Lesson 15: Nucleus
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Questions and Answers

What is the main function of the nuclear lamins in the nucleus?

  • They facilitate cellular respiration.
  • They provide structural support and shape to the nucleus. (correct)
  • They synthesize ribosomal RNA.
  • They regulate the movement of proteins across the nuclear envelope.

How does the outer nuclear membrane interact with the endoplasmic reticulum (ER)?

  • The ER is embedded within the outer membrane.
  • They are completely separate compartments.
  • The outer membrane is continuous with the ER. (correct)
  • They communicate only via nuclear pores.

What genetic mutation is associated with Hutchinson–Gilford syndrome?

  • A duplication of chromosome 21.
  • A mutation in a lamin gene affecting a single amino acid. (correct)
  • A deletion of the entire lamin gene.
  • A mutation affecting mitochondrial DNA.

How do nuclear lamins contribute to gene expression beyond structural support?

<p>They provide a connection for chromosome attachment and may influence gene regulation. (D)</p> Signup and view all the answers

What is a symptom of progeria?

<p>Premature aging characteristics. (D)</p> Signup and view all the answers

Which of the following correctly describes the nuclear pores?

<p>They selectively allow proteins and mRNA to pass. (B)</p> Signup and view all the answers

What role do nuclear lamins play during interphase?

<p>They help with chromosome attachment and gene expression. (C)</p> Signup and view all the answers

What is a characteristic shape seen in the nuclei of progeria patients?

<p>Abnormal shapes indicating structural instability. (C)</p> Signup and view all the answers

What is the role of telomeres during DNA replication?

<p>They contain DNA sequences that enable complete replication of chromosome ends. (D)</p> Signup and view all the answers

What characterizes heterochromatin in the nucleus?

<p>It consists of few genes and is located mainly around the periphery of the nucleus. (B)</p> Signup and view all the answers

How do interphase chromosomes occupy space within the nucleus?

<p>They occupy distinct territories without overlapping. (B)</p> Signup and view all the answers

What is the primary function of the nucleolus?

<p>To assemble ribosomal RNA genes and produce ribosomes. (B)</p> Signup and view all the answers

What occurs at the centromere during cell division?

<p>It holds duplicated chromosomes together until they are ready to separate. (D)</p> Signup and view all the answers

Which type of microscopy can be used to visualize nucleosomes?

<p>Transmission electron microscopy. (D)</p> Signup and view all the answers

What visual characteristic may indicate the presence of nucleosomes in chromatin?

<p>A speckled mass with clearly defined fibers. (A)</p> Signup and view all the answers

During replication, how does DNA progress from the origins?

<p>Bidirectionally from each origin towards the telomeres. (A)</p> Signup and view all the answers

What is the primary role of histone tail modifications?

<p>To regulate chromatin structure (D)</p> Signup and view all the answers

Which of the following modifications can be made to histone tails?

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

What do 'writers', 'erasers', and 'readers' refer to in the context of histone modifications?

<p>Enzymes that add, remove, and interpret histone modifications (C)</p> Signup and view all the answers

How can lysine residues at specific positions in histone tails be modified?

<p>By trimethylation or acetylation, but not both simultaneously (A)</p> Signup and view all the answers

What impact can the combination of histone tail modifications have on chromatin?

<p>It confers specific meanings for chromatin structure or gene expression (A)</p> Signup and view all the answers

What is the structure of a nucleosome composed of?

<p>DNA wrapped around a protein core of eight histone molecules (D)</p> Signup and view all the answers

What role does histone H1 play in chromatin structure?

<p>It provides additional packaging of nucleosomes (B)</p> Signup and view all the answers

How does the SMC ring complex compact chromatin?

<p>By forming loops through ATP hydrolysis (D)</p> Signup and view all the answers

What is the primary function of cohesins during interphase?

<p>To organize interphase chromatin (B)</p> Signup and view all the answers

What mechanism is proposed in the inchworm model of the SMC complex?

<p>It moves along DNA by altering its grip after hydrolyzing ATP (B)</p> Signup and view all the answers

What is the length of DNA that wraps around each nucleosome core particle?

<p>147 nucleotide pairs (D)</p> Signup and view all the answers

What happens to linker DNA during the digestion of nucleosomes?

