Gene expression
Process by which genes produce functional products.
Gene expression is how a cell turns the instructions in a gene into something functional—either a protein or a non-coding RNA molecule that does a job on its own. This process lets cells use their genetic blueprint to perform countless biological tasks. How much a gene is expressed can shift depending on what the cell needs or what’s happening in its environment, though a few genes are always active at a steady level.
**Mechanism**
**Transcription** Making an RNA copy from a DNA strand is called transcription. RNA polymerases do this work, adding one ribonucleotide at a time to the growing RNA chain, following the rule that bases pair up—except that uracil (U) replaces thymine (T) in the RNA, and occasional errors occur. In bacteria, a single type of RNA polymerase handles all transcription. To start, it must latch onto a DNA sequence called the Pribnow box, helped by a protein called sigma factor (σ factor). In eukaryotes, transcription happens inside the nucleus and involves three different RNA polymerases. Each one needs a specific promoter sequence and a set of DNA-binding proteins—transcription factors—to kick things off (more on that under regulation of transcription). RNA polymerase I transcribes ribosomal RNA (rRNA) genes. RNA polymerase II (Pol II) handles all protein-coding genes plus some non-coding RNAs (like snRNAs, snoRNAs, or long non-coding RNAs). RNA polymerase III transcribes 5S rRNA, transfer RNA (tRNA) genes, and a few small non-coding RNAs (such as 7SK). Transcription stops when the polymerase hits a terminator sequence.
**mRNA processing** In prokaryotes, the messenger RNA (mRNA) made during transcription is ready to be translated into protein right away. But in eukaryotes, transcription produces a primary transcript (pre-RNA) that must be modified before it becomes mature RNA. The exact steps and machinery differ for coding versus non-coding pre-RNAs—for instance, both pre-mRNA and pre-tRNA undergo splicing, but the processes are not the same. Non-coding RNA maturation is covered below.
For pre-mRNA, the first modification is 5′ capping: a set of enzymes adds a 7-methylguanosine (m7G) cap to the 5′ end. This cap protects the RNA from being chewed up by exonucleases. It then gets bound by a cap-binding complex (CBP20/CBP80), which helps export the mRNA to the cytoplasm and prevents decapping.
Another change is 3′ cleavage a
- field
- Molecular biology
- known_for
- Transcription, mRNA processing, non-coding RNA maturation, translation, and regulation of gene expression
Lore & Background
Non-coding RNA genes are transcribed as precursors that undergo further processing. Ribosomal RNA precursors are cleaved and modified by snoRNAs; transfer RNA precursors have their ends removed and a CCA tail added; microRNA precursors are processed by Drosha and Pasha in the nucleus and by Dicer in the cytoplasm. Translation of messenger RNA into protein occurs via ribosomes and transfer RNA, with prokaryotes often translating co-transcriptionally and eukaryotes translating in the cytoplasm or at the endoplasmic reticulum. Regulation of gene expression controls the amount and timing of functional gene products.
Reader's Guide
Gene expression is a fundamental biological process that allows cells to produce proteins and functional RNAs from genetic information. Its mechanisms—transcription, RNA processing, and translation—are conserved across life, with key differences between prokaryotes and eukaryotes, such as the nuclear separation of transcription and translation in eukaryotes enabling extensive RNA processing. The regulation of expression is critical for cellular adaptation and specialization, and alternative splicing in eukaryotes greatly expands the proteome from a limited number of genes. Understanding gene expression has implications for medicine, biotechnology, and basic biology, as disruptions can lead to disease. The article details the steps and machinery involved, from RNA polymerases and transcription factors to spliceosomes and ribosomes, highlighting the complexity and precision of this process.
Did You Know?
- In bacteria, transcription is carried out by a single type of RNA polymerase, which needs to bind a Pribnow box with the help of the sigma factor protein.
- Eukaryotic pre-mRNA undergoes 5' capping, 3' cleavage and polyadenylation, and RNA splicing, with alternative splicing creating different transcripts from a single gene.
- Non-coding RNA maturation includes cleavage and modification of rRNA by snoRNAs, and processing of miRNA by Drosha, Pasha, and Dicer.
- In prokaryotes, translation generally occurs co-transcriptionally, while in eukaryotes translation can occur in the cytoplasm or at the endoplasmic reticulum.
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