Cell Biology: Nucleus and Nuclear Envelope

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Questions and Answers

What primary role does the nucleus serve in a cell?

Facilitating the transport of ions across the membrane

Storing nutrients for cellular metabolism

Producing ATP through cellular respiration

Regulating gene expression and cell activities (correct)

What characterizes the structure of the nuclear envelope?

A single-layer membrane surrounding the nucleus

A series of interconnected vesicles

A double-layered membrane complex (correct)

A mass of chromatin fibers

What is the role of nuclear pore complexes (NPCs)?

To facilitate exchange between the nucleus and cytoplasm (correct)

To synthesize mRNA from DNA

To protect the nucleus from viral infections

To maintain the structural integrity of the nuclear envelope

Which structure connects the two layers of the nuclear envelope?

Perinuclear space Signup and view all the answers

Which type of transport is used for small molecules and ions through the nuclear pore complex?

Passive transport Signup and view all the answers

What is the average density range of nuclear pore complexes per 1 mm²?

30 to 50 pores Signup and view all the answers

Which of the following accurately describes the nuclear transport receptor involved in importing macromolecules?

Importin β Signup and view all the answers

In which type of cells can multiple nuclei be found?

Muscle cells Signup and view all the answers

What is the primary consequence of histone hypo-acetylation?

Condensation of chromatin and gene silencing Signup and view all the answers

Which component is responsible for keeping sister chromatids attached after DNA replication?

Cohesin Signup and view all the answers

Which of the following structures ensures proper alignment of chromosomes during cell division?

Kinetochore Signup and view all the answers

What is the main role of telomeres in eukaryotic cells?

Protecting chromosome ends from degradation Signup and view all the answers

Which type of chromatin is primarily associated with gene repression?

Heterochromatin Signup and view all the answers

What characterizes the structure of a metaphase chromosome?

Contains two sister chromatids joined at the centromere Signup and view all the answers

What do nucleolus organizing regions (NOR) primarily contain?

Coding sequences for rRNA Signup and view all the answers

In terms of karyotype, what does '2n = 46' signify for human somatic cells?

Human's diploid number of chromosomes is 46 Signup and view all the answers

What is the function of condensins during cell division?

Condense and stabilize chromosomes Signup and view all the answers

How do constitutive and facultative heterochromatin primarily differ?

Constitutive is permanently inactive, facultative can be active Signup and view all the answers

What condition is linked to mutations in the DNMT3B gene?

ICF Syndrome Signup and view all the answers

What does the process of acetylation typically do to DNA-histone interactions?

Loosens the interaction, promoting transcription Signup and view all the answers

What is the primary function of exportin in the nuclear transport system?

To bind macromolecules for transport out of the nucleus Signup and view all the answers

Which chromosomes contain pseudoautosomal regions (PAR)?

Sex chromosomes X and Y Signup and view all the answers

What is the primary function of the kinetochore during mitosis?

To connect sister chromatids to the spindle apparatus Signup and view all the answers

What type of lamins are encoded by the LMNA gene?

A-type lamins Signup and view all the answers

Which of the following nuclear processes are nuclear lamins NOT involved in?

Photosynthesis Signup and view all the answers

What is a notable feature of the DNA sequence known as CEN DNA?

It is essential for centromere formation and function Signup and view all the answers

What condition is characterized by chronic joint stiffness and progressive muscle weakness due to mutations in lamin genes?

Emery-Dreifuss Muscular Dystrophy Signup and view all the answers

Which proteins form the main structure of the LINC complex?

Nesprin and SUN proteins Signup and view all the answers

What is the major component of nucleoplasm?

Nucleoproteins Signup and view all the answers

How many nucleotides of DNA wrap around the histone octamer in a nucleosome?

147 nucleotides Signup and view all the answers

What distinguishes euchromatin from heterochromatin?

Euchromatin is loosely packed and active in gene expression Signup and view all the answers

What type of DNA methylation is typically associated with facultative heterochromatin?

Methylated on gene control regions (CpG islands) Signup and view all the answers

Which statement about non-histone proteins is true?

They are primarily small, acidic proteins with enzymatic functions. Signup and view all the answers

What function does Emerin primarily perform within the nucleus?

