and prokaryotes. Eukaryotic mRNA requires extensive processing and transport while prokaryotic mRNA does not. Prokaryotic mRNA synthesis takes place in the cytoplasm itself, and once made, it is immediately ready to undergo translation without processing.

What are the Similarities Between pre-mRNA and mRNA?

Pre-mRNA converts into mRNA after processing.
Both pre-mRNA and mRNA are products of gene transcription.
They are single-stranded RNA.
Both are synthesized in the cell nucleus.
They are found in eukaryotic cells.
Both contain a complementary base sequence to that of the DNA sequence of the gene.

What is the Difference Between pre-mRNA and mRNA?

Pre-mRNA is the primary transcript which contains both coding and non-coding sequences. mRNA is the mature messenger RNA which contains only the coding sequence of a gene. So, this is the key difference between pre-mRNA and mRNA. Pre-mRNA is subject to several processing steps, while mRNA is the product that results from the processing. Moreover, pre-mRNA does not travel to the cytoplasm while mRNA goes to the cytoplasm in order to produce a protein.

The below info-graphic tabulates more differences between pre-mRNA and mRNA.

Summary – pre-mRNA vs mRNA

Pre-mRNA is the immediate product of transcription of a gene. It consists of coding and non-coding sequences of a gene. Several processing steps facilitate the converting of pre-mRNA into mRNA. mRNA is the actual RNA sequence that contains the coding sequence of the gene. It contains the genetic code of a protein. Both pre-mRNA and mRNA are single-stranded RNA sequences that have a complementary sequence to one of the DNA strands of a gene. Thus, this summarizes the difference between pre-mRNA and mRNA.

Reference:

1. “Translation: DNA to MRNA to Protein.” Nature News, Nature Publishing Group, Available here.
2. “Primary Transcript.” Wikipedia, Wikimedia Foundation, 3 Dec. 2020, Available here.

Image Courtesy:

1. “Pre-mRNA” By Nastypatty – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Ribosome mRNA translation en” By LadyofHats – Own work based on:[1], [2], [3], [4], [5], [6], [7], and [8]. (Public Domain) via Commons Wikimedia