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Summary Chapter 12 - Gene Transcription and RNA Modification
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Chapter 12 of Genetics by Brooker.
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Chapter 16 - Gene Regulation in Eukaryotes II: Epigenetics16.1 Overview of Epigenetics
Epigenetics is the study of mechanisms that lead to changes in gene expression that can be
passed from cell to cell and are reversible, but do not involve a change in the sequence of
DNA. This type of change may also be called epimutation - a heritable change in gene
expression that does not alter the sequence of DNA.
In multicellular species that reproduce via gametes, an epigenetic change that is passed
from parent to offspring is called epigenetic inheritance, or transgenerational epigenetic
inheritance.
Different types of molecular changes underlie epigenetic gene regulation. The most
common types of molecular changes that underlie epigenetic control are DNA methylation,
chromatin remodeling, covalent histone modification, the localization of histone variants, and
feedback loops, see table 16.1!
Epigenetic changes may be targeted to specific genes by transcription factors or non-
coding RNAs.
1. Transcription factors may bind to a specific gene and initiate a series of events that
leads to an epigenetic modification. Figure 16.1a
2. Non-coding RNAs (ncRNAs) - RNAs that do not encode polypeptides - are involved
in establishing an epigenetic modification.
Epigenetic changes may be maintained by cis- or trans-epigenetic mechanisms.
- Cis-epigenetic mechanism, the epigenetic change at a given site is maintained only
at that site; it does not affect the expression of the same gene located elsewhere in
the cell nucleus.
- Trans-epigenetic mechanisms are maintained by diffusible molecules, such as
proteins or ncRNAs. An epigenetic change is established by activating a gene that
encodes a transcription factor. (figure 16.3)
Epigenetic gene regulation may occur as a programmed developmental change or be
caused by environmental agents. Many epigenetic modifications that regulate gene
expression are programmed changes that occur at specific stages of development (see table
16.2). During embryonic development, certain genes undergo epigenetic changes that affect
their expression throughout the rest of development.
16.2 Epigenetics and development
Genomic imprinting occur during gamete formation. Methylation may occur at two sites:
1. Imprinting control region (ICR)
2. Differentially methylated region (DMR)
During oogenesis methylation does not occur at either site. A protein called the CTC-binding
factor (CTCF) binds to a DNA sequence containing CTC that is found in both the ICR and
DMR. The CTCFs bound to these sites may bind to each other to form a loop in the DNA. In
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