Introns Flashcards
what are introns
non-coding sequences in gene, not in mature mRNA and they interrupt coding sequence
what are Lariats
discarded bioproducts of RNA splicing
what are snRNPs
small nuclear ribonuceloproteins
what is a spliceosome
molecular machine that removes introns
what are endonucleases
enzymes that internally cut DNA
what are consensus sequences
calculated order of most frequent residues (nt, nucleotides or aa) found in a sequence alignment at each position
what are enhancers
short region (50-1500bp) of DNA that can bind to activator proteins, trigger transcription gene
what is alternative splicing
allows DNA to code for more than one protein by varying exons in mRNA
what are protein domains
regions in proteins that fold up in semi-independent way and have independent function
what is a transposon
genetic element that ‘jumps’ to different locations in genome – jumping genes
what is illegitimate recombination
when recombination happens despite little or no sequence homology presence
what is exon shuffling
molecular mechanism that allow new gene formation
how many different types of introns are there
4
where and what is group 1 introns splicing mechanism
found in nuclear, mitochondrial and chloroplast genes coding for rRNAs, mRNAs and tRNAs
- No ATP
- Splicing needs transesterification reaction steps
- Introns self-splice
where and what is group2 introns splicing mechanism
found in primary transcripts of mitochondrial, chloroplast mRNAs in fungi, algae and plants. A lariat is formed before splicing occurs
- No ATP
- Splicing needs transesterification reaction steps
- Introns self-splice
where and what is group 3 introns splicing mechanism
largest class of introns. In nuclear mRNA primary transcripts. A lariat is formed and a RNA-protein complex of nuclear ribonucleoproteins (SnRNPs) is needed. RNA is called small nuclear RNA (snRNA and there are 5 units: U1-U6, no U5) - Need ATP
where and what is group 4 introns splicing mechanism
in certain tRNAs. Splicing endonuclease cleaves phosphodiester bond at both ends of intron. Two exons joined by mechanism similar to DNA ligase reaction
- Need ATP
- Splicing needs an endonuclease
what is intron structure like
starts with GU (5’ donor) ends with AG (3’ acceptor)
Sequences next to GU and AG and also third sequence in middle (branch point) all similar in each intron (they’re consensus sequences)
- GU and AG are the recognition points
Branch point is a fixed distance from 3’ end
what is the splicing of introns and alternative splicing like
spliceosome cuts introns and re-join exons in mRNA consists of several RNAs (U1, U2, U4, U5, U6) and also several proteins
what percent does alternative splicing occur in genes
40% human genes
how many proteins can a cell encode for with one gene in alternative splicing, where is spliced
- cell can encode many related proteins with one gene
- intron boundaries that are spliced decided by protein factors that bind to intron boundaries and either help or hinder spliceosome cutting
what is the impact of alternative splicing 5UTR’
enhancer regulatory element
lead to altered translation and change in protein expression levels
what must sometimes happen for proteins to become functional
need to be processed to become functional
- Phosphorylation
- Glycosylation
- Methylation
- Sometimes cleavage – cut bit of protein to be functional
what is the evolution of introns
early evolution, proteins may have been small, simple functions
introns might have made it easier for DNA sequences encoding individual protein domains to be joined together in many different combinations
what does exon shuffling form and what occurs during
molecular mechanism, form new genes
two or more exons from different genes can be brought together ectopically, or the same exon can be duplicated to create new exon-intron structure
what are the three mechanisms for exon shuffling
- Transposons mediated exon shuffling
- Crossover during sexual recombination of parental genomes
- Illegitimate recombination
impact of alternative splicing Protein coding sequence
altered protein structure and function, change in protein ratios
impact of alternative splicing 3UTR’
stabilising domain
mRNA instability, change in protein expression levels
