Epigenetics 27-33 Flashcards
What is Epigenetics?
phenotypic variation —> variation in gene expression
What does gene expression changes mean?
gene expression change = chromatin structure change
What are some characteristics of Chromatin Structure?
- cell division stabilized (mitosis)
- passed on (meiosis)
- influenced by environment
What is DNA Modification?
Methylation of nucleotide bases (mostly cytosine)
—– CpG dinucleotides
—– both strands of DNA modified
Lowers gene expression
What is Histone Modification?
- Methylation, phosphorylation, acetylation, ubiquitination
- affects binding to DNA
- some lower some higher gene expression
How is level of DNA Methylation maintained after DNA replication?
-both DNA molecules are hemimethylated
—-methylated on 1 DNA strand
—-
-special methyltransferase enzymes recognize hemimethylated DNA
Does DNA Methylation increase or decrease gene expression?
Decrease
What are the 2 ways DNA methylation down-regulates gene expression?
- Methyl group in major groove prevents binding of proteins that stimulate transcription
- creates binding site for histone deacetylase enzymes
Why are transcription factors important?
They turn on gene expression
What are histone deacetylase enzymes do?
remove acetyl groups from histone proteins
— histones bind DNA more tightly
— chromatin structure is more condensed
— gene expression downregulated
What are characteristics of Histone modifications?
- Heritable
- positively charged tails
— groups of amino acids at end of histones
— wraps around DNA & interact w/phosphates in backbone
What does Histone Acetylation do?
-masks positive charges
-binding to DNA weaker
-chromatin structure less condensed
-stimulates gene expression higher
What does Histone Methylation do?
-creates binding sites for transcription factors
- can “mark” an area for increased or decreased gene expression
—–How? –> specific amino acids in certain histones
Is Histone modification heritable?
Yes
What are some examples of Epigenetics?
- Queen v.s. worker bees
- X-inactivation
- Genomic imprinting
- Behavioral epigenetics
Explain DNA Methylation in Honeybees
- Queen and worker bees develop from genetically identical eggs.
- Differences are caused by changes in gene expression (Diet)
- Queen is the only one that eats royal jelly
-royal jelly = Dnmt3 gene repressed = Less methylation= increased gene expression
Explain X-Inactivation
-random event early in development
- stable changes in gene expression
- daughter cells after mitosis
- histone modification important
What are the important genes of X-inactivation?
X-ist = X-inactivation specific transcript
important for keeping one X activated:
Tsix
Jpx
Xite
What is Xist gene?
-encodes long RNA that does not code for protein
- binds X-chromosome from which it was made
-attracts protein complexes (PRC2:PRC1)
—methylate histones to turn off gene expression
—“epigenetic mark” for repressed gene expression
What is Tsix gene?
- Keeps ative X from being inactiviated
- encodes long RNA that does not code for protein
— anti-sense to Xist RNA
— prevents Xist expression/function on activated X
What is Jpx gene?
-encodes long RNA that does not code for protein
- stimulates synthesis of Xist on inactive chromosome
What is Xite gene?
-encodes long RNA that does not code for protein
- sustains Tsix expression on active X chromosome
Explain Genomic Imprinting
-expression affected by whether mom or dad
- results is autosomal traits that show different results for reciprocal crosses
- clusters 3-12
—encode proteins and noncoding RNAs
—chromatin modifications differ
- genetic conflict hypothesis attempts to explain
hinny vs mule
