6.1 Nucleus and nuclear envelope
3 min read•july 22, 2024
The nucleus, a cell's command center, houses DNA and orchestrates genetic activities. Its double-membrane envelope, perforated by nuclear pores, acts as a gatekeeper, controlling molecular traffic between the nucleus and cytoplasm. This selective barrier maintains distinct environments crucial for cellular functions.
Inside the nucleus, organization plays a key role in . , less condensed and more accessible, allows active . , tightly packed, represses gene activity. This dynamic structure influences how genes are read and expressed, shaping cellular behavior.
Nucleus and Nuclear Envelope
Structure and function of nucleus
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- Nucleus largest organelle in eukaryotic cells contains genetic material (DNA) and nucleoli
- Surrounded by double-membrane nuclear envelope
- Outer membrane continuous with endoplasmic reticulum (ER)
- Inner membrane lined with provides structural support
- perforate nuclear envelope regulate transport of molecules between nucleus and cytoplasm
- Surrounded by double-membrane nuclear envelope
- Nucleolus dense, non-membrane bound structure within nucleus site of ribosomal RNA () synthesis and ribosome subunit assembly
- Chromatin DNA and associated proteins () within nucleus
- Euchromatin less condensed, transcriptionally active (genes actively expressed)
- Heterochromatin highly condensed, transcriptionally inactive (genes repressed)
Role of nuclear envelope
- Nuclear envelope acts as selective barrier controls entry and exit of molecules between nucleus and cytoplasm maintains distinct nuclear and cytoplasmic environments
- Nuclear pore complexes (NPCs) mediate transport
- Allow passive diffusion of small molecules (ions, metabolites)
- Facilitate active transport of larger molecules (proteins, RNA) through (NLS) or (NES)
- gradient drives directional transport
- Ran-GTP (nucleus) and Ran-GDP (cytoplasm) gradient
- bind cargo with NLS in cytoplasm release it in nucleus upon Ran-GTP binding
- bind cargo with NES in nucleus release it in cytoplasm upon Ran-GTP hydrolysis
- Ran-GTP (nucleus) and Ran-GDP (cytoplasm) gradient
Proteins in nuclear pore complexes
- (Nups) building blocks of NPCs over 30 different Nups in vertebrates (humans)
- Structural Nups provide scaffold maintain NPC architecture (Nup107-160 complex, Nup93 complex)
- FG-Nups contain phenylalanine-glycine (FG) repeats form selective permeability barrier interact with transport receptors (Nup98, Nup153, Nup214)
- Transmembrane Nups anchor NPCs to nuclear envelope (Pom121, Ndc1, gp210)
- Ran GTPase and its regulators drive directional transport
- Ran small GTPase
- RCC1 Ran guanine nucleotide exchange factor (GEF) generates Ran-GTP in nucleus
- RanGAP Ran GTPase-activating protein stimulates Ran-GTP hydrolysis in cytoplasm
Chromatin organization and gene expression
- Chromatin organization DNA wrapped around histone octamers forms nucleosomes further compacts into higher-order structures (chromatin fibers, loops, domains)
- Euchromatin less condensed, more accessible enriched in actively transcribed genes associated with histone modifications that promote transcription (H3K4me3, H3K36me3)
- Heterochromatin highly condensed, less accessible contains fewer actively transcribed genes associated with histone modifications that repress transcription (H3K9me3, H3K27me3)
- Chromatin remodeling complexes alter chromatin accessibility by repositioning, evicting, or exchanging nucleosomes (, , , families)
- Chromatin accessibility influences transcription factor binding and gene expression
- Open chromatin (euchromatin) facilitates transcription factor access and gene activation
- Closed chromatin (heterochromatin) hinders transcription factor access and gene repression
- Nuclear bodies distinct nuclear domains associated with specific functions
- involved in snRNP biogenesis
- involved in transcription and apoptosis
- involved in pre-mRNA splicing
Key Terms to Review (27)
Cajal Bodies: Cajal bodies are small, spherical structures found within the nucleus of eukaryotic cells, playing a key role in the biogenesis of ribonucleoprotein complexes and in the regulation of gene expression. Named after Santiago Ramón y Cajal, they are involved in the processing and assembly of small nuclear RNAs (snRNAs) and other essential RNA molecules, contributing to the overall function of the nucleus and its envelope.
Chd: CHD, or congenital heart disease, refers to a range of structural heart defects that are present at birth. These defects can affect the walls of the heart, the valves of the heart, or the arteries and veins near the heart. Understanding CHD is essential because it can impact normal blood flow through the heart and result in various health issues throughout life.
