DNA Packaging & Chromatin Architecture

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Questions and Answers

What is the fundamental unit of chromatin?

  • The chromosome
  • The histone octamer
  • The DNA linker
  • The nucleosome core particle (correct)

What is the role of the DNA linker?

  • To provide structural support for the chromosome
  • To bind to the histone octamer
  • To form the histone fold
  • To connect adjacent nucleosomes (correct)

Which of the following histones is NOT part of the histone octamer?

  • H2B
  • H2A
  • H3
  • H1 (correct)

What is the arrangement of the histone octamer?

<p>A tripartite arrangement with a H2A-H2B dimer flanked to a centrally located H3-H4 tetramer (C)</p> Signup and view all the answers

What is the histone fold?

<p>A region of DNA that interacts with histones (B), A type of enzyme that modifies histones (D)</p> Signup and view all the answers

Which of the following statements about histone-DNA interactions is TRUE?

<p>Histones interact with the minor groove of DNA (C)</p> Signup and view all the answers

What is the significance of the term 'chromatin'?

<p>It refers to the material that makes up chromosomes (A)</p> Signup and view all the answers

How many minor DNA grooves face towards the histone octamer in the 147bp of DNA wrapped around it?

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

What is the primary function of histone chaperones?

<p>To regulate gene expression by controlling access to DNA. (B)</p> Signup and view all the answers

What is the difference between the one-start helix and two-start helix models of the 30nm chromatin fiber?

<p>The two-start helix model involves a zigzag arrangement of nucleosomes, while the one-start helix model has nucleosomes stacked directly on top of each other. (A)</p> Signup and view all the answers

How does the positive charge of the N-terminal histone tails contribute to chromatin structure?

<p>They interact with and stabilize the negative charge of phosphate groups on DNA, contributing to the compaction of chromatin. (B)</p> Signup and view all the answers

What is the relationship between heterochromatin and gene expression?

<p>Heterochromatin is a region of inactive gene expression, as its tightly packed structure prevents access to DNA by transcription machinery. (B)</p> Signup and view all the answers

Which of the following is NOT a characteristic of histone chaperones?

<p>They are highly positively charged proteins. (B)</p> Signup and view all the answers

How do the one-start and two-start helix models of the 30nm chromatin fiber differ in terms of nucleosome packing density?

<p>The two-start helix model has a lower nucleosome packing density than the one-start helix model. (C)</p> Signup and view all the answers

What is the significance of linker DNA in chromatin structure?

<p>Linker DNA helps to connect adjacent nucleosomes, forming the 30nm chromatin fiber. (A)</p> Signup and view all the answers

Which of the following techniques is commonly used to visualize heterochromatin and euchromatin?

<p>Giemsa or quinacrine staining (A)</p> Signup and view all the answers

Which type of chromatin is characterized by its loose packing and high gene concentration?

<p>Euchromatin (D)</p> Signup and view all the answers

What is the main function of heterochromatin?

<p>Protection of DNA from endonuclease damage (A)</p> Signup and view all the answers

Which of the following is NOT a characteristic of euchromatin?

<p>Contains inactive genes (D)</p> Signup and view all the answers

What is the primary function of euchromatin in gene expression?

<p>Transcribing active genes into mRNA (A)</p> Signup and view all the answers

Which type of heterochromatin is associated with permanently inactive genes?

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

What is the role of siRNA in the formation of facultative heterochromatin?

<p>Silencing gene expression (A)</p> Signup and view all the answers

What is the main difference between euchromatin and heterochromatin?

<p>The level of DNA packing (A)</p> Signup and view all the answers

What is the result of the transformation of euchromatin into heterochromatin?

<p>Silencing of gene expression (C)</p> Signup and view all the answers

Flashcards

Heterochromatin

A dense form of chromatin that is transcriptionally inactive and contains silenced genes.

Facultative Heterochromatin

Type of heterochromatin that contains inactive genes, dynamically changing based on cell conditions.

Constitutive Heterochromatin

Type of heterochromatin containing repetitive genes like telomeres, permanently inactive in the genome.

Euchromatin

Loosely packed chromatin rich in genes, active during transcription, covering most of the genome.

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Histone Methylation

A process that silences genes leading to facultative heterochromatin formation.

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Transcription

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

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Endonuclease Damage

DNA damage that heterochromatin protects against due to its compact structure.

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Gene Regulation

The mechanism by which euchromatin can convert to heterochromatin, affecting gene activity.

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Positively charged regions

Locations in histone tails and nucleosome core that attract negatively charged DNA backbone.

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Histone Chaperones

Acidic proteins that assist in chromatin assembly and disassembly during transcription and replication.

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Chromatin assembly

The process of forming higher-order structures of DNA and histones into chromatin.

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Chromatin disassembly

The process of breaking down chromatin into its components, allowing access to DNA.

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One-start helix model

A chromatin fiber model where nucleosomes are adjacent, achieving a packing density of 11 nucleosomes.

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Two-start helix model

A chromatin fiber model with a zigzag arrangement of nucleosomes, having a packing density of 5 to 6 nucleosomes.

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Nucleosome Structure

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

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Histone Octamer

An octamer composed of four core histones (H2A, H2B, H3, H4) forming the nucleosome.

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Histone Fold

A common motif in histones aiding in their dimerization during the formation of the octamer.

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Nucleosome Core Particle (NCP)

The arrangement of DNA and histones in a nucleosome, specifically 147 bp of DNA around the octamer.

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DNA Linker

The short DNA sequence that connects adjacent nucleosomes, associated with linker histone H1.

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Histone-DNA Interaction

The interaction between histones and DNA in the nucleosome, where specific grooves face the histones.

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Study Notes

DNA Packaging & Chromatin Architecture

  • DNA is packaged into chromatin, a complex of DNA and proteins, to fit within the nucleus.
  • Nucleosomes are the fundamental structural units of chromatin.
  • A nucleosome comprises DNA wrapped around a core of histone proteins (an octamer of histones H2A, H2B, H3, and H4).
  • DNA linker sections connect adjacent nucleosomes.
  • Nucleosomes coil into a 30-nm chromatin fiber.
  • Further coiling and folding results in condensed chromosomes during mitosis and meiosis.
  • Euchromatin is less condensed and transcriptionally active, while heterochromatin is more condensed and typically inactive.
  • Histones have a conserved histone fold motif.
  • Histones interact with each other and with DNA through specific interactions.
  • Chromatin assembly and disassembly are critical processes for gene regulation and DNA replication.
  • Histone chaperones facilitate these dynamic processes.

The Chromosome

  • Chromosomes are highly condensed structures of DNA and proteins.
  • Levels of organization include base pairs in a double helix, formation of nucleosomes, 30-nm chromatin fibers, and loops of supercoiled fibers.
  • The 30 nm fiber can be organized into loops, allowing individual genes or sets of genes to be accessed without unpacking the entire chromosome.
  • The chromosome structure is dynamic, changing during different stages of a cell's life cycle.

Heterochromatin & Euchromatin

  • Heterochromatin is a tightly packed, condensed form of chromatin that's largely inactive in gene transcription.
  • Euchromatin is a less condensed form of chromatin that's more involved in gene transcription.
  • Types of heterochromatin include facultative and constitutive, distinguished by their permanence and specific location.
  • Euchromatin is rich in gene concentrations and is active during transcription.
  • Heterochromatin is rich in repetitive sequences and is generally inactive.
  • Heterochromatin protects DNA from damage, prevents inappropriate gene activation, and is essential for chromosome stability.
  • Euchromatin allows the expression of genes essential for cellular function.

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