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Unformatted text preview: NA: Small Nuclear RNAs •  Introns –  Removed during pre ­mRNA maturation through two transesteriGication reactions. •  Pre ­spliceosome –  5′ ­end of the U1 snRNA base pairs with the 5′ junction (donor site or 5′ splice site) –  Disruption of the U1 snRNA – 5 junction base pairing… –  Allows other spliceosomal components U4/U5/U6 to interact with the donor site •  Spliceosome –  Catalyzes intron removal and splicing of exons –  Complex of snRNAs and snRNPs (small nuclear ribonucleoproteins) –  5 snRNPs U1, U2, U4, U5 and U6 are involved in the majority of the pre ­ mRNA splicing. Survival of motor neuron (SMN) complex •  SMN mediates assembly of snRNA and proteins (snRNPs) in eukaryotic cells. •  Assembly apparatus composed of –  SMN and the Gemins •  Binds to the Sm proteins and snRNAs •  constructs the heptameric rings  ­ common cores of Sm proteins •  on the Sm site (AU(56)G) of the snRNAs. SMN protein assembles snRNP complex Mutations in the SMN Gene causes spinal muscular atrophy(SMA) Spinal Muscular Atrophy •  Normal SMN1 gene –  codes for the survival of motor neuron protein (SMN) which plays a crucial role in survival of motor neurons •  Normal SMN2 gene –  variation in a single nucleotide (840.C→T)  ­ undergoes alternative splicing at the junction of intron 6 to exon 8 –  25% of SMN2 transcripts – full length –  75% of transcripts  ­truncated protein compound missing exon 7 (SMNΔ7) which is rapidly degraded in the cell •  SMA ­affected individuals –  SMN1 is mutated and therefore have only 10 ­20% of the normal levels of SMN protein from SMN2 gene •  Reduced availability of the SMN protein –  Gradual death of motor neuron cells in the anterior horn of spinal cord and the brain. –  Progressive atrophy of motor...
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This document was uploaded on 01/18/2014.

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