<p>It is cleaved by a nuclease (A)</p> Signup and view all the answers

What effect does the SMC ring complex have on mitotic chromosomes?

<p>It compactly organizes them (C)</p> Signup and view all the answers

What role do clamp proteins play in chromatin loop regulation?

<p>They bind to specific DNA sequences and limit loop enlargement. (A)</p> Signup and view all the answers

How do cohesins and condensins differ in their function during cell division?

<p>Cohesins maintain chromatin structure while condensins condense it. (D)</p> Signup and view all the answers

What is the primary function of ATP-dependent chromatin-remodeling complexes?

<p>To shift nucleosomes and regulate DNA accessibility. (D)</p> Signup and view all the answers

Which of the following best describes how condensins contribute to chromosome structure?

<p>They create a hierarchy of loops within chromatin. (A)</p> Signup and view all the answers

Which protein is involved in both chromatin loop formation and condensation?

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

What is the result of ATP hydrolysis by chromatin-remodeling complexes?

<p>Shifting of DNA around nucleosomes. (D)</p> Signup and view all the answers

What structure is primarily responsible for the initial formation of large chromatin loops during mitosis?

<p>Condensin II (C)</p> Signup and view all the answers

Which protein is crucial for drawing together DNA at the base of each loop in chromatin?

<p>Clamp proteins (C)</p> Signup and view all the answers

What distinguishes facultative heterochromatin from constitutive heterochromatin?

<p>Facultative heterochromatin is temporarily condensed. (A)</p> Signup and view all the answers

Which type of chromatin is associated with genes that are actively expressed?

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

What is the main role of histone modifications in heterochromatin formation?

<p>To attract and propagate modifying enzymes to neighboring nucleosomes (B)</p> Signup and view all the answers

What happens to one of the two X chromosomes in mammalian females during embryonic development?

<p>One X chromosome is inactivated through heterochromatin formation. (B)</p> Signup and view all the answers

How does heterochromatin spread along chromatin?

<p>Via adjacent histones with specific modifications. (D)</p> Signup and view all the answers

What distinguishes inactive euchromatin from active euchromatin?

<p>Active euchromatin has more extended stretches of chromatin. (C)</p> Signup and view all the answers

What is the primary function of constitutive heterochromatin in chromosomes?

<p>To protect chromosome integrity during cell division. (D)</p> Signup and view all the answers

How does the inactivation of one X chromosome get passed on during cell division?

<p>The inactivated X chromosome remains condensed in all descendant cells. (C)</p> Signup and view all the answers

Flashcards

Nuclear Envelope

The double membrane surrounding the nucleus; separating the nucleus from the cytoplasm

Nuclear Lamins

Intermediate filaments that form a network just beneath the nuclear membrane, supporting nucleus shape

Progeria

A genetic condition causing premature aging, related to mutations in lamin genes.

Eukaryotic Genes

Genes that code for proteins in eukaryotic cells.

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Nuclear Pores

Channels in the nuclear envelope that allow movement of molecules between the nucleus and cytoplasm.

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Chromosome

Organized structure of DNA containing genes.

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Genetic Material

DNA organized into chromosomes.

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Premature Aging

Physical changes associated with aging, happening at a faster rate than expected.

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DNA Replication

The process where DNA makes a copy of itself, beginning at origins of replication, proceeding bidirectionally along the chromosomes to the telomeres.

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Telomeres

DNA sequences at the ends of chromosomes that allow complete replication of chromosome ends.

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Centromere

The structure that attaches duplicated chromosomes to the mitotic spindle, ensuring equal distribution of chromosomes to daughter cells during cell division.

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Interphase Chromosomes

Chromosomes that occupy specific territories within the nucleus during interphase.

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Homologous Chromosomes

Pairs of chromosomes that are similar in shape and size, but not generally located in the same position in interphase.

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Nucleolus

A prominent structure in the interphase nucleus that contains the genes for ribosomal RNA (rRNA), and plays a role in ribosome formation.

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Heterochromatin

Dense regions of chromatin within the nucleus, containing few genes and located primarily around the nuclear periphery.

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Nucleosome

The fundamental repeating unit of eukaryotic chromosome structure, consisting of DNA wrapped around histone proteins.

