Genetics Exam 3

F 25

What is epigenetics?

Study of mechanisms that lead to changes in gene expression that can pass from cell to cell, are reversible, but don’t involve changes to DNA

What is epimutation?

A heritable change in gene expression that does not alter the dequence of DNA

What is epigenetic inheritance/ transgenerational epigenetic inheritance?

Epigenetic changes that are passed from parent to offspring

How are cis-epigenetic changes maintained?

At a specific site

How are trans-epigenetic changes maintained?

Maintained by diffusable factors, such as transcription factors

What can a cis epigenetic change affect?

It may affect only one copy of gene but not the other copy

What can a trans-epigenetic change affect?

Both copies of a gene

How can nucleosomes compact?

Forming a zig-zag

What is a nucleosome made up of?

8 histone proteins and 146 bp of DNA 

What gets packaged into chromatin?

Chromosomal DNA in eukaryotic cells

What is euchromatin?

Regions that are not stained during interphase

What are two features of euchromatin

Transcriptionally active and occupies a cnetral position in the nucleus

What is heterochromatin

Regions that are stained throughout the cell cycle

What are three features of heterochromatin?

Greater level of compaction, inhibitory effect on gene expression, and localized along the periphery of the nucleus; attached to nuclear lamina 

What are some roles heterochromatin plays?

Gene slicing, prevention of transposable element movement, and prevention of viral proliferation

What is gene slicing?

Inhibition of transcription; may limit access of activators or inhibit other aspects

What is the prevention of transposable element movement?

Genes needed for transposition are silenced

What is the prevention of viral proliferation?

Genes needed to produce more viruses are silenced 

What is constitutive heterochromatin?

Heterochromatic at the same location in all cell types

What is facultative heterochromatin?

Locations vary among different cell types

What does facultative heterochromatin allow?

Silencing of genes in a cell specific manner 

What is the chromosomal location of constitutive heterochromatin?

Close to centromere and telomere

What are the repeat sequences like in constitutive heterochromatin?

Generally composed of many short tandemly repeated sequences

What does DNA methylation look like in constitutive heterochromatin?

Highly methylated on cytosines in vertebrates and plants

What does histone modifications look like in constitutive heterochromatin?

Amino terminal tails

What is H3K9me3 common in?

Constitutive heterochromatin in yeast and animals

What is H3K9me2 common in?

Constitutive heterochromatin in plants

What are reader domains?

Sometimes PTM recruits other proteins to modify histones further, these proteins bind to PTM via the reader domains

What do reader domains recognize?

Appropriate shape

What is formation like in facultative heterochromatin?

It’s reversible, but it depends on stage of development or cell type

Where is facultative heterochromatins chromosomal location?

Multiple sites between centromere and telomere

What is facultative heterochromatins repeat sequences like?

LINE-type sequences

What is DNA methylation in facultative heterochromatin like?

Methylation at CpG islands in gene regulatory regions— silences genes

What is the histone modification of facultative heterochromatin?

H3K9me

What is higher order structure? 

Assemblage of nucleosomes that assumes a reproducible 3D conformation

What is heterochromatin formation thought to involve?

Histone PTMs, binding of proteins to nucleosomes, chromatin remodeling, DNA methylation, and binding of non-coding RNAs

What are four structural features of higher order structure?

Closer more stable contacts of nucleosomes, closer loop domains, binding to nuclear lamina, may undergo liquid-liquid phase separation

What happens in higher order structure having closer and more stable contacts of nucleosomes?

H3K9me recognized by HP1, HP1 bridges nucleosomes and makes them more compact

What happens in higher order structure forming loop domains?

SMCs promotes structure, CTCFS form a crosslink that stabilize the loops