BMB 251 Lecture Points 7 Chromatin Structure and Function

How does Acetylation of Histone's Lysine occur?

-A differing set of Histone Acetyl Transferases (HATs) add Acetyl groups to specific Lysines -A set of Histone Deacetylase Complexes (HDACs) removes Acetyls, reversing the modification Note: A Methylated Lysine CANNOT be Acetylated

How does Methylation of Histone's Lysine occur?

-A differing set of Histone Methyl Transferases (HMTs) add Methyl groups to specific Lysines -A set of Histone Demethlases (HDMs) removes Methyls, reversing the modification Note: An Acetylated Lysin CANNOT be Methylated

Characteristics of Lampbrush Chromosomes

-Believed that all interphase Eukaryote chromosomes are arranged in loops -Genes present in the loops are being expressed, while genes not in the loop are condensed and not expressed -Largest Chromosomes known, with Chromosomes arranged in Chromatin Loops

What is the mechanism behind the Histone Code Hypothesis?

-Since N-tails (with the modifications) extend outward, they should always be accessible. -Code-Reader Protein Complexes can bind to specific markings/modifications on Chromatin to perform functions at the right time.

Mono-Methylation is associated with-

-Transcriptional Activation -Euchromatin

Di/Tri-Methylation is associated with-

-Transcriptional Repression -Heterochromatin

How does Chromatin Structure spread?

1. A modifying Enzyme is attracted to a sequence specific Protein on the DNA/Protein Nucleosome 2. "Code Writer" Enzyme works with a "Code Reader" Protein within the same complex after Writer modifies the markings. 3. Code Reader's reader module recognizes/binds to newly modified Nucleosome, positioning its Writer by an adjacent nucleosome 4. Cycle repeats, powered by an ATP-driven Chromatin Remodelling Protein to either compress or decompress Chromatin chains.

What are the Mechanisms of Barrier Sequences?

1. Tethering of Chromatin to a fixed site (Ex: Nuclear Pore Complex) prevents Heterochromatin spread 2. Tight binding of barrier Proteins to a Nucleosome group can compete with Writers, stopping Heterochromatin spread 3. Highly active Histone-modifying Enzymes can erase the Histone marks left by Writers, stopping the Read/Write cycle

How condensed are Mitotic Chromosomes?

A normal Interphase Chromosome can be compacted by tenfold by organizing Chromatin Loops from a central scaffold -Becomes the "X" shape we all know

What is responsible for Histone modification?

A specific Enzyme is responsible for adding Side Chain Modifications, while another different Enzyme is responsible for removing them. -Enzymes used for specific sites are at defined times of the cell's life

What effects does Acetylation have on Chromatin?

Acetylation loosens Chromatin since by removing + charge, you remove affinity for the N-Tail and DNA -Associated with Transcriptional Activation, as it attracts specific Proteins like Transcriptional Activators -Therefore, Chromatin structure determines how the genes are packaged

What separates different Chromatin Domains?

Barrier DNA Sequences separate different domains

What causes Nucleosomes to pack so tightly?

By Nucleosome-Nucleosome linkages that occur through Tail interaction with the DNA of neighboring Nucleosomes.

Why does a change in Chromatin Structure affect phenotype?

Chromatin Structure controls gene expression, by changing its structure specific traits are changed. (Cell memory is based on inherited protein structure)

How are Chromatin Structure changed?

Covalent modification of Histones within Nucleosomes causes other protein complexes to bind there, producing effects and changing Chromatin Structure. -Since these Histone modifications can be passed down, they play a central role in development, growth, and maintenance of Eukaryotes.

How are Chromatin Structures inherited?

During DNA replication, parental histones are distributed between the 2 daughter DNA strands. -In the daughter cells, Histone modifications (markings) can be recognized and reestablished by spreading the same markings to new Nucleosomes.

What is Epigenetics/Epigenetic Inheritance? Whats its difference from Genetic Inheritance?

Epigenetics is the study of inheritable phenotype changes that do not involve DNA changes. -Epigenetic Inheritance: Change in Chromatin Structure thats passed down -Genetic Inheritance: Mutation in DNA thats passed down

What are the 2 Types of Chromatin?

Heterochromatin -Highly condensed -DNA in Heterochromatin contains very few genes -More than 10% of Genome packaged like this -Concentrated at Centromeres/Telemeres -Euchromatic Genes packaged into this are "Turned off" or silenced Euchromatin -Not highly condensed (everything not heterochromatin) -Genes can be active or silenced by mechanisms not related to structure -Regions of Euchromatin can differ in activity, composition, and structure

Other than ________, all Histones have small variations of the other core Histones have evolved.

Histone H4

What effects do Methylation have on Chromatin's gene expression? How does it do so?

Methylation of Histones can increase OR decrease Gene Transcription -Methylation weakens attraction between Histone Tails and DNA, increasing transcription as DNA uncoils and becomes accessible to Transcription Proteins and RNA Polymerase -Methylation of Histone Tails attracts Proteins/Protein Complexes that regulates Chromatin activation/inactivation

Where do modifications to Histone occur?

Most modifications occur on the 8 N-terminal Histone Tails, though direct modifications to the core can occur. These modifications are reversible. Modifications Include: -Acetylation of Lysines (K) -Mono/Di/Tri-methylation of Lysines (K) -Phosphorylation of Serines (S)

Where are the Gene-Poor Regions in the Nucleus (for Humans)?

Regions close to the nuclear membrane. (Photo: Red= Gene Poor, Green= Gene Rich)

Histone Code Hypothesis

The hypothesis that the pattern of Histone modification acts like a language/code in specifying alterations in chromatin structure. (Ex: A set of modifications can signal needed DNA repair while another signals when Gene Expression takes place)

How do Chromosomes avoid tangling?

They are organized in Fractal Domains, with Heterochromatic areas located close to the Nuclear Membrane.

How can Chromatin positions within the Nucleus affect Gene Expression?

When Activation is needed, Chromosomes leave their chromosomal territory (often near the Nuclear Membrane) and expand.