Genetics Terms Starting With U
Genetics Glossary: U
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Uniparental Disomy
/ yoo-nih-puh-REN-tul dy-SOH-mee / · Latin: uni (one) + parens (parent) + Greek: disomia (two bodies)
Uniparental Disomy is a condition in which both copies of a chromosome pair are inherited from the same parent rather than one from each parent, potentially disrupting imprinted gene expression even when chromosome number appears normal.
UPD can arise from trisomy rescue, in which a trisomic embryo loses one chromosome, or from gamete complementation, in which two abnormal gametes each contribute the same chromosome. Isodisomy, in which both chromosomes are identical copies of one parental chromosome, can expose recessive mutations if that parent is a carrier. Heterodisomy, in which both homologs from one parent are present, preserves heterozygosity but still disrupts imprinting.
The clinical significance of UPD depends entirely on whether the affected chromosome carries imprinted genes whose expression is controlled by parent of origin.
The first confirmed case of UPD in humans was reported in 1988 when a girl with cystic fibrosis was found to carry two copies of chromosome 7 from her mother and none from her father. Her mother carried only one cystic fibrosis mutation, but maternal isodisomy caused the daughter to inherit two copies of the same mutated allele.
UPD does not always cause disease. UPD affecting chromosomes that carry no imprinted genes may produce no clinical consequence, particularly when isodisomy does not unmask a recessive allele carried by the contributing parent.
Autosomal Recessive Inheritance →UPD of chromosome 15 causes Prader-Willi syndrome when both copies originate from the mother, because the paternally expressed genes in the 15q11-q13 region are absent and the maternally derived copies of those same genes are silenced by imprinting. A standard karyotype showing 46 chromosomes gives no indication that anything is wrong.
Upstream Sequence
/ UP-streem SEE-kwens / · English: upstream + sequence
Upstream Sequence is a DNA region located on the 5-prime side of a gene's transcription start site that contains regulatory elements such as promoters, enhancers, and transcription factor binding sites that control the rate and timing of transcription.
By convention, upstream refers to the direction opposite to transcription, toward lower nucleotide position numbers on the coding strand. Core promoter elements such as the TATA box are typically located 25 to 35 base pairs upstream of the transcription start site and position RNA polymerase for accurate initiation. More distal upstream enhancer sequences can lie hundreds to thousands of base pairs away and dramatically increase transcription when bound by activator proteins.
Because upstream position is defined relative to the direction of transcription, an element that is upstream of one gene may simultaneously be downstream of a neighboring gene transcribed in the opposite direction.
The term upstream in molecular biology is directional and defined relative to the direction of transcription, not by absolute chromosomal position, so an upstream element for one gene may be downstream of a neighboring gene.
Building Blocks of Nucleic Acids →Upstream does not mean the same thing in all biological contexts. In cell signaling, upstream describes earlier steps in a biochemical cascade, while in molecular genetics it specifically refers to the 5-prime direction relative to a gene's transcription start site.
The human erythropoietin gene carries a hypoxia response element approximately 120 base pairs upstream of its transcription start site that binds the transcription factor HIF-1 alpha when oxygen tension falls below roughly 5 percent. Binding of HIF-1 alpha to this upstream element increases erythropoietin mRNA output by more than 100-fold in kidney and liver cells, driving red blood cell production.
How To Become A Hepatologist? →Uracil
/ YOOR-uh-sil / · Latin: urea + acid
Uracil is a pyrimidine nitrogenous base found in RNA that replaces thymine found in DNA, pairing with adenine through two hydrogen bonds during complementary base pairing.
Uracil lacks the methyl group present on thymine at the 5-carbon position, making it metabolically less costly to synthesize but also less stable in a hereditary molecule. Because cytosine spontaneously deaminates to uracil at a measurable rate, estimated at roughly 100 to 500 events per cell per day in mammals, the presence of uracil in DNA would be ambiguous and mutagenic. This chemical instability is one reason thymine, the methylated form, replaced uracil in DNA during evolution, while RNA retained uracil because its transcripts are short-lived and do not serve as the permanent genetic record.
Uracil appears in mRNA codons, tRNA anticodons, ribosomal RNA, and many regulatory non-coding RNAs.
The chemotherapy drug 5-fluorouracil, introduced clinically in 1962, mimics uracil closely enough to be incorporated into RNA and to block thymidylate synthase, the enzyme that converts uracil nucleotides into thymine nucleotides, starving cancer cells of the building blocks needed for DNA replication.
Uracil is not found in DNA under normal circumstances. When uracil does appear in DNA through cytosine deamination, the enzyme uracil-DNA glycosylase removes it before replication can fix a C-to-T mutation permanently into the sequence.
In the mRNA of Saccharomyces cerevisiae (baker's yeast), all three stop codons, UAA, UAG, and UGA, contain uracil rather than thymine, reflecting the RNA composition of the transcript. Yeast cells produce roughly 300,000 mRNA molecules at any given moment, each carrying uracil-containing codons that ribosomes read with the same fidelity as thymine-containing DNA codons are read during replication.
Building Blocks of Nucleic Acids →