Stabilizes the nuclear membrane against mechanical stress Signup and view all the answers

Which of the following correctly describes constitutive heterochromatin?

Composed of repetitive, genetically inactive DNA Signup and view all the answers

How do hemosiderin and nuclear lamins differ in their functions?

Hemosiderin is a storage form of iron while nuclear lamins provide structural support Signup and view all the answers

Which of the following is NOT a function associated with the nuclear lamina?

Transporting proteins into the nucleus Signup and view all the answers

What is the primary function of Barr bodies in female somatic cells?

Silence one of the X chromosomes to maintain dosage compensation Signup and view all the answers

Which of the following accurately describes autosomes?

There are 22 pairs in humans and they carry genes unrelated to sex Signup and view all the answers

How do pseudoautosomal regions (PARs) function during meiosis?

They promote recombination between X and Y chromosomes Signup and view all the answers

What is the role of supercoiling in prokaryotic DNA?

To compact and organize DNA within the nucleoid Signup and view all the answers

What type of DNA is found in mitochondria, and how is it inherited?

Circular DNA, inherited maternally Signup and view all the answers

What is the defining feature of the inner membrane of mitochondria?

It is highly folded into structures called cristae for ATP production Signup and view all the answers

How do plasmids contribute to bacterial genetics?

They provide additional genetic capabilities, like antibiotic resistance Signup and view all the answers

Which accurately describes the structure of the matrix in mitochondria?

It contains enzymes for the citric acid cycle and mtDNA Signup and view all the answers

What is the consequence of defects in lamins associated with the nuclear envelope?

Development of diseases like Emery-Dreifuss muscular dystrophy Signup and view all the answers

What is the general characteristic of prokaryotic chromosomes?

Circular structure not associated with histones Signup and view all the answers

How do mitochondria aid in apoptosis?

By releasing pro-apoptotic molecules activating apoptotic pathways Signup and view all the answers

Which function is primarily performed by the outer membrane of mitochondria?

Regulating the passage of ions and small molecules Signup and view all the answers

What is the role of DNA gyrase in the nucleoid?

To introduce negative supercoils and compact DNA Signup and view all the answers

What is a characteristic of mitochondrial ribosomes?

They are similar in size to bacterial ribosomes Signup and view all the answers

Study Notes

Nucleus: The Control Center

The nucleus is the largest membrane-bound organelle in a eukaryotic cell.
It contains linear DNA molecules and acts as the cell's control center.
Red blood cells lack a nucleus, while some cells, like muscle cells, can have multiple nuclei.

Nuclear Envelope

The nuclear envelope is a double-layered membrane complex surrounding the nucleus.
It physically separates nuclear contents (DNA and chromatin) from the cytoplasm.
The outer membrane is connected to the rough endoplasmic reticulum (ER) and contains ribosomes.
The inner membrane contains proteins that connect to chromatin and the nuclear lamina.
The perinuclear space, a fluid-filled compartment between the membranes, is a continuation of the ER lumen.

Nuclear Pore Complex (NPC)

The NPC is a large protein complex forming a channel through the nuclear envelope.
It is made up of proteins called nucleoporins (Nups).
Its function is to control the exchange of components between the nucleus and cytoplasm.
Small molecules and ions move passively through the NPC.
Macromolecules (proteins, RNA) require active transport with the help of nuclear transport receptors:
- Importin β transports molecules into the nucleus.
- Exportin transports molecules out of the nucleus.

Nuclear Lamina

The nuclear lamina is a network of proteins that lines the inner nuclear membrane.
It is composed of proteins called lamins.
Two types of lamins exist:
- A-type lamins (lamin A and C) are encoded by the LMNA gene.
- B-type lamins are encoded by the LMNB1 and LMNB2 genes.
Lamins form intermediate filaments, providing structural support to the nucleus.
The lamina is involved in various nuclear processes:
- DNA replication
- Transcription
- Nuclear and chromatin organization
- Cell cycle regulation
- Cell development and differentiation
- Nuclear migration
- Apoptosis

Laminopathies

Laminopathies are inherited disorders caused by mutations in the genes coding for lamins or lamina-interacting proteins.
An example is Emery-Dreifuss Muscular Dystrophy, characterized by:
- Chronic joint stiffness
- Progressive muscle weakness
- Cardiac issues in adulthood
The molecular mechanism involves dysfunction in the LINC/emerin/lamin protein complex:
- Emerin is an integral protein of the inner nuclear membrane interacting with the nuclear lamina.
- Emerin stabilizes the nuclear membrane against mechanical stress, particularly in muscle cells during contractions.

