A cell’s DNA contains the complete set of instructions for building and operating an organism. These instructions are not directly used for cellular functions. Instead, they serve as a master blueprint, requiring a two-step molecular process to transform genetic information into functional proteins. This intricate process involves transcription and translation, forming the core mechanism by which life’s instructions are carried out.
Transcription: Creating the Messenger
The initial step, transcription, involves creating a temporary messenger RNA (mRNA) copy from a specific DNA segment. This takes place within the nucleus of eukaryotic cells, safeguarding the original DNA blueprint. A specialized enzyme, RNA polymerase, initiates this copying process by recognizing and binding to a specific region on the DNA called the promoter.
Once bound, RNA polymerase unwinds a section of the DNA double helix, exposing the nucleotide sequence of one DNA strand. The enzyme then moves along this DNA template strand, synthesizing a complementary RNA molecule by adding RNA nucleotides in a 5′ to 3′ direction. This newly formed mRNA molecule acts as a working copy of a single gene, ready to carry the genetic message out of the nucleus.
Translation: Building the Protein
Following its creation, the mRNA molecule exits the nucleus and enters the cytoplasm, where translation occurs. Here, the mRNA encounters ribosomes, which are dedicated to protein synthesis. Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome.
As the ribosome moves along the mRNA strand in a 5′ to 3′ direction, it reads the mRNA’s nucleotide sequence in successive groups of three bases, called codons. Each tRNA molecule possesses a complementary three-base sequence, known as an anticodon, which precisely matches an mRNA codon. When a tRNA’s anticodon pairs with an mRNA codon, the ribosome catalyzes the formation of a peptide bond between the amino acid carried by that tRNA and the growing polypeptide chain, which then folds into a functional protein.
The Genetic Code
The genetic code dictates how information in DNA or mRNA sequences is interpreted into proteins. This code is organized into units called codons, each consisting of three consecutive nucleotides. There are 64 possible codon combinations derived from the four different nucleotides. Of these, 61 codons specify one of the 20 common amino acids, while the remaining three serve as specific signals.
The codon AUG signals the beginning of protein synthesis and also codes for the amino acid methionine. Conversely, three specific codons—UAA, UAG, and UGA—do not code for any amino acid and act as “stop” signals, indicating the end of a polypeptide chain. The genetic code is nearly universal, meaning that almost all living organisms, from bacteria to humans, use the same set of codon-amino acid assignments.
Impact of Errors in the Process
Accuracy in transcription and translation is important, as errors can lead to significant consequences for cellular function. Mutations, permanent alterations in the DNA sequence, can affect the resulting protein. A change in the DNA can lead to an incorrect nucleotide being incorporated into the mRNA during transcription.
This altered mRNA codon can cause a different amino acid to be inserted during translation, called a missense mutation, changing the protein’s shape and function. A mutation might convert an amino acid-coding codon into a stop codon, resulting in a nonsense mutation and a prematurely shortened, nonfunctional protein. If nucleotides are inserted or deleted in numbers not divisible by three, a frameshift mutation occurs, shifting the entire reading frame of the mRNA and drastically altering all subsequent amino acids, leading to a nonfunctional protein. Such errors are linked to various genetic disorders, including sickle cell anemia.