The blueprint for all living things resides in deoxyribonucleic acid (DNA), which contains the instructions for cellular processes and characteristics. These instructions must be converted into functional molecules, primarily proteins, to carry out various tasks within an organism. This conversion involves two distinct processes: transcription, which transforms DNA into messenger RNA (mRNA), and translation, which then uses mRNA to build proteins.
From DNA to mRNA: The Transcription Process
Transcription, the initial step in utilizing genetic information, copies a specific segment of DNA into an RNA molecule. This process begins when an enzyme called RNA polymerase binds to a DNA region known as the promoter, signaling the start of a gene. RNA polymerase then unwinds the DNA double helix, creating a “transcription bubble” and exposing the DNA template strand. It moves along this template strand, synthesizing a complementary RNA strand by adding nucleotides in a 5′ to 3′ direction.
During this elongation phase, RNA polymerase ensures that each adenine (A) on the DNA template pairs with uracil (U) in the growing RNA strand, while guanine (G) pairs with cytosine (C), and thymine (T) pairs with adenine (A). Transcription continues until the RNA polymerase encounters specific DNA sequences called terminators, which signal the end of the gene. At this point, the newly formed RNA molecule detaches from the DNA template, and the DNA strands re-form their double helix structure.
Preparing mRNA for Action
Once transcribed, the newly synthesized RNA molecule often undergoes several modifications before becoming mature messenger RNA (mRNA). These post-transcriptional modifications protect the mRNA from degradation, facilitate its transport out of the nucleus, and ensure efficient translation. One modification is the addition of a 5′ cap, a modified guanine nucleotide, to the beginning of the mRNA molecule. This cap protects the mRNA from enzymes and plays a role in ribosome recognition during translation.
Another modification involves adding a poly(A) tail, a long chain of adenine nucleotides, to the 3′ end of the mRNA. This tail protects the mRNA from degradation and aids in its export from the nucleus to the cytoplasm. Additionally, many pre-mRNAs undergo splicing, where non-coding regions called introns are removed, and coding regions, known as exons, are joined. This ensures only relevant genetic information is carried forward for protein synthesis.
Building Proteins: The Translation Process
The final stage in expressing genetic information is translation, where the mature mRNA molecule is used as a template to build a protein. This complex process occurs on structures called ribosomes, found in the cytoplasm of the cell. Ribosomes read the mRNA sequence in groups of three nucleotides, called codons. Each codon specifies a particular amino acid, the building blocks of proteins.
Transfer RNA (tRNA) molecules play a mediating role in translation, acting as adaptors. Each tRNA molecule carries a specific amino acid at one end and has a three-nucleotide sequence called an anticodon at the other. The anticodon on the tRNA is complementary to a specific codon on the mRNA. As the ribosome moves along the mRNA, tRNAs bring the correct amino acids, matching their anticodons to the mRNA codons.
The ribosome then catalyzes the formation of peptide bonds between the incoming amino acids, linking them to form a polypeptide chain. This process continues until the ribosome encounters a stop codon on the mRNA, signaling the termination of protein synthesis and the release of the completed polypeptide chain. The polypeptide then folds into a specific three-dimensional structure to become a functional protein.