The Process of Eukaryotic Transcription and Translation
Explore how genetic information is expressed in eukaryotes. This overview details the compartmentalized and controlled pathway from a DNA blueprint to a protein product.
Gene expression is the process of converting genetic information in DNA into functional products like proteins. This pathway has two main stages: transcription and translation. Transcription creates a copy of a gene’s DNA sequence into an RNA molecule. This RNA then serves as a template for translation, where the genetic code is used to synthesize a protein.
The Process of Eukaryotic Transcription
In eukaryotic cells, transcription occurs within the nucleus where DNA is housed. The process is carried out by an enzyme called RNA polymerase and unfolds in three stages. Initiation begins when transcription factors bind to a specific DNA sequence called a promoter, located near the start of a gene. This binding recruits RNA polymerase to the site and signals the DNA to unwind.
Once RNA polymerase is positioned, the elongation phase begins. The enzyme moves along the DNA’s template strand, reading its nucleotide sequence. As it travels, it synthesizes a complementary strand of pre-messenger RNA (pre-mRNA) by adding corresponding RNA nucleotides. This new RNA is a copy of the other DNA strand, except the base uracil (U) is used instead of thymine (T).
The final stage is termination, where the RNA polymerase encounters a terminator sequence in the DNA. This signals the polymerase to detach from the DNA and release the new pre-mRNA molecule. This pre-mRNA is not yet ready for protein synthesis and must undergo further modifications to prepare it for export from the nucleus.
To become mature messenger RNA (mRNA), the pre-mRNA undergoes several processing steps. A protective 5′ cap is added to the beginning of the strand, and a 3′ poly-A tail is attached to the end. These additions protect the mRNA from degradation and aid its export from the nucleus. Another modification is RNA splicing, where non-coding regions called introns are removed, and the remaining coding regions, known as exons, are joined together.
The Process of Eukaryotic Translation
After processing, the mature mRNA molecule is transported from the nucleus to the cytoplasm to begin translation. This process occurs on molecular machines called ribosomes. The primary molecules involved are the mRNA template, the ribosome which serves as the site of synthesis, and transfer RNA (tRNA), which carries specific amino acids.
The genetic information on the mRNA is read in three-nucleotide units called codons. Each codon corresponds to a particular amino acid, the building blocks of proteins. This genetic code dictates the sequence in which amino acids are added to the growing polypeptide chain.
Like transcription, translation proceeds in three stages. Initiation begins when the small ribosomal subunit binds to the mRNA and finds the start codon. This recruits the large ribosomal subunit to form a complete ribosome and the first tRNA molecule carrying its specific amino acid.
During the elongation stage, the ribosome moves along the mRNA molecule one codon at a time. For each codon, the corresponding tRNA molecule with its attached amino acid is brought into the ribosome. The ribosome catalyzes the formation of a peptide bond between the new amino acid and the growing polypeptide chain.
Termination occurs when the ribosome encounters a stop codon in the mRNA. These codons do not code for an amino acid but signal the end of protein synthesis. The completed polypeptide chain is then released from the ribosome and may undergo folding to become a fully functional protein.
Regulation of Gene Expression
Cells tightly control which genes are turned on or off, allowing them to respond to their environment and perform specialized functions. Much of this control occurs at the level of transcription. Specific DNA sequences known as enhancers can be bound by activator proteins to increase the rate of transcription, while silencers can be bound by repressor proteins to decrease it.
The physical state of the DNA also regulates gene access. DNA in eukaryotic cells is wound around proteins to form a compact structure called chromatin. When chromatin is tightly packed, transcriptional machinery cannot access the gene’s promoter, silencing the gene. Chemical modifications can cause the chromatin to relax, making the DNA accessible for transcription.
Gene expression can also be controlled after mRNA has been synthesized, a process known as translational regulation. Small regulatory molecules called microRNAs (miRNAs) can bind to specific mRNA molecules in the cytoplasm. This binding can either mark the mRNA for destruction or physically block the ribosome from attaching and initiating translation.
Key Distinctions from Prokaryotic Processes
The gene expression process in eukaryotic cells is distinct from that in prokaryotic cells, like bacteria, which lack a nucleus. This structural difference leads to major variations in how genetic information is processed.
In prokaryotes, transcription and translation occur concurrently in the cytoplasm. Because there is no nucleus, translation of an mRNA molecule can begin even before its transcription from the DNA is complete. This “coupled” process allows prokaryotic cells to rapidly synthesize proteins.
Another distinction is RNA processing. Prokaryotic mRNA is generally ready for immediate translation after being transcribed. In contrast, eukaryotic pre-mRNA undergoes significant modifications, including splicing and the addition of a 5′ cap and 3′ poly-A tail, all of which are absent in prokaryotes. The molecular machinery also differs, with eukaryotes using larger ribosomes and more complex transcription factors.