LINC Complex (Linkers of Nucleoskeleton and Cytoskeleton)

The LINC complex connects the nuclear envelope to the cytoskeleton.
It is formed by Nesprin and SUN proteins.
The LINC complex has several functions:
- Positions the nucleus within the cell
- Coordinates nuclear and cytoplasmic activities
- Helps position the centrosome near the nucleus
- Transmits mechanical forces from the cytoskeleton to the nucleus

Nucleoplasm

Nucleoplasm is the protoplasm within the nucleus.
It is a gel-like substance containing chromatin, the nucleolus, and various proteins and nucleic acids.
Nucleoproteins, consisting of proteins and nucleic acids, are the major component.
Proteins are categorized as non-histones and histones.
- Non-histone proteins are small, acidic proteins, including phosphoproteins and enzymes.
- Histone proteins are basic proteins containing arginine, lysine, and histidine.
  - They bind to DNA via ionic bonds, forming nucleosomes.
  - Histones can be enzymatically modified (methylated, acetylated).

Chromatin Overview

Chromatin is a complex of DNA and nucleoproteins.
The basic unit of chromatin is the nucleosome:
- It consists of 8 histone proteins (2 copies each of H2A, H2B, H3, H4) wrapped by 147 base pairs of DNA.
- Linker DNA (60 base pairs) connects nucleosomes, and histone H1 binds to it to form a chromatosome.
During interphase, chromatin has a long, stretched, and coiled linear structure.
Chromosomes within an interphase nucleus occupy specific spaces called chromosomal territories.
Chromosomes are subdivided into Topologically Associated Domains (TADs):
- Repressed TADs are associated with the nuclear lamina.
- Active TADs reside in the nuclear interior.

Types of Chromatin

Euchromatin is less compact, active in gene expression, and comprises ~5% of chromatin.
Heterochromatin is tightly packed, limited or inactive in gene expression, and represents ~95% of chromatin.
- Constitutive heterochromatin is genetically inactive, contains repetitive DNA sequences, and is stable during development.
- Facultative heterochromatin becomes heterochromatic in specific cells and is reversible, potentially becoming transcriptionally active.

Heterochromatin Features

DNA methylation and histone modifications play a role in heterochromatin formation.
- Constitutive heterochromatin is heavily methylated on cytosine.
- Facultative heterochromatin is methylated in gene control regions (CpG islands).
- Hypo-acetylation of histones condenses chromatin, leading to gene silencing, a characteristic of heterochromatin.
Heterochromatin does not undergo recombination.

Heterochromatin Functions

Heterochromatin has multiple functions:
- Centromere function
- Nuclear domain organization
- Gene repression (epigenetic regulation)

Metaphase Chromosome Overview

During cell division, chromatin condenses into visible metaphase chromosomes.
Condensins help compact and stabilize chromosomes during division.
Cohesin holds sister chromatids together after DNA replication until they separate during cell division.
Sister chromatids are two identical copies of a chromosome formed during DNA replication.
The centromere is the region where sister chromatids attach. It consists of chromatin and centromere-associated proteins.
- The DNA sequence in this region is called CEN DNA.
Kinetochores are multiprotein complexes linking sister chromatids to the mitotic spindle to ensure accurate segregation.

Specialized Chromosome Features

Nucleolus Organizing Regions (NORs):
- They are specific chromosomal regions that form the nucleolus.
- They contain rRNA genes (5.8S, 18S, and 28S rRNA).
- They are located on the short arms of chromosomes 13, 14, 15, 21, and 22 in humans.
Telomeres:
- They are the ends of linear chromosomes, consisting of repeated nucleotide sequences (TTAGGG in humans).
- They protect chromosome ends and prevent end-to-end fusion.
- Telomerase maintains telomere length.
- Telomere length is essential for chromosomal stability and acts as a "counting mechanism" for cellular aging.