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Nucleosome Structure

DNA wrapped around a histone protein core, forming a "beads-on-a-string" structure.

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Linker DNA

DNA segments between nucleosomes, easily digested by nucleases.

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Histone H1

Histone that further compacts nucleosomes, tightening chromatin structure.

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SMC Ring Complex

Protein complex using ATP to form chromatin loops, crucial for chromosome organization.

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Cohesins Role

SMC complex organizing interphase chromatin.

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Condensins Role

SMC complex condensing mitotic chromosomes.

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"Inchworm" Model

Proposed mechanism where SMC ring complexes move along DNA, creating loops.

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Nucleosome Core Particle Length

147 nucleotide pairs of DNA wrapped around the histone octamer.

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Histone code

Patterns of modifications of histone tails that regulate chromatin and gene expression.

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Histone tails

Protruding parts of histone proteins, sites of modifications controlling chromatin structure.

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Chromatin Modifications

Changes to chromatin structure, often by adding chemical groups to histone tails, altering gene activity.

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Histone modification writers

Enzymes that add modifications to histone tails.

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Histone modification readers

Enzymes that recognize and interpret specific combinations of histone tail modifications, affecting chromatin structure and gene expression.

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Heterochromatin

Highly condensed chromatin, containing inactive genes.

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Euchromatin

Less condensed chromatin, associated with active genes.

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Constitutive Heterochromatin

Permanently condensed heterochromatin at telomeres and centromeres.

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Facultative Heterochromatin

Temporarily condensed heterochromatin.

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X Chromosome Inactivation

One X chromosome is condensed (and inactivated) in each female cell.

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Histone Modifications

Changes to histone proteins, influencing chromatin structure.

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Chromatin Spreading

Heterochromatin expanding to neighboring regions.

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Barrier DNA Sequences

DNA segments that prevent chromatin spreading.

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Cohesins' Role

SMC complex that organizes chromatin loops in interphase.

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Condensins' Role

SMC complex that condenses mitotic chromosomes.

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Chromatin Loops

Structures formed by looping DNA segments. Important for regulating DNA access.

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Clamp Proteins

Proteins that regulate the size of chromatin loops, bound to specific DNA sequences.

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Chromatin Remodeling Complexes

Enzymes that reposition nucleosomes along DNA using ATP.

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SMC Ring Complexes

Protein complexes that form chromatin loops using ATP. Included in cohesins and condensins.

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Mitotic Chromosome

Highly condensed chromosome structure in mitosis.

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Nucleosome

Fundamental repeating unit of eukaryotic chromosomes—DNA coiled around histone proteins.

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Study Notes

Exam 2 Results

  • Exam 2 had a mean score of 77.62%
  • Standard deviation was 14.74%
  • Reliability minimum was 0.81
  • Median score was 81.5%
  • Maximum possible score was 100%
  • Exam scores varied between 33.0% and 100.0%

The Nucleus

  • The nucleus contains the cell's genetic material (DNA)
  • DNA is organized into chromosomes
  • The nucleus is surrounded by two membranes (inner and outer)
  • Outer membrane is continuous with the endoplasmic reticulum (ER)
  • Nuclear pores allow proteins and mRNA to move between the nucleus and cytoplasm
  • Nuclear lamins form a network of filaments protecting the nucleus from mechanical forces
  • Lamin proteins are intermediate filaments in eukaryotic cells
  • Lamins form a network within the nucleus just under the nuclear membrane
  • These lamins provide a place for attaching chromosomes during an interphase cell cycle
  • Lamins may be involved in gene expression during an interphase cell cycle

Mutations in Lamins

  • Mutations in lamin genes are associated with Hutchinson-Gilford syndrome (progeria)
  • Progeria causes premature aging symptoms
  • Symptoms include hair loss, wrinkled skin, atherosclerosis, blindness, kidney failure, and cardiovascular disease
  • Progeria patients rarely live past their teenage years
  • Progeria is often caused by mutations in lamin genes—a single amino acid residue.
  • These mutations cause altered gene expression patterns.

Gene Structure and Function

  • Most Genes Contain Information to Make Proteins
  • DNA is transcribed into RNA, which is translated into proteins.
  • Several genes contribute to producing a single protein.

