Themes: RNA is very versatile! RNA and RNA Processing Chapter 14 RNA-RNA interactions are very important! Prokaryotes and Eukaryotes have many important differences. Messenger RNA (mrna) Carries genetic information from DNA to a ribosome Helps assemble amino acids in the correct order What is a gene? Colinearity: a direct correspondence between DNA sequence and amino acid sequence of a protein. So, a gene must be a stretch of nucleotides that codes for a protein. Figure 14.1 1
Figure 14.2 Heat Single stranded fragments Non Coding Regions of DNA look like loops DNA-DNA Cool DNA-RNA Introns Segments of DNA within a gene that do not code for amino acids Transcribed into RNA, and then removed by RNA processing Common in eukaryotic genes, rare in bacterial genes Number and size vary from gene to gene Colinearity?? Not Always! The presence of introns means that EUKARYOTIC genes are frequently NOT colinear with the proteins they encode. Figure 14.3 Code for polypeptides 2
Humans: Average Human Gene: 10 to 15 exons Encodes 3 or more alternatively spliced proteins Each amino acid in a protein is specified by a three nucleotide sequence in the mrna called a CODON. Three Primary Regions of Mature RNA: Figure 14.5 Three Primary Regions of Mature RNA: Figure 14.5 Involved in Ribosome Binding Involved in Ribosome Binding Codon: a sequence of three nucleotides that codes for a specific amino acid Codon: a sequence of three nucleotides that codes for a specific amino acid Pre-mRNA Processing Bacteria: Transcription and Translation occur simultaneously No time for RNA modification Foreshadowing: An mrna with several ribosomes attached. 3
Pre-mRNA Processing Eukaryotes Transcription in nucleus Translation in cytoplasm Plenty of opportunity for RNA modification prior to translation Eukaryotic Pre-mRNA Processing Addition of the 5 Cap A modified nucleotide and several methyl groups are added to the 5 end Facilitates binding of ribosome, increases RNA stability, influences intron removal Addition of the Poly(A) tail Confers stability on some mrnas Figure 14.7 4
RNA Splicing Removal of Introns 3 sequences required 5 Splice site (consensus sequence GU) 3 Splice site (consensus sequence AG) Branch point (Adenine 18 to 40 nucleotides upstream of 3 splice site) Splicing occurs in a large complex called the spliceosome A SPLICEOSOME is a structure consisting of several RNA molecules (snrnas) and many different proteins. Figure 14.10 Figure 14.11 Figure 14.12 Figure 14.12 snrnps: small ribonucleoprotien particles 5
Figure 14.10 BRCA1 Normal protein: lower incidence of breast and ovarian cancer What if we had a mutation (Chapter 17) inside the intron? A mutation in an exon of BRCA1 is associated with increased cancer incidence. This mutation doesn t change the amino acid sequence, but affects intron splicing. Alternative Processing Pathways Figure 14.16a Alternative Splicing: Exons can be spliced together in different combinations: Produces different proteins from THE SAME GENE Multiple 3 Cleavage sites: Pre-mRNA can be cleaved at different sites: Produces mrnas of different lengths, and thus different proteins from THE SAME GENE We estimate that 40% to 60% of all human genes are alternatively spliced. Dscam Drosophila homologue of the human Down s syndrome adhesion molecule Alternative splicing leads to ~40,000 different mature transcripts 6
Figure 14.16b The Calcitonin Gene RNA editing Coding sequence of mrna is CHANGED AFTER TRANSCRIPTION. Resultant protein has an amino acid sequence different from that encoded in the DNA. Relies on GUIDE RNAs (grnas) RNA processing Summary 5 Cap Poly (A) tail Intron Splicing Alternative processing pathways Alternative Splice Variants Multiple 3 Cleavage sites RNA editing Figure 14.19 7
What is a gene? A DNA sequence that codes for an RNA molecule. Includes exons, introns, rrnas, trnas, Promoter? Terminator? Monday: trnas rrnas sirnas and RNA Interference RNAi and Fluffy 8