Messenger ribonucleic acid (mRNA) is a single-stranded molecule found in cells that plays a significant role in gene expression. It carries genetic information, transferring instructions from DNA to the protein-making machinery. Understanding its production and use is key to comprehending these fundamental biological processes.
The Blueprint for Life
The flow of genetic information follows the Central Dogma of molecular biology: DNA creates RNA, which then produces proteins. DNA, or deoxyribonucleic acid, contains all the instructions to build and maintain an organism. These instructions are organized into genes, which dictate the proteins a cell produces.
DNA resides in a protected compartment within eukaryotic cells. Since DNA cannot directly leave this compartment, mRNA acts as a temporary copy of specific gene instructions. This allows genetic information to be accessed and translated into functional molecules without compromising the original DNA.
Making the Messenger
mRNA creation, called transcription, occurs within the nucleus of eukaryotic cells. Inside the nucleus, RNA polymerase accesses the DNA template. This enzyme binds to a DNA region, signaling the start of a gene, and unwinds a section of the DNA, exposing its nucleotide sequence.
One DNA strand, the template strand, guides mRNA synthesis. RNA polymerase moves along this template, adding complementary RNA nucleotides to build a new mRNA molecule. This process ensures the mRNA carries an accurate copy of the genetic instructions from the DNA.
Readying the Message
After synthesis, the new mRNA molecule, called pre-mRNA, undergoes modifications within the nucleus. These alterations are important for the mRNA’s stability, transport out of the nucleus, and proper use in protein production.
A protective 5′ cap is added to one end, shielding the mRNA from degradation and assisting its recognition by protein-making machinery. Splicing removes non-coding regions (introns) from the pre-mRNA. Coding regions (exons) are then joined to form a continuous sequence, ensuring only relevant genetic information is carried forward.
Finally, a poly-A tail is added to the other end, contributing to mRNA stability and export from the nucleus. Once these modifications are complete, the mature mRNA is prepared to leave the nucleus.
Delivering the Instructions
After modification, mature mRNA exits the nucleus and travels into the cytoplasm. This transport occurs through nuclear pore complexes, channels in the nuclear membrane.
In the cytoplasm, mRNA finds ribosomes, the cellular structures that synthesize proteins. Ribosomes are composed of ribosomal RNA (rRNA) and proteins, found freely floating or attached to the endoplasmic reticulum.
Ribosomes read mRNA instructions and build a protein through translation. The mRNA sequence is read in groups of three nucleotides, called codons, each corresponding to a specific amino acid. Transfer RNA (tRNA) molecules bring the correct amino acids to the ribosome, matching them to the mRNA codons.
As the ribosome moves along the mRNA, it links these amino acids, forming a polypeptide chain that folds into a functional protein. Proteins perform many functions, including acting as enzymes, providing structural support, transporting molecules, and facilitating cell signaling. This completes the journey from genetic information in DNA to the creation of proteins that carry out cellular processes.