Transcript Chapter 19

Chapter 19: Eukaryotic Genomes
Most
gene expression regulated through
transcription/chromatin structure
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Histones (+ proteins) bind to – DNA
Nucleosome- DNA wound around histones separated
by linker DNA strands
Telomere & centromere regions=heterochromatin
inaccessible to polymerases
Differential gene expression cell specialization
Gene expression control: regulation
of chromatin structure
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Heterochromatin never expressed
Expression of genes through chemical
modifications of histones
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Histone acetylation:
(-COCH3) is attached to + histone tail
 Histone tail is neutralized
 histone no longer binds
 Allows polymerases gene access
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De-acetylation allows recoiling of histone
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DNA methylation
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Methyl groups added to cytosine inactivate
gene expression inactivated long term
Methylation pattern passed through genomic
imprinting
Gene expression control: regulating
transcription initiation
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General transcription factors assembly
of transcription initiation complex (low
rate of transcription)
specific transcription factors affect
transcription rate
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Activators- binds to enhancer stimulate
transcription
Repressors- inhibit expression by
preventing activator binding
 Inhibiting the enhancer
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Gene expression control: Posttranscriptional regulation
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RNA processing
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Alternative RNA splicing
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Segments of primary transcript treated as exons & introns
varies
regulatory proteins determine intron-exon choices
mRNA degradation
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Enzymes degrade mRNA by
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shortening poly-A tail
removing 5’ cap
Nuclease destroys the mRNA
microRNAs bind to complementary mRNA sequence
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dicer enzyme cuts this double stranded RNA
RNA interference (RNAi)
Gene expression control: Posttranscriptional regulation
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Blocking translation initiation
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Regulatory proteins block ribosome
attachment
Environmental factors regulate initiation
factors
Protein processing & degradation
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Delay of protein modification/activation
Enzymes destroy protein
Eukaryotic Genomes: non-coding DNA
sequences
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Non-coding regions
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“junk DNA”
Code for rRNA & tRNA
Repetitive DNA
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Sequences with multiple copies in the genome
Mostly transposable elements
Transposable Elements
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Transposons
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Move within genome
“cut & paste”
Retro-transposons
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Move within the genome
” copy & paste”
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Repetitive DNA
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Repeated sequences of 15 to 500 nucleotides
The number of times the repeat occurs varies
Often found at centromeres & telomeres
Genes & Multi-gene families
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Multi-gene families
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Identical or very similar genes
Identical genes = histone proteins or RNA
Non-identical genes code for families of
proteins (ex. Globins myoglobin and
hemoglobin)
Genome Evolution
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Evolution occurs through
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Duplications
Failure during meiosis ex. Polyploidy, unequal
crossing over, slippage
 Example: globin genes
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Rearrangements
Exon duplication
 Exon shuffling
 Transposable elements
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mutations