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  1. What is RNA splicing?
  2. Genetic information is transferred from genes to the proteins they encode via a “messenger” RNA intermediate DNA GENE messenger RNA (mRNA) protein transcription translation
  3. Most genes have their protein-coding information interrupted by non-coding sequences called “introns”. The coding sequences are then called “exons” DNA GE NE intron exon 1 exon 2 transcription precursor-mRNA (pre-mRNA) intron
  4. The intron is also present in the RNA copy of the gene and must be removed by a process called “RNA splicing” protein translation mRNA RNA splicing pre-mRNA intron
  5. Splicing a pre-mRNA involves two reactions pre-mRNA intron branchpoint A spliced mRNA Step 2 intermediates Step 1 A
  6. Splicing occurs in a “spliceosome” an RNA-protein complex (simplified) pre-mRNA spliced mRNA spliceosome (~100 proteins + 5 small RNAs) Splicing works similarly in different organisms, for example in yeast, flies, worms, plants and animals.
  7. RNA is produced in the nucleus of the cell. The mRNA has to be transported to the cytoplasm to produce proteins Ribosomes are RNA-protein machines that make proteins, translating the coding information in the mRNA
  8. Pre-messenger RNA Processing cytoplasm nucleus mRNA RNA splicing M7G AAAAAAA200 pre-mRNA intron exon exon AAAAAAA200 M7G transport M7G AAAAAAA200 ribosomes protein cap poly(A) tail
  9. Alternative splicing In humans, many genes contain multiple introns 3 4 5 1 2 1 2 3 5 4 intron 2 intron 3 intron 4 intron 1 Usually all introns must be removed before the mRNA can be translated to produce protein
  10. However, multiple introns may be spliced differently in different circumstances, for example in different tissues. 1 2 3 5 Heart muscle mRNA 1 4 3 5 Uterine muscle mRNA Thus one gene can encode more than one protein. The proteins are similar but not identical and may have distinct properties. This is important in complex organisms 3 5 4 2 1 pre-mRNA
  11. Different signals in the pre-mRNA and different proteins cause spliceosomes to form in particular positions to give alternative splicing
  12. 7 6 5 7 5 6 5 7 Fas pre-mRNA APOPTOSIS Alternative splicing can generate mRNAs encoding proteins with different, even opposite functions (programmed cell death) Fas ligand Soluble Fas (membrane) Fas Fas ligand (membrane- associated) (+) (-)
  13. Alternative splicing can generate tens of thousands of mRNAs from a single primary transcript 12 48 33 2 Combinatorial selection of one exon at each of four variable regions generates more than 38,000 different mRNAs and proteins in the Drosophila cell adhesion molecule Dscam The protein variants are important for wiring of the nervous system and for immune response protein mRNA pre-mRNA
  14. The Genetic Code • Describes how nucleotide sequence is converted to protein sequence • Unit of three nucleotides = a codon • A codon codes for a specific amino acid (structural component of protein)
  15. • The four bases can form 64 different codons • 20 amino acids are found from the nature • Regulatory codons
  16. The Nature of the Genetic Code • A group of three bases codes for one amino acid • The code is not overlapping • The base sequence is read from a fixed starting point, with no punctuation • The code is degenerate (in most cases, each amino acid can be designated by any of several triplets
  17. Features of the Genetic Code • All the codons have meaning: 61 specify amino acids, and the other 3 are "nonsense" or "stop" codons • The code is unambiguous - only one amino acid is indicated by each of the 61 codons • The code is degenerate - except for Trp and Met, each amino acid is coded by two or more codons • Codons representing the same or similar amino acids are similar in sequence