U1 spliceosomal RNA is the small nuclear RNA

Small nuclear RNA

Small nuclear ribonucleic acid is a class of small RNA molecules that are found within the nucleus of eukaryotic cells. They are transcribed by RNA polymerase II or RNA polymerase III and are involved in a variety of important processes such as RNA splicing , regulation of transcription factors ...

 (snRNA) component of U1 snRNP

SnRNP

snRNPs , or small nuclear ribonucleoproteins, are RNA-protein complexes that combine with unmodified pre-mRNA and various other proteins to form a spliceosome, a large RNA-protein molecular complex upon which splicing of pre-mRNA occurs...

 (small nuclear ribonucleoprotein), an RNA-protein complex that combines with other snRNPs, unmodified pre-mRNA, and various other proteins to assemble a spliceosome

Spliceosome

A spliceosome is a complex of snRNA and protein subunits that removes introns from a transcribed pre-mRNA segment. This process is generally referred to as splicing.-Composition:...

, a large RNA-protein molecular complex upon which splicing

RNA splicing

In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...

 of pre-mRNA occurs. Splicing, or the removal of introns, is a major aspect of post-transcriptional modification

Post-transcriptional modification

Post-transcriptional modification is a process in cell biology by which, in eukaryotic cells, primary transcript RNA is converted into mature RNA. A notable example is the conversion of precursor messenger RNA into mature messenger RNA , which includes splicing and occurs prior to protein synthesis...

, and takes place only in the nucleus

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 of eukaryotes.

The U1 snRNP recognizes

Complementarity

-Mathematics:*Complementary angles, in geometry* Complementarity theory, a concept related to optimization -Physical sciences:* Complementarity , a property of nucleic acid molecules in molecular biology...

 and binds to the sequence of the 5'-splice site of an intron in a strand of pre-mRNA. Experimentation has demonstrated that the binding of U1 snRNA to the 5'-splice site is required, but not sufficient, to begin spliceosome assembly.

There are significant differences in sequence and secondary structure

Secondary structure

In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...

 between metazoan and yeast U1 snRNAs, the latter being much longer (568 nucleotide

Nucleotide

Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

s as compared to 164 nucleotides in humans). Nevertheless, secondary structure predictions suggest that all U1 snRNAs share a 'common core' consisting of helices I, II, the proximal region of III, and IV. This family does not contain the larger yeast sequences.

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