Chromatin: Chromatin is a complex of DNA and proteins found in the nucleus of eukaryotic cells, responsible for packaging the long strands of DNA into a more compact, organized structure. This organization is crucial for DNA replication, gene expression, and maintaining the integrity of the genetic material. Chromatin exists in two forms: euchromatin, which is less condensed and active in transcription, and heterochromatin, which is more condensed and typically inactive.
Euchromatin: Euchromatin is a loosely packed form of chromatin that is associated with active gene transcription and is more accessible for transcription factors and RNA polymerase. This form of chromatin allows DNA to be readily available for the processes of transcription, making it crucial for gene expression. The distribution of euchromatin within the nucleus can indicate the level of metabolic activity in a cell, with regions rich in euchromatin typically correlating with active cellular functions.
Exportins: Exportins are specialized transport proteins that facilitate the export of molecules, particularly RNA and proteins, from the nucleus to the cytoplasm. They play a crucial role in maintaining cellular function by ensuring that essential molecules are appropriately transported to their sites of action outside the nucleus, highlighting their importance in the dynamics of nuclear transport.
Gene expression: Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically proteins, which ultimately influence the phenotype of an organism. This process involves several key steps including transcription of DNA into messenger RNA (mRNA) and translation of mRNA into proteins, influenced by various signaling pathways and cellular mechanisms.
Heterochromatin: Heterochromatin is a tightly packed form of DNA that is generally transcriptionally inactive, meaning it is not actively expressed as genes. This structure plays a crucial role in maintaining genome stability, regulating gene expression, and organizing the nuclear architecture. It contrasts with euchromatin, which is less condensed and associated with actively transcribed genes, emphasizing the dynamic nature of chromatin in cellular function and differentiation.
Histones: Histones are small, positively charged proteins that play a crucial role in the organization and packaging of DNA within the nucleus of eukaryotic cells. They help to form a structure known as nucleosomes, which consists of DNA wrapped around a core of histone proteins, thereby compacting the genetic material and allowing it to fit within the limited space of the nucleus. This packaging is vital for regulating gene expression and DNA replication.
Importins: Importins are proteins that facilitate the transport of other proteins into the nucleus by recognizing nuclear localization signals (NLS) on the cargo proteins. They bind to these proteins in the cytoplasm, allowing them to cross the nuclear envelope through the nuclear pore complexes, which are large protein structures that span the nuclear membrane. This transport mechanism is crucial for cellular function, as it regulates the import of various nuclear proteins necessary for processes such as gene expression and DNA replication.
Ino80: Ino80 is a multi-subunit protein complex that plays a crucial role in chromatin remodeling, which is essential for various DNA-related processes such as transcription, replication, and repair. This complex is specifically involved in the ATP-dependent repositioning of nucleosomes, which helps regulate access to the underlying DNA, making it an important player in maintaining genomic integrity and facilitating gene expression.
Iswi: Iswi refers to a family of ATP-dependent chromatin remodelers that play a critical role in the organization and accessibility of chromatin within the nucleus. These proteins are essential for maintaining proper gene expression, as they help reposition nucleosomes and alter chromatin structure, facilitating the access of transcription factors and other regulatory proteins to DNA. Their activity is vital for numerous cellular processes, including DNA repair, replication, and cell differentiation.
Nuclear Division: Nuclear division is the process by which a cell's nucleus divides, ensuring that each daughter cell receives an identical set of chromosomes. This process is crucial for maintaining genetic continuity during cell division and can occur through different mechanisms such as mitosis and meiosis. Each mechanism serves distinct purposes in growth, development, and reproduction, influencing the organization of genetic material within the cell.
Nuclear Export Signals: Nuclear export signals (NES) are specific amino acid sequences found in proteins that direct their transport from the nucleus to the cytoplasm. These signals play a critical role in cellular processes by ensuring that proteins are relocated to where they are needed, allowing for proper gene expression regulation and response to various cellular conditions. The presence of these signals is key for the function of many proteins involved in signaling pathways, transcription regulation, and stress responses.
Nuclear Lamina: The nuclear lamina is a dense fibrillar network inside the nucleus, composed primarily of intermediate filaments called lamins. It provides structural support to the nucleus, maintains its shape, and plays a crucial role in organizing chromatin and anchoring nuclear components. This network is closely associated with the inner membrane of the nuclear envelope and is essential for various nuclear processes, including DNA replication and cell division.
Nuclear Localization Signals: Nuclear localization signals (NLS) are specific amino acid sequences within proteins that direct their transport from the cytoplasm into the nucleus. These signals play a crucial role in the cellular process of protein trafficking, ensuring that proteins required for nuclear functions, such as transcription factors and histones, reach their target destination within the nucleus through the nuclear pore complex.