Eukaryotic Protein-Coding Gene Structure

  • Protein-coding genes have a characteristic structure
  • Include regulatory sequences such as promoters and enhancers
  • Coding regions (exons) specify amino acid sequences
  • Non-coding regions (introns) are removed from mRNA before translation
  • Regulatory sequences control gene expression

Non-Coding DNA

  • Eukaryotic genomes have a significant amount of non-coding DNA
  • Most DNA is non-coding—not involved in making proteins
  • Non-coding DNA may play regulatory roles, such as in transcriptional regulation and gene expression

Chromosome Duplication and Segregation

  • Chromosomes duplicate and segregate during the cell cycle.
  • Chromosome duplication occurs during an interphase stage
  • Mitosis is the division of the nucleus for cell division.
  • The mitotic spindle forms from microtubules to separate the duplicated chromosomes.

Fluorescence In Situ Hybridization (FISH)

  • FISH uses fluorescent probes to visualize chromosomes.
  • Probes bind to specific DNA sequences on chromosomes to allow for individual chromosomes' visualization in a cell.

Chromosomes

  • Chromosomes condense for nuclear division (e.g., mitosis).
  • Individual chromosomes can be identified in a karyotype—the array of chromosomes in a cell lined up.

DNA Packing

  • DNA is packaged in a highly compact way in chromosomes.
  • Eukaryotic DNA is packaged into multiple chromosomes.
  • Chromosomes contain multiple origins of replication, centromeres, and telomeres.

Chromatin Organization

  • Interphase chromosomes occupy distinct territories within the nucleus.
  • Chromosomes are not randomly arranged within the nucleus but occupy specific, non-overlapping, regions.
  • Chromosomes may exist in differing states of condensation (e.g., euchromatin, heterochromatin)

The Nucleolus

  • The nucleolus is the most prominent structure within the interphase nucleus.
  • The nucleolus contains the genes for ribosomal RNA (rRNA)
  • Subcompartment for ribosomal RNA (rRNA) synthesis, as well as special proteins
  • The presence of rRNA genes in clustered regions suggests they require similar structural arrangement and regulation.

Nucleosomes

  • Nucleosomes are the basic units of eukaryotic chromosome structure.
  • DNA is wrapped around a protein core called histones (eight histone molecules total).
  • DNA wrapped around histones form nucleosomes that give chromatin a "beads-on-a-string" appearance.

Histone Modifications

  • Histone tails can be modified (e.g., acetylation, methylation).
  • Modifications to histone tails influence how cells handle chromatin structure.

Chromatin Structure and Modification

  • The pattern varies along a single interphase chromosome
  • Heterochromatin and euchromatin represent levels of condensation
  • Heterochromatin is highly condensed and often contains inactive genes
  • Euchromatin is less condensed and contains active genes.
  • Heterochromatin can be influenced by various factors, including histone modifications and environmental cues.
  • Heterochromatin formation can be inherited across subsequent cell divisions
  • Chromatin structure can be modified by enzyme processes

X-inactivation

  • One X chromosome in mammalian females is inactivated to control gene expression.
  • Inactivation occurs during early embryonic development.
  • The inactivated X chromosome forms a Barr body.
  • Inactivation patterns are passed on to daughter cells.
  • This is a mechanism to balance dosage between males and females.

X-inactivation Example

  • Tortoiseshell cats have coat color patterns due to X-chromosome inactivation.
  • Different X chromosomes are inactivated in different cells, resulting in distinctive color patches on the cat.

Chromatin Looping

  • Sequence-specific clamp proteins regulate the size of chromatin loops.
  • Cohesins and condensins are SMC complexes that fold chromatin into loops.
  • Condensation processes are driven by interactions with SMC complexes.

ATP-Dependent Chromatin Remodeling

  • ATP-dependent chromatin remodeling complexes reposition DNA around nucleosomes
  • These complexes use ATP hydrolysis to alter DNA positioning around nucleosomes, enabling greater protein access.

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The Nucleus PDF

Description

Explore the anatomy of the nucleus and the role of lamins in eukaryotic cells. This quiz covers the structure of the nucleus, the organization of DNA, and the implications of mutations in lamin genes. Test your knowledge on cell biology concepts related to genetic material and cellular mechanics.

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