Molecular Biology Terms Starting With G
Molecular Biology Glossary: G
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Gene Regulation
/ jeen reg-yoo-LAY-shun / · English: gene + Latin: regulatio (rule)
Gene Regulation is the collection of mechanisms by which a cell controls the timing, location, and level of gene expression, ensuring the right genes are active at the right times in the right cell types.
Transcriptional regulation involves sequence-specific transcription factors that bind enhancers and promoters to activate or repress RNA polymerase II recruitment, with some enhancers acting from more than one million base pairs away from the genes they control. Post-transcriptional regulation includes alternative splicing, mRNA stability control, microRNA-mediated silencing, and translational repression by RNA-binding proteins. Epigenetic mechanisms, including DNA methylation at CpG islands and histone modifications such as H3K27 trimethylation, provide heritable layers of control that can be maintained across cell divisions without changes to the underlying DNA sequence.
A single human cell type expresses roughly 10,000 to 15,000 genes at any given moment, even though all somatic cells carry the same approximately 20,000 protein-coding genes. Disruption of gene regulatory networks underlies developmental disorders, autoimmune diseases, and most cancers.
The same stretch of DNA can regulate multiple genes simultaneously through long-range chromatin looping. A single super-enhancer in mouse embryonic stem cells (Mus musculus) can contact and activate five or more target genes at once by drawing distant promoters into a shared transcriptional hub.
Every gene in a cell is active all the time, and regulation only determines how much protein is made. Regulation operates at multiple levels, including whether a gene is transcribed at all, whether the resulting RNA is processed and exported, and whether the mRNA is translated or degraded.
Human globin genes are regulated in a strict developmental sequence controlled by the locus control region on chromosome 11. Embryonic epsilon-globin is expressed in yolk-sac erythroid cells during the first weeks of gestation, fetal gamma-globin dominates from roughly 10 weeks until birth, and adult beta-globin becomes the predominant form within the first six months of postnatal life.
Gene Silencing
/ jeen SY-len-sing / · English: gene + silencing
Gene Silencing is the suppression of gene expression through mechanisms that reduce transcription, destabilize mRNA, or block translation without necessarily changing the DNA sequence.
Transcriptional silencing can occur through DNA methylation, repressive histone modifications such as H3K27me3, and chromatin compaction that prevents RNA polymerase access. Post-transcriptional silencing includes microRNA and siRNA pathways that guide Argonaute-containing complexes to target mRNAs for degradation or translational repression. X-chromosome inactivation silences most genes on one X chromosome in female mammals through XIST RNA coating, Polycomb recruitment, and DNA methylation.
Therapeutic silencing is now clinically important: inclisiran received its first approval in the European Union in 2020 and U.S. FDA approval in 2021 to lower LDL cholesterol by silencing PCSK9 mRNA in liver cells.
Paramutation in maize shows that silencing information can pass between alleles at the same locus. A silenced allele can convert an active allele into a silenced state that remains heritable even after the original allele is no longer present.
A silenced gene has been deleted from the genome. Silencing leaves the DNA sequence present but reduces or blocks expression through epigenetic marks, RNA-guided repression, or mRNA decay.
In female cats, X-chromosome inactivation silences either the orange or black coat-color allele in each melanocyte lineage. Because the decision occurs early in development and is clonally inherited, tortoiseshell patches often span 1 to several centimeters across the coat.
Kupffer Cells →Genome Editing
/ JEE-nohm ED-ih-ting / · Greek genea, birth; nomos; Old English editan
Genome Editing is a set of molecular techniques that introduce targeted insertions, deletions, substitutions, or regulatory changes at chosen DNA sites in living cells.
Genome editing tools combine a programmable targeting component with an enzyme or effector that changes DNA or gene activity at a selected locus. Zinc finger nucleases and TALENs use engineered proteins to recognize DNA, while CRISPR-Cas systems use guide RNAs to direct nucleases such as Cas9 to matching sequences. After a double-strand break, cells repair the site through non-homologous end joining, which often creates small insertions or deletions, or through homology-directed repair when a donor template is available.
Base editors and prime editors expand the toolkit by making specific nucleotide changes without relying on a standard double-strand break. In December 2023, the U.S. FDA approved Casgevy as the first FDA-approved CRISPR-based gene-editing therapy for sickle cell disease, a milestone that showed edited human cells can be used as a licensed treatment.
Zinc finger nucleases preceded CRISPR as programmable genome-editing tools. They were used in early clinical trials to disrupt the CCR5 gene in patient T cells, but each target required custom protein engineering rather than a simple guide RNA change.
Genome editing always inserts a new gene into the genome. Many editing strategies instead disrupt a gene, correct a single nucleotide, alter a regulatory sequence, or change gene expression without adding any foreign coding sequence.
CRISPR-Cas9 genome editing has been used to alter hematopoietic stem cells from patients with sickle cell disease by disrupting an erythroid enhancer of BCL11A. Reducing BCL11A reactivates fetal hemoglobin, and treated patients in clinical studies often reached fetal hemoglobin levels above 30 percent of total hemoglobin after edited cells engrafted.
Guide RNA
/ gyd ar-en-AY / · English: guide + RNA abbreviation
Guide RNA is an engineered RNA molecule that directs a nuclease such as Cas9 to a specific genomic target by base-pairing with the complementary DNA sequence, enabling site-specific cleavage.
In the CRISPR-Cas9 system, the single guide RNA is a fusion of the CRISPR RNA and the trans-activating CRISPR RNA, combining target recognition and Cas9 recruitment into one molecule. A 20-nucleotide spacer sequence at the 5-prime end of the guide RNA determines target specificity by base-pairing with the protospacer in the genome, while a protospacer adjacent motif, typically the trinucleotide NGG for Streptococcus pyogenes Cas9, must be present immediately downstream for cleavage to occur. Mismatches between the guide RNA and the target DNA are tolerated to varying degrees depending on their position; mismatches within the seed region, the 10 to 12 nucleotides nearest the protospacer adjacent motif, most severely reduce cleavage efficiency.
Guide RNA design algorithms score candidate spacers for predicted off-target sites across the genome, and chemical modifications such as 2-prime-O-methyl groups at the termini improve stability and reduce innate immune activation in therapeutic applications. Truncated guide RNAs of 17 to 18 nucleotides rather than the standard 20 can improve specificity by reducing tolerance for mismatches at off-target sites without substantially lowering on-target cleavage.
Catalytically dead Cas9 guided by a standard guide RNA, a system called CRISPRi, can silence a target gene without cutting DNA at all; when fused to a transcriptional repressor domain, the complex blocks RNA polymerase access and reduces target gene expression by more than 1,000-fold in bacterial cells.
Building Blocks of Nucleic Acids →Cas9 locates its target sequence by scanning the genome independently of any guide. Cas9 has no intrinsic DNA sequence preference; the guide RNA provides all target specificity by base-pairing with the protospacer, and Cas9 will not cleave any sequence the guide RNA does not match.
In a 2022 clinical trial for transthyretin amyloidosis, a guide RNA directed the Cas9 nuclease to the TTR gene in liver cells following intravenous delivery of lipid nanoparticles. A single infusion reduced serum transthyretin protein levels by approximately 87% at 12 months, demonstrating durable in vivo editing from one treatment.