Epigenetics and Genetics Study Notes
Genetics vs Epigenetics
Genetics: study of inheritance and variation through DNA sequence (genotype) and how it determines phenotype.
Epigenetics: heritable changes in gene expression that do not involve changes to the DNA sequence itself.
Core idea: gene activity can be turned on or off by chemical modifications and pathways that regulate chromatin and transcription.
Key mechanisms (as introduced in the transcript): histone modifications, DNA methylation, and RNA interference (RNAi, including microRNA or miRNA).
Epigenetic changes can be influenced by the cellular environment and developmental context; some changes can be reversible.
Epigenetics links to inheritance (transgenerational effects) and real-world relevance (development, disease, and response to environment).
Types of Epigenetic Mechanisms
A. Histone modifications
Histone tails (e.g., on H2A, H2B, H3, H4) are subject to post-translational modifications.
Common modifications include acetylation and methylation.
Enzymes involved:
HATs (Histone acetyltransferases): add acetyl groups to lysine residues in histone tails.
HDACs (Histone deacetylases): remove acetyl groups.
Consequence:
Histone acetylation generally correlates with open chromatin (euchromatin) and active transcription.
Deacetylation is associated with tighter packing (heterochromatin) and reduced transcription.
Other chromatin regulators mentioned in the transcript: TRX-COMPASS, PRC2, PRC1, Ring1a/b, and associated histone marks (e.g., Me3) contributing to activation or repression.
Structural context: acetylation reduces histone–DNA interactions, promoting a more accessible chromatin state.
B. DNA methylation
Covalent addition of a methyl group to the 5' position of cytosine bases.
Occurs largely at CpG dinucleotides.
CpG islands: regions of DNA > base pairs with more than CG content, typically found in gene promoters.
Catalyzed by DNMTs (DNA methyltransferases).
General rule: higher DNA methylation corresponds to lower gene expression.
Mechanistic consequence: methylation can block transcription factor binding and recruit methyl-binding domain (MDB) proteins that compact chromatin.
C. RNAi (RNA interference)
RNA inhibitors that suppress gene expression post-transcriptionally.
Includes microRNAs (miRNA) as a major class.
Pathway overview (miRNA):
miRNA genes are transcribed in the nucleus as primary miRNAs (pri-miRNA).
Drosha processes pri-miRNA to precursor miRNA (pre-miRNA).
Exportin-5 (EXPO5) exports pre-miRNA to the cytoplasm.
Dicer cleaves pre-miRNA to mature miRNA.
Mature miRNA associates with the RISC (RNA-induced silencing complex).
The miRNA guides RISC to target mRNAs, causing translational repression or mRNA degradation.
Note: miRNA can be encoded in our genome and can also be obtained from the diet (e.g., milk exosomes).
How DNA Fits Inside the Nucleus
6-foot-long DNA molecule packaging in the nucleus occurs through hierarchical organization:
Histones form nucleosomes around which DNA is wound.
Nucleosomes organize into chromatin fibers.
Chromatin fibers further condense into chromosomes during mitosis/meiosis.
Chromatin States: Euchromatin vs. Heterochromatin
Interphase chromatin exists in two main states: euchromatin and heterochromatin.
Euchromatin
“Active” chromatin.
Generally gene-rich and transcriptionally active.
Heterochromatin
“Silent” chromatin.
Typically gene-poor or transcriptionally repressed.
Conceptual images show the spatial organization of DNA within the nucleus and how chromatin state correlates with gene expression.
Histone Acetylation
Key players:
HATs (Histone acetyltransferases): add acetyl groups to lysine residues on histone tails.
HDACs (Histone deacetylases): remove acetyl groups from lysine residues.
Consequences:
Acetylation reduces positive charge on histones, weakening DNA–histone interactions.
Leads to a less compact chromatin structure (euchromatin) and increased transcription.
Deacetylation generally promotes chromatin compaction (heterochromatin) and reduced transcription.
Histone Methylation
Key players:
HMTs (Histone methyltransferases): add methyl groups to lysine or arginine residues on histone tails.
The methyl donor is SAM (S-adenosyl methionine).
Consequences:
Histone methylation is often associated with transcriptional repression, though the effect depends on the specific residue and context (e.g., H3K4me3 often marks active promoters, while H3K27me3 is repressive).
Note from transcript: generally, histone methylation correlates with reduced gene expression, depending on marks.
DNA Methylation in Detail
Mechanism:
Covalent addition of a methyl group to the 5' carbon of cytosine bases.
Primarily occurs at CpG sites.
CpG islands:
Regions > with > , typically located in gene promoters.
Enzymes:
DNMTs (DNA methyltransferases) catalyze methylation.
Functional outcome:
Increased DNA methylation generally leads to decreased gene expression.
Interaction with transcription factors:
Methylated DNA can prevent transcription factor binding, contributing to gene silencing.
DNA Methylation and Transcription Factor Binding
Methylation can block transcription factor access to DNA.
MDB proteins (Methyl-Binding Domain) recognize methylated DNA and recruit repressive complexes.
Reference highlighted: Qian, J. et al. (2016) Nat Rev Genet; TF = transcription factor; MDB = methyl-binding domain protein.
Epigenetic Patterns and Gene Expression
Epigenetic marks influence chromatin structure and gene activity:
DNA methylation and histone methylation typically promote nucleosome packing (compact chromatin) and gene repression.
Histone acetylation promotes looser nucleosome packing and gene activation.
Mechanistic linkage:
Methylation of DNA and histones can cause nucleosomes to pack tightly, hindering transcription factor binding and leading to gene silencing.
Acetylation of histones leads to more accessible DNA and active transcription.
miRNA and RNA Interference (RNAi)
miRNA are a type of RNAi that regulate gene expression post-transcriptionally.
Biogenesis and function (summary from transcript):
miRNA genes are transcribed in the nucleus as pri-miRNA.
Drosha cleaves pri-miRNA to pre-miRNA in the nucleus.
Pre-miRNA is exported to the cytoplasm by Exportin-5 (EXPO5).
Dicer processes pre-miRNA to mature miRNA.
Mature miRNA incorporates into RISC (RNA-induced silencing complex).
The miRNA–RISC complex binds target mRNA, causing translational repression or mRNA degradation.
Role in gene regulation:
miRNA can downregulate expression of specific genes by interfering with mRNA stability and translation.
Dietary and Endogenous miRNA
miRNAs are encoded in our genome, but we can also acquire miRNAs from our diet.
Milk exosomes are mentioned as a source of dietary miRNA, illustrating potential cross-species regulatory effects.
Epigenetics and Inheritance
Generational scope of epigenetic effects (as presented):
Mother — 1st generation (influences the fetus in the intrauterine environment).
Fetus — 2nd generation (developmental effects during gestation).
Reproductive cells — 3rd generation (germline epigenetic changes).
Implication:
Epigenetic patterns can be inherited across generations, influencing traits beyond the immediate individual.
Key Takeaways and Connections
Epigenetic regulation provides a layer of control over gene expression beyond DNA sequence alone.
The main mechanisms—histone modifications, DNA methylation, and RNAi/miRNA—interact to modulate chromatin structure and transcription.
The same marks can have different outcomes depending on the specific residue, context, and cellular environment (e.g., certain methyl marks can activate or repress transcription).
Epigenetic inheritance introduces the idea that environmental exposures and cellular states can have lasting effects across generations, though the extent and mechanisms are an active area of research.