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