DNA Packaging and Chromatin Architecture

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Questions and Answers

What is the fundamental unit of chromatin?

  • Chromatin fiber
  • DNA linker
  • Histone octamer
  • Nucleosome core particle (NCP) (correct)

Which of the following is NOT a core histone?

  • H2A
  • H1 (correct)
  • H2B
  • H3

What is the histone fold?

  • A region of the histone that interacts with the DNA linker
  • A specific DNA sequence that binds to histones
  • A structural motif that allows histones to bind to each other (correct)
  • A type of protein that modifies histone tails

How many minor grooves of the DNA are facing towards the histone octamer in the nucleosome core particle?

<p>12 (A)</p> Signup and view all the answers

What is the function of the DNA linker?

<p>To connect adjacent nucleosomes (D)</p> Signup and view all the answers

What is the function of linker histone H1?

<p>To bind to the DNA linker and help compact chromatin (A)</p> Signup and view all the answers

What is the difference between euchromatin and heterochromatin?

<p>Euchromatin is less condensed than heterochromatin and is actively transcribed. (D)</p> Signup and view all the answers

What is the term for the arrangement of the histone octamer?

<p>Tripartite arrangement (A)</p> Signup and view all the answers

What type of heterochromatin is associated with inactive genes and is not a permanent characteristic of every cell's nucleus?

<p>Facultative heterochromatin (C)</p> Signup and view all the answers

Which type of chromatin is characterized by dense packing and protects DNA from endonuclease damage?

<p>Constitutive heterochromatin (A)</p> Signup and view all the answers

Which feature distinguishes euchromatin from heterochromatin?

<p>Euchromatin is rich in gene concentration. (A)</p> Signup and view all the answers

What percentage of the entire human genome is estimated to be composed of euchromatin?

<p>90% (A)</p> Signup and view all the answers

What is the primary function of euchromatin during the transcription process?

<p>To allow DNA to be readily available for transcription (D)</p> Signup and view all the answers

How does euchromatin participate in gene regulation?

<p>By transforming into heterochromatin (B)</p> Signup and view all the answers

What role does histone methylation play in facultative heterochromatin?

<p>It silences genes, making them inactive. (B)</p> Signup and view all the answers

Which cellular component aids in making euchromatin more accessible during transcription?

<p>Histones (B)</p> Signup and view all the answers

What is the role of histone chaperones during DNA processes?

<p>They assist in chromatin assembly and disassembly. (A)</p> Signup and view all the answers

Which model describes a packing density of 11 nucleosomes?

<p>One-start helix model (C)</p> Signup and view all the answers

Why is heterochromatin considered inaccessible to gene expression proteins?

<p>It is tightly packed and highly condensed. (D)</p> Signup and view all the answers

What distinguishes the two-start helix model from the one-start helix model?

<p>The two-start model has a zigzag stacking of nucleosomes. (B)</p> Signup and view all the answers

Which of the following is a common characteristic of heterochromatin?

<p>Intense staining with DNA-specific stains. (C)</p> Signup and view all the answers

What is the primary determinant for the need of different compaction in DNA?

<p>The accessibility required for transcription. (D)</p> Signup and view all the answers

What is nucleoplasmin known for?

<p>It is the first isolated histone chaperone. (B)</p> Signup and view all the answers

What type of interactions occur between positively charged regions and the phosphate backbone in nucleosomes?

<p>Attractive interactions. (D)</p> Signup and view all the answers

Flashcards

Nucleosome

The fundamental unit of chromatin consisting of DNA wrapped around a histone octamer.

Histone Octamer

An octamer made up of four core histones: H2A, H2B, H3, and H4.

Chromatin

The material of chromosomes, consisting of DNA and proteins, primarily histones.

Euchromatin

A less condensed form of chromatin that is active in transcription.

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Heterochromatin

Tightly packed chromatin that is generally inactive in transcription.

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Histone Fold

A structural motif in histones, essential for forming dimers during octamer formation.

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Nucleosome Core Particle (NCP)

A structure comprised of 147 bp of DNA wrapped around the histone octamer.

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DNA Linker

A short section of DNA connecting nucleosomes, associated with linker histone H1.

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Positively charged regions

Positively charged areas in the N-terminal histone tails and nucleosome core that interact with negatively charged DNA.

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Histone chaperones

Acidic proteins that bind histones, aiding chromatin assembly and disassembly during DNA processes.

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Nucleoplasmin

The first identified histone chaperone, isolated from Xenopus laevis egg extracts.

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Chromatin structure

The arrangement of DNA into chromatin, which includes different models of nucleosome packing.

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One-start helix model

Model where nucleosomes are adjacent, leading to higher packing density of 11 nucleosomes.

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Two-start helix model

Model where nucleosomes stack in a zigzag arrangement, with 5 to 6 nucleosomes density.

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Facultative heterochromatin

Heterochromatin that contains temporarily inactive genes, can change based on conditions.

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Constitutive heterochromatin

Heterochromatin that is always inactive and contains repetitive sequences like telomeres.

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Transcription

The process of converting DNA into RNA, occurring mainly in euchromatin.

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Histone methylation

A process that modifies histones, leading to gene silencing in facultative heterochromatin.

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Endonuclease damage prevention

One of the main functions of heterochromatin to protect DNA by its compact structure.

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Gene regulating mechanism

The process of converting euchromatin to heterochromatin or vice versa to regulate gene activity.

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Study Notes

DNA Packaging & Chromatin Architecture

  • DNA is packaged into a highly organized structure called chromatin
  • This packaging allows the vast amount of DNA to fit within the nucleus of a cell
  • The fundamental unit of chromatin is the nucleosome
  • Nucleosomes consist of a core of histone proteins around which DNA is wrapped
  • The nucleosome core particle (NCP) contains the histone octamer and 147 base pairs of DNA
  • Histones are proteins that have a positive charge, enabling them to bind to the negatively charged DNA
  • The linker DNA connects adjacent nucleosomes
  • The 10nm chromatin fiber (beads-on-a-string) is a result of the nucleosomes and linker DNA
  • The 30nm chromatin fiber is formed by the folding of the 10nm fiber. Various model exist, with different dimension and packing densities
  • This further compaction results in a more condensed structure necessary for cell division
  • Euchromatin is loosely packed chromatin active in transcription
  • Heterochromatin is tightly packed chromatin, generally inactive in transcription. There are two types, constitutive and facultative

Histone Octamer

  • The histone octamer is composed of two copies each of four core histone proteins: H2A, H2B, H3, and H4.
  • The core histones associate to form a dimer of H2A-H2B and a tetramer of H3-H4
  • These components form a protein complex that is crucial to DNA packaging.

Histone Fold

  • Histones have conserved motifs known as the histone fold
  • The histone fold motif is a common structural motif in histones (across species)
  • The histone fold motif is crucial for the dimerization (joining) of histones during octamer formation
  • The motif contains a long α-helix connected to two short helices and strands (HSH) at opposite ends. This motif is important for histone-histone interactions

Euchromatin and Heterochromatin

  • Euchromatin is lightly packed chromatin, which is active in transcription and contains genes that are being expressed.
  • Heterochromatin is tightly packed chromatin, which is generally inactive in transcription and contains repetitive DNA sequences
  • There are two types of heterochromatin: facultative (can change) heterochromatin and constitutive (permanent) heterochromatin. Constitutive heterochromatin typically contains regions like telomeres and centromeres and is often repetitive sequence

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