Ch. 11: Gene regulations How is cloning possible? Every cell has the same chromosomes Then….. Why does a heart muscle cell look different from a skin cell? Organisms respond to their environment by altering gene expression Central question: what regulates gene expression?
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Ch. 11: Gene regulations How is cloning possible? Every cell has the same chromosomes Then….. Why does a heart muscle cell look different from a skin.
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Ch. 11: Gene regulationsHow is cloning possible?
Every cell has the same chromosomes
Then….. Why does a heart muscle cell look different from a skin cell?
Organisms respond to their environment by altering gene expression
Central question: what regulates gene expression?
Differentiation
Differentiation is controlled by turning specific sets of genes on or off
DNA Packing
eukaryotic chromosomes condense during prophase of Mitosis
helps regulate gene expression by preventing transcription
– Nucleosomes
– Tight helical fiber =
– Supercoil = coiling of the tight helical fiber
DNA double helix(2-nm diameter)
“Beads ona string”
Linker
Histones
Metaphasechromosome
Tight helical fiber(30-nm diameter)
Nucleosome(10-nm diameter)
Supercoil(300-nm diameter)
700 nm
Animation: DNA Packing
X-chromosome inactivation
– female mammals
– one of the two X chromosomes is highly compacted and transcriptionally inactive (Barr body)
– Occurs early in embryonic development, thus all cellular descendants have the same inactivated chromosome
– Tortoiseshell fur coloration is due to inactivation of X chromosomes in heterozygous female cats
Two cell populationsin adult
X chromosomes
Early embryo
Allele forblack fur
Inactive X
Black furAllele fororange fur
Orange fur
Cell divisionand random
X chromosomeinactivation Active X
Inactive X
Active X
Eukaryotic gene expression
– Each gene has its own promoter and terminator
– Are controlled by interactions between numerous regulatory proteins and control sequences
Regulatory proteins • Transcription factors - help RNA
polymerase bind to the promoter• Activators –
• Silencers - Control sequences
– Promoter– Enhancer
– Related genes located on different chromosomes can be controlled by similar enhancer sequences
Animation: Initiation of Transcription
Enhancers
Otherproteins
DNA
Transcriptionfactors
Activatorproteins
RNA polymerase
Promoter
Gene
Bendingof DNA
Transcription
Alternative RNA splicing
– Can involve removal of an exon with the introns on either side
Animation: RNA Processing
1
or
Exons
DNA
RNA splicing
RNAtranscript
mRNA
2 3 4 5
1 2 3 4 5
1 2 4 51 2 3 5
Small RNAs control gene expression
RNA interference (RNAi)
– small, complementary RNAs bind to mRNA transcripts, blocking translation
MicroRNA (miRNA)
– MicroRNA + protein complex binds to complementary mRNA transcripts, blocking translation