Epigenetics and Imprinting - Exam 4

Epigenetics

heritable changes in gene expression that do not change the DNA sequence

modifications alter the accessibility of genes and their promoters by altering chromatin structure

effects may be reversible

Primary Mechanisms

ncRNA expression, histone tail modifications, and DNA methylation

Somatic

cell differentiation and x-inactivation

Transgenerational

genetic imprinting

grandparents life and environmental impacts can affect their grandkids genomes - ex: stress and smoking

Programmed

changes to gene expression that occur during development controlled by genetic code

ex: imprinting, x-inactivation, and cell differentiation

Environmental

external factors controlling gene expression

ex: smoking, prenatal vitamins, and stress

Chromatin Remodeling

DNA methylation, ncRNAs and histone modifications that affect gene expression levels

DNA Methylation

condenses chromatin structure and lowers gene expression levels

Tail Acetylation

relaxes chromatin structures leading to higher levels of gene expression

Histone Tail Modifications

different amino acids on them can be phosphorylated, acetylated, methylated, and ubiquitinated

Writers

introduce modifications on DNA and histone tails

Readers

recognize modifications and recruit chromatin remodeling enzymes, or recruit transcription factors or ncRNAs

Erasers

remove modifications introduced by writers

lcrRNAs

recruit remodeling complexes that can condense or decondense chromatin

different histone tail modifications recruit different remodeling complexes

CpG

targets for methylation

not uniformly distributed - most found around first exon

moving 5’ to 3’ a C then a G

5-cytosine phosphate guanine-3

CpG Islands

clusters of CpG sequences frequently located in promoter regions

methylation here inhibits transcription through inhibition or recruitment

not all promoters have them and not all of them are methylated and no one alone controls expression

Methylation Inheritance

methylated DNA acts as a template for methyl transferases

after interphase methylated DNA on old strands recruits DNA methylases that add methyl groups to newly synthesized strand

Methyltransferases

enzymes that add methyl groups

Histone Modifications

its thought patterns on original chromatin structures are read and then written onto new chromatin complexes

X Inactivation

an epigenetic process where daughter cells remember which x chromosome is condensed into a barr body

the center encodes RNAs that run in opposite directions - xist and tsix

Escape Genes

a small number of genes on the inactivated x chromosome can still be expressed in certain circumstances

X Inactivation Skewing

can result in different degrees of x linked disease presentation

Imprinting

leads to monoallelic expression - 1% of mammal genes

a biological phenomenon in which certain genes are expressed in a parent of origin specific manner due to epigenetic modifications

occurs during gametogenesis and resets during this too

erased during first steps of meiosis and reset based on sex of parent

occurs independent of alleles and every gamete from an individual has the same imprinting pattern

Maternal Imprint

genes turned off in the chromosomes inherited from our mother

paternal allele will be expressed

methylated during oogenesis

Paternal Imprint

genes turned off in the chromosomes inherited from our father

maternal allele will be expressed

methylated during spermatogenesis

More Growth Factor

leads to a larger than normal animal

More Receptor

leads to a smaller than normal animal

PW Angelman

pw genes are maternally imprinted

angelman genes are paternally imprinted

loss of function alleles where the absence of a gene causes the disease