Chromosomal Instability

ICF Syndrome (Immunodeficiency-Centromeric Instability-Facial Anomalies Syndrome):
- This syndrome is caused by mutations in DNMT3B, a gene responsible for methylating centromeric DNA.
- It leads to facial dysmorphism, immune deficiency, and centromeric instability in chromosomes 1, 9, and 16.
- The loss of DNA methylation disrupts centromere organization, causing instability during cell division.

Human Karyotype

A karyotype is the complete set of chromosomes in a cell.
The normal human karyotype is 46, XX (female) or 46, XY (male).
Autosomes are the non-sex chromosomes (22 pairs).
Sex chromosomes determine biological sex.
Pseudoautosomal Regions (PARs) are homologous regions on the X and Y chromosomes that allow recombination during meiosis.

The Nucleolus

The nucleolus is a dense, non-membrane bound structure within the nucleus.
It is the site of rRNA transcription, pre-rRNA processing, and ribosome subunit assembly.
The nucleolus consists of three components:
- Fibrillar centers: Contain rRNA genes and transcription enzymes.
- Dense fibrillar component: Contains proteins bound to rRNA for processing.
- Granular component: Involved in assembling ribosome subunits.

DNA Condensation and Metaphase Chromosome Formation

DNA is initially packed around histone proteins to form nucleosomes.
These nucleosomes undergo further folding to form chromatin fibers.
During mitosis, chromatin condenses further, facilitated by condensins, resulting in visible metaphase chromosomes.

Prokaryotic and Mitochondrial Chromosomes

Prokaryotic cells:
- The nucleoid is a region containing DNA, RNA, and nucleoproteins. It is not membrane-bound.
- Prokaryotic DNA is usually circular and not associated with histones.
- Plasmids are small, circular DNA molecules carrying additional genetic information.
  - They can transfer between bacteria and integrate into or exit the nucleoid, contributing to genetic recombination.
Supercoiling of DNA:
- Supercoiling compacts and organizes the nucleoid's DNA.
- DNA gyrase introduces negative supercoils, while DNA topoisomerase I prevents excessive supercoiling.
Mitochondria:
- Mitochondria are thought to have originated from α-purple bacteria through endosymbiosis.
- They have two membranes:
  - Outer membrane: Permeable to small molecules and ions.
  - Inner membrane: Highly folded into cristae, increasing the surface area for ATP production.
- The matrix, enclosed by the inner membrane, contains enzymes for ATP synthesis, mitochondrial ribosomes, tRNAs, and mtDNA.
- Mitochondrial DNA (mtDNA) is circular and negatively supercoiled, similar to bacterial DNA.
  - It exists in multiple copies per mitochondrion.
  - Top1mt, Top3α, and Top2β (topoisomerases) are found in mitochondria.
Mitochondrial functions:
- Self-replication
- Cytoplasmic inheritance
- Aerobic ATP production
- Ion homeostasis
- Steroid biogenesis
- Apoptosis

Key Definitions for Chromosome Structures

Nucleosome: A structural unit of chromatin composed of DNA wrapped around histone proteins, a fundamental unit for DNA packing.
Chromatid: One of the two identical halves of a replicated chromosome, ensuring proper segregation during cell division.
Metaphase chromosome: Highly condensed chromosome observed during metaphase of cell division, crucial for accurate distribution of genetic material.

Structure and Function of Metaphase Chromosomes

Centromere: The region where sister chromatids are joined, essential for chromosome alignment and segregation.
Kinetochore: A protein complex attached to the centromere, responsible for linking chromosomes to spindle fibers during mitosis for proper segregation.
Telomere: The protective cap at the end of each chromosome, safeguarding chromosome ends from degradation and ensuring stability.

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Description

This quiz explores the nucleus and its structures, including the nuclear envelope and nuclear pore complex. Understand the role of the nucleus as the control center of a cell and how it interacts with other cellular components. Perfect for students studying cell biology.