13. Control of eukaryotic gene expression 2

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  • 13. Control of eukaryotic gene expression 2
    • Chromatin
      • Trans acting transcription factors (activators or repressors) bind to cis acting dna sequence in chromatin
      • Chromosomes organised, supercoiled, nucleoprotein, fibrous 3x109 bp
      • Building blocks of chromatin are nucleosomes146bp DNA per nucleosome. Wrapped around an octamer of core histones
        • Nucleosomescan be covalently modified. Structural changes affects transcription
      • Structure; efficient and ordered packaging of large amounts of DNA. Structure makes it difficult for transcription factors to find target genes.
    • Histones
      • 1. Chromatin remodelling factors - coil and uncoil chromatin fibres (reversible packaging). This move histones along the dna molecules opening and closing access to specific dna sequences
        • 2. Covalent modificationsof specific regions on dna and histones "flagged". Direct transcription factors here.
      • Nucleosomescan be fixed into position within chromatin by linker histones H1 which facilitates chromatin compaction and organisation but limit accessibility
        • CRF displace linker and core histones from stable interaction with dna
      • Acetylation of lysin residues in the N terminal tails of core histones. This acts as flags that recruit transcriptional activators keeping genes on
        • Methylation of dna in cytosine bases in cpg dinucleotidesis a hallmark of transcriptionally inactive chromatin; helps to switch gene off
          • Causes loss of transcription activators. Methylation carried out by enzymes. Causes recruitment of methyl-c binding proteins and hence a repressor
      • Changes to chromatin structure that effects expression of genes without altering nucleotide sequence are known as epigenetic changes
        • Tortoise shell cats exhibit an epigenetic phenotype caused by random epigenetic silencing of the coat colour genes in patches of skin pigment cells
    • Post transcriptionalcontrols
      • RNA transcript - splicing - 5' capping polyadenylation - nuclear export - mrna localisation in cytoplasm - mrna translation - mrna stability
      • RNA splicing involves removal of introns and the ligation of  exons. Controls sex determinationin drosophila
        • Transformer protein (only in females) is a splicing activator. Removal of intro (DSXF) results in female. DSXM results in male.
        • Alternative splicing controls the synthesis of multiple protein isoforms from a single gene as produce different exon-intron junction in rna transcripts
      • 5' cap structure required for translation initiation. Initiation factors regulate translation e.g. phosphorylation
        • Translation of mrna encoding the iron storage protein ferritin is positively regulated by the free ion concentration. Low iron; aconitase binds to ferritin mrna and inhibits translation. High; it dissociates so translation can occur

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