Gene expression is the process where information from a gene is used to create a functional product, such as a protein or RNA molecule. This mechanism ensures cells function correctly and adapt to various conditions. All known life forms use gene expression to generate the machinery necessary for life. It allows the genetic information stored in DNA (genotype) to manifest as observable traits (phenotype).
Distinguishing Eukaryotic Gene Expression
Eukaryotic gene expression differs from prokaryotic gene expression due to cellular organization and genome complexity. A notable difference is compartmentalization: transcription (making RNA from DNA) occurs in the nucleus, while translation (synthesizing protein from RNA) takes place in the cytoplasm. This physical separation allows for additional layers of regulation not possible in prokaryotes, where these processes can occur almost simultaneously.
DNA packaging also distinguishes eukaryotes. Eukaryotic DNA is wound around histones, forming chromatin. This structure influences gene accessibility: tightly packed regions (heterochromatin) are less accessible for transcription, while relaxed regions (euchromatin) are more active.
Eukaryotic genes often contain non-coding introns interspersed within coding exons. These introns must be removed from the initial RNA transcript to create a functional protein. Prokaryotic genes typically lack introns, necessitating additional RNA processing steps unique to eukaryotes.
Eukaryotic genomes are larger and more complex, often containing substantial non-coding DNA, including long sequences between genes. These features result in a more intricate and multi-layered regulatory system for gene expression.
The Journey from Gene to Protein
The journey from a eukaryotic gene to a functional protein is a multi-step process, beginning in the nucleus and concluding in the cytoplasm. The first stage, transcription, synthesizes a preliminary RNA molecule (pre-mRNA) from a DNA template. RNA polymerase carries out this process, reading the DNA sequence and building a complementary RNA strand.
Following transcription, the pre-mRNA undergoes extensive RNA processing within the nucleus. One modification is 5′ capping, where a modified guanine nucleotide is added to the 5′ end. This cap protects the mRNA from degradation and aids in its recognition by ribosomes during translation.
A poly-A tail is simultaneously added to the 3′ end of the pre-mRNA in a process called polyadenylation. This tail enhances mRNA stability and assists in its export from the nucleus. Splicing is another aspect of RNA processing, where non-coding introns are removed, and coding exons are joined to form a continuous mature messenger RNA (mRNA) sequence.
Once fully processed, mRNA is transported from the nucleus into the cytoplasm through nuclear pores. In the cytoplasm, the mature mRNA encounters ribosomes, cellular machinery for protein synthesis. This next stage is translation, where the genetic code carried by the mRNA is read in three-nucleotide units called codons. Each codon specifies an amino acid, and transfer RNA (tRNA) molecules bind to their corresponding codons. The ribosome then links these amino acids, forming a polypeptide chain that folds into a functional protein.
Orchestrating Gene Activity
Eukaryotic gene activity is controlled through various regulatory mechanisms. One level of control is chromatin remodeling, which alters chromatin structure to make genes more or less accessible for transcription. Chromatin remodeling complexes reposition or eject nucleosomes, exposing or concealing DNA sequences for transcription factors. Histone modifications, such as adding acetyl, methyl, or phosphate groups, can also relax or compact chromatin, directly influencing gene expression.
Transcriptional control is a key point of regulation, governing how frequently a gene is transcribed into RNA. This involves transcription factors binding to specific DNA sequences called promoter regions near the gene. Other regulatory DNA sequences, like enhancers and silencers, located far from the gene, can also bind proteins to boost or suppress transcription. The recruitment of RNA polymerase II, which synthesizes RNA, is tightly regulated by these factors.
Regulation also occurs after transcription, known as post-transcriptional control. This includes mechanisms affecting mRNA stability, determining how long an mRNA molecule persists in the cytoplasm. A longer lifespan allows more protein production from a single mRNA. RNA interference (RNAi) is another post-transcriptional control, where small non-coding RNA molecules, like microRNAs (miRNAs), bind to specific mRNA sequences. This binding can lead to mRNA degradation or block its translation, silencing gene expression.
Translational control regulates the rate at which mRNA is translated into protein. This often involves controlling the formation of the initiation complex, proteins that assemble on the mRNA to begin translation. Modifications to these initiation factors can increase or decrease protein synthesis speed. Post-translational modification involves changes to the protein after synthesis. These modifications, such as phosphorylation, glycosylation, or ubiquitination, can alter a protein’s activity, location, or lifespan, rapidly adjusting cellular function.