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GENETICS A Conceptual Approach
Benjamin A. Pierce GENETICS A Conceptual Approach FIFTH EDITION CHAPTER 21 Epigenetics © 2014 W. H. Freeman and Company
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Through epigenetics, crop abundance and failure can have health effects that persist for several generations
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21.1 What is Epigenetics? How, through the process of development, a genotype produces a phenotype “epigenesis”–how an embryo develops “genetics”–the study of genes and heredity Changes that are heritable
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Figure Carl Waddington first used the word epigenetics to refer to how a phenotype develops from a genotype.
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21.2 Several Molecular Processes Lead to Epigenetic Changes
Changes in chromatin structure, which alter gene expression Molecular mechanisms that alter chromatin structure: Changes in patterns of DNA methylation Chemical modification of histone proteins RNA molecules that affect chromatin structure and gene expression
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21.2 Several Molecular Processes Lead to Epigenetic Changes
DNA methylation: addition of methyl groups to nucleotide bases Most common: methylation of cytosine to produce 5-methylcytosine
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Figure 21.2 DNA methylation is a common epigenetic modification of chromatin.
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Figure 21.3. DNA methylation is stably maintained through DNA replication.
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Figure 21.4 Epigenetic changes are responsible for differences in the phenotypes of honeybee (Apis mellifera) queens (left) and workers (right).
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21.2 Several Molecular Processes Lead to Epigenetic Changes
Histone modifications: more than 100 different posttranslational modifications of histone proteins Modifications include addition of: Phosphates Methyl groups Acetyl groups ubiquitin
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21.2 Several Molecular Processes Lead to Epigenetic Changes
Epigenetic effects by RNA molecules Examples: X inactivation by Xist Paramutation in corn by siRNAs
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21.3 Epigenetic Processes Produce a Diverse Set of Effects
Paramutation: an interaction between two alleles that leads to a heritable change in expression of one of the alleles Examples: Paramutation in corn Paramutation in mice
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Figure 21.5 In paramutation at the b1 locus in corn, a copy of the B’ allele converts the B-I allele to B’’, which has the same phenotype as B’.
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Figure 21.6 Paramutation at the b1 locus in corn requires the presence of seven tandem repeats upstream of the b1 locus.
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Figure 21.7 Paramutation at the Kit locus in mice.
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21.3 Epigenetic Processes Produce a Diverse Set of Effects
Behavioral epigenetics: life experiences, especially early in life, have long-lasting effects on behavior Epigenetic changes induced by maternal behavior Epigenetic effects of early stress in humans Epigenetics in cognition
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Figure 21.8 Young rats exposed to more licking and grooming from their mothers develop different patterns of DNA methylation, which alters the expression of stress-response genes and makes them less fearful as adults.
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21.3 Epigenetic Processes Produce a Diverse Set of Effects
Epigenetic effects of environmental chemicals Transgenerational epigenetic effects on metabolism Epigenetic effects in monozygotic twins X inactivation
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Figure In X inactivation, the Xist gene on the inactive X produces a long noncoding RNA that coats the inactive X chromosome and suppresses transcription.
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Figure Several genes within the X-inactivation center interact to bring about inactivation of one X chromosome while keeping the other X chromosome active.
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21.3 Epigenetic Processes Produce a Diverse Set of Effects
Epigenetic Changes Associated with Cell Differentiation Genomic Imprinting Epigenetic effects in monozygotic twins
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Figure Differentiated adult cells can be reprogrammed to form induced pluripotent stem cells (iPSCs), which are capable of differentiating into many different types of cells.
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Figure In genomic imprinting, the expression of an allele depends on whether it is inherited from the male or female parent.
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21.4 The Epigenome Epigenome: overall pattern of chromatin modifications possessed by each individual organism Detecting DNA methylation Restriction endonucleases Bisulfate sequencing Detecting histone modifications ChIP Genome-wide epigenetic marks
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Figure 21.13 Bisulfate sequencing can be used to determine the locations of 5-methylcytosines.
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