Transcription FactoriesThe processes of replication and transcription are spatially separateThetranscriptome is likely immobile RNA would get tangled with DNA if it rotated around the DNA helixTranscriptionMergedImagesDNAReplication
Transcription Factories ◼ The processes of replication and transcription are spatially separate ◼ The transcriptome is likely immobile RNA would get tangled with DNA if it rotated around the DNA helix
Transcription FactoriesTranscription Needa startsiteNeed to melt DNA口Need to initiateRNA synthesis口Need to extend the RNA transcript口Need to modify the transcript口 Need to signal to stop
Transcription Factories ◼ Transcription Need a start site Need to melt DNA Need to initiate RNA synthesis Need to extend the RNA transcript Need to modify the transcript Need to signal to stop
ChromatinRemodelingModify binding of histones to DNA to access template fortranscription Methylation (-CH,) Acetylation (-COCH,) Phosphorylation (-OPO,2-)Adding or removing chemical groups is often the switchfrom transcriptionally silent → active chromatin → activechromatin → and vice versaTABLE21-2ITranscriptionalActivityandHistoneModificationTranscriptionallySilentChromatinH3K9me2Lys9of histone H3 is dimethylatedH3K27me3Lys27of histoneH3 istrimethylatedTranscriptionallyActive ChromatinH3K4me3Lys4of histoneH3 is trimethylatedH3K9acLys9ofhistoneH3isacetylatedH4K16acLys16ofhistoneH4isacetylated
Chromatin Remodeling ◼ Modify binding of histones to DNA to access template for transcription Methylation (-CH3 ) Acetylation (-COCH3 ) Phosphorylation (-OPO3 2- ) ◼ Adding or removing chemical groups is often the switch from transcriptionally silent → active chromatin → active chromatin → and vice versa
ChromatinRemodelingAcetylation Catalyzed by HATs (histone acetyl transferases) Added to Lys residues from acetyl-CoA Reversed by histone deacetylaseHCHAcetyl-Lys
Chromatin Remodeling ◼ Acetylation Catalyzed by HATs (histone acetyl transferases) Added to Lys residues from acetyl-CoA Reversed by histone deacetylase
ChromatinRemodelingChromatin remodeling Remodeling protein complex driven by ATP hydrolysis Loosens segment of DNA from nucleosomeAllows DNA to be transcribed Nucleosomes appear to slide along DNAClosedOpenNucleosomeboundtochromatin-remodeling complex
Chromatin Remodeling ◼ Chromatin remodeling Remodeling protein complex driven by ATP hydrolysis Loosens segment of DNA from nucleosome • Allows DNA to be transcribed Nucleosomes appear to slide along DNA