Nuclear matrix: The nuclear matrix is a network of fibers within the nucleus of a cell that provides structural support and organization to the genetic material and other components. This framework plays a crucial role in maintaining the shape of the nucleus, organizing chromatin, and anchoring nuclear proteins, which all contribute to essential cellular functions such as gene expression and DNA replication.
Nuclear Pore Complexes: Nuclear pore complexes (NPCs) are large protein structures that span the nuclear envelope, serving as gateways for the transport of molecules between the nucleus and the cytoplasm. These complexes allow for the selective exchange of proteins, RNA, and other substances, ensuring that essential cellular functions are maintained while protecting the genetic material within the nucleus.
Nuclear speckles: Nuclear speckles are dynamic, irregularly shaped structures found within the nucleus of eukaryotic cells that serve as storage and processing sites for pre-mRNA splicing factors. These structures are associated with active transcription and play a critical role in gene expression regulation by organizing the splicing machinery and facilitating the maturation of mRNA. Their presence is closely linked to the functional organization of the nucleus, particularly in relation to transcription and mRNA processing.
Nuclear transport: Nuclear transport refers to the process by which molecules, including proteins and RNA, move between the nucleus and the cytoplasm through the nuclear envelope. This transport is essential for cellular function, as it allows for the regulation of gene expression and the maintenance of cellular homeostasis by controlling the localization of various macromolecules. The nuclear envelope, which consists of two lipid bilayer membranes, is perforated by nuclear pore complexes that facilitate this bidirectional transport.
Nucleoporins: Nucleoporins are a group of proteins that make up the nuclear pore complexes (NPCs) which span the nuclear envelope. These proteins play a critical role in regulating the transport of molecules between the nucleus and the cytoplasm, ensuring that only specific substances can enter or exit the nucleus. By forming a selective barrier, nucleoporins help maintain the distinct environments necessary for various cellular processes, including gene expression and ribosome assembly.
PML Bodies: PML bodies, or promyelocytic leukemia bodies, are distinct nuclear structures involved in various cellular processes such as transcription regulation, DNA repair, and apoptosis. These small, spherical bodies contain various proteins, including the PML protein, which plays a crucial role in the formation of these structures and is implicated in multiple cellular pathways. The presence and function of PML bodies are closely linked to the dynamics of the nuclear envelope and overall nuclear organization.
Ran GTPase: Ran GTPase is a small GTP-binding protein that plays a critical role in the transport of proteins and RNA across the nuclear envelope. It operates by switching between an active GTP-bound form and an inactive GDP-bound form, facilitating the movement of molecules between the nucleus and cytoplasm, particularly during processes such as nuclear import and export.
RNA processing: RNA processing refers to the series of modifications that a primary RNA transcript undergoes to become a mature RNA molecule ready for translation. This includes the addition of a 5' cap, splicing out introns, and adding a poly-A tail at the 3' end. These modifications are crucial as they enhance RNA stability, facilitate nuclear export, and ensure the proper translation of mRNA into proteins.
RRNA: rRNA, or ribosomal RNA, is a type of RNA that plays a crucial role in the synthesis of proteins by forming the core of ribosome structures and catalyzing the translation process. It serves as a structural and functional component of ribosomes, which are the cellular machinery responsible for translating messenger RNA (mRNA) into proteins. This connection to protein synthesis highlights its essential function in gene expression and cellular activity.
Swi/snf: swi/snf is a chromatin remodeling complex that plays a crucial role in the regulation of gene expression by altering the structure of chromatin, allowing access to DNA for transcription and other nuclear processes. This complex utilizes energy from ATP hydrolysis to reposition nucleosomes, which are the basic units of chromatin, facilitating the recruitment of transcription factors and other regulatory proteins essential for gene activation.
Transcription: Transcription is the process by which the genetic information encoded in DNA is copied into messenger RNA (mRNA) to be used in protein synthesis. This process connects the genetic blueprint stored in the DNA with the functional roles of proteins, facilitating the flow of genetic information within a cell. Understanding transcription is crucial as it occurs within the nucleus, involves specific DNA sequences, and varies between prokaryotic and eukaryotic organisms.
Transcription Factors: Transcription factors are proteins that regulate the transcription of specific genes by binding to nearby DNA. They play a critical role in controlling gene expression, influencing cellular processes such as growth, differentiation, and response to environmental signals. Their function is essential in both prokaryotic and eukaryotic cells, and they interact with the nuclear envelope and various RNA polymerases during the transcription process.