分子生物学（张海红）CHAPTER 17.pptVIP

A histone code Different patterns of modification on histone tails can be “read” to mean different things. The “meaning” would be the result of the direct effects of these modifications on chromatin density and form. But in addition, the particular pattern of modifications at any given location would recruit specific proteins. Modification of the histone N-terminal tails alters the function of chromatin Histone modification and the histone code hypothesis Transcription can also be silenced by methylation of DNA by enzymes called DNA methylases. This kind of silencing is not found in yeast but is common in mammalian cells. Methylation of DNA sequence can inhibit binding of proteins, including the transcriptional machinery, and thereby block gene expression. DNA methylases. Switching a gene off : A mammalian gene marked by methylation of nearby DNA sequence recognized by DNA-binding proteins recruit histone deacetylases and histone methylases modify nearby chromatin This gene is completely off. Switching a gene off Patterns of gene expression must sometimes be inherited. These may remain for many cell generations, even if the signal that induced them is present only fleetingly. This inheritance of gene expression patterns is called epigenetic regulation. . epigenetic regulation Nucleosome and DNA modifications can provide the basis for epigenetic inheritance. DNA methylation is even more reliably inherited, but far more efficiently is the so-called maintenance methylases modify hemimethylated DNA－the very substrate provided by replication of fully methylated DNA. Patterns of DNA methylation can be maintained through cell division Mechanisms of Eukaryotic Regulators Signal Transduction and the Control of Transcriptional Regulators Gene “Silencing” by Modification of Histones and DNA Eukaryotic Gene Regulation at Steps after Transcription Initiation After transcription initiation After transcription Translation In eukaryotic ce