Chapter 19: Eukaryotic GenomesChapter 19: Eukaryotic Genomes
Most gene expression regulated Most gene expression regulated through transcription/chromatin through transcription/chromatin structurestructure
• Histones (+ proteins) bind to – DNAHistones (+ proteins) bind to – DNA• Nucleosome- DNA wound around histones Nucleosome- DNA wound around histones
separated by linker DNA strandsseparated by linker DNA strands• Telomere & centromere Telomere & centromere
regions=heterochromatinregions=heterochromatin inaccessible to inaccessible to polymerasespolymerases
Differential gene expressionDifferential gene expression cell cell specializationspecialization
Gene expression control: Gene expression control: regulation of chromatin regulation of chromatin
structurestructure Heterochromatin never expressed Heterochromatin never expressed Expression of genes through Expression of genes through
chemical modifications of histoneschemical modifications of histones Histone acetylation: Histone acetylation:
(-COCH(-COCH33) is attached to + histone tail ) is attached to + histone tail Histone tail is neutralized Histone tail is neutralized histone no longer binds histone no longer binds Allows polymerases gene accessAllows polymerases gene access
De-acetylation allows recoiling of De-acetylation allows recoiling of histonehistone
DNA methylationDNA methylation Methyl groups added to cytosineMethyl groups added to cytosine
inactivate gene expression inactivated inactivate gene expression inactivated long termlong term
Methylation pattern passed through Methylation pattern passed through genomic imprintinggenomic imprinting
Gene expression control: Gene expression control: regulating transcription regulating transcription
initiationinitiation General transcription factorsGeneral transcription factors assembly of transcription initiation assembly of transcription initiation complex (low rate of transcription)complex (low rate of transcription)
specific transcription factorsspecific transcription factors affect affect transcription ratetranscription rate Activators- binds to enhancerActivators- binds to enhancer
stimulate transcriptionstimulate transcription Repressors- inhibit expression byRepressors- inhibit expression by
preventing activator binding preventing activator binding Inhibiting the enhancerInhibiting the enhancer
Gene expression control: Post-Gene expression control: Post-transcriptional regulationtranscriptional regulation
RNA processingRNA processing Alternative RNA splicing Alternative RNA splicing
Segments of primary transcript treated as exons & Segments of primary transcript treated as exons & introns variesintrons varies
regulatory proteins determine intron-exon choicesregulatory proteins determine intron-exon choices
mRNA degradationmRNA degradation Enzymes degrade mRNA by Enzymes degrade mRNA by
shortening poly-A tail shortening poly-A tail removing 5’ cap removing 5’ cap Nuclease destroys the mRNANuclease destroys the mRNA
microRNAs bind to complementary mRNA microRNAs bind to complementary mRNA sequence sequence
dicer enzyme cuts this double stranded RNAdicer enzyme cuts this double stranded RNA RNA interference (RNAi)RNA interference (RNAi)
Gene expression control: Post-Gene expression control: Post-transcriptional regulationtranscriptional regulation
Blocking translation initiationBlocking translation initiation Regulatory proteins block ribosome Regulatory proteins block ribosome
attachmentattachment Environmental factors regulate initiation Environmental factors regulate initiation
factorsfactors Protein processing & degradationProtein processing & degradation
Delay of protein modification/activationDelay of protein modification/activation Enzymes destroy proteinEnzymes destroy protein
Eukaryotic Genomes: non-coding Eukaryotic Genomes: non-coding DNA sequencesDNA sequences
Non-coding regionsNon-coding regions ““junk DNA”junk DNA” Code for rRNA & tRNA Code for rRNA & tRNA Repetitive DNA Repetitive DNA
Sequences with multiple copies in the genomeSequences with multiple copies in the genome Mostly transposable elementsMostly transposable elements
Transposable ElementsTransposable Elements TransposonsTransposons
Move within genomeMove within genome ““cut & paste”cut & paste”
Retro-transposonsRetro-transposons Move within the genomeMove within the genome ” ” copy & paste”copy & paste”
Repetitive DNARepetitive DNA Repeated sequences of 15 to 500 Repeated sequences of 15 to 500
nucleotidesnucleotides The number of times the repeat occurs The number of times the repeat occurs
variesvaries Often found at centromeres & telomeres Often found at centromeres & telomeres
Genes & Multi-gene familiesGenes & Multi-gene families
Multi-gene familiesMulti-gene families Identical or very similar genesIdentical or very similar genes Identical genes = histone proteins or Identical genes = histone proteins or
RNARNA Non-identical genes code for families of Non-identical genes code for families of
proteins (ex. Globinsproteins (ex. Globins myoglobin and myoglobin and hemoglobin)hemoglobin)
Genome EvolutionGenome Evolution Evolution occurs throughEvolution occurs through
DuplicationsDuplications Failure during meiosis ex. Polyploidy, Failure during meiosis ex. Polyploidy,
unequal crossing over, slippageunequal crossing over, slippage Example: globin genesExample: globin genes
RearrangementsRearrangements Exon duplicationExon duplication Exon shufflingExon shuffling Transposable elementsTransposable elements
mutationsmutations