04 - Cell Nucleus

nucleus

defining organelle of eukaryotes that functions to:

• store genetic information

• regulate gene expression

• coordinate DNA replication and repair

• organize chromatin

• create ribosome in nucleolus

nuclear envelope

double-membrane system that functions to separate nuclear contents from cytoplasm, provide structural support, and house nuclear pore complexes for selective transport

• outer membrane → continuous with rough ER

• inner membrane → anchored to nuclear lamina

nuclear pore complex

large protein assemblies spanning both membranes of nucleus

• bidirectional transport → import proteins and export of RNA and ribosomal subunits

• selective transport → requires nuclear localization signal for import and nuclear export signal for export

• driven by Ran GTPase system → Ran-GTP in nucleus and Ran-GDP in cytoplasm creates directionality

  • Ran-GTP binds importin inside nucleus to release cargo

• transport is active, signal-dependent, and tightly regulated

• small molecules can diffuse freely

nuclear lamina

meshwork of lamin intermediate filaments under membrane

• functions for structural support of envelope, and anchors chromatins and nuclear pore complex

• dynamic

  • phosphorylation → disassembly during mitosis

  • dephosphorylation → reassembly in daughter cells

Hutchinson-Gilford Progeria Syndrome

mutations in nuclear lamins causes premature aging

chromatin organization

chromatin accessibility directly regulates gene activity

• euchromatin: loosely packed, transcriptionally active, light staining in light microscopy

• heterochromatin: densely packed, transcriptionally silent, dark staining in light microscopy

• epigenetic modifications

  • histone acetylation → loosens chromatin to turn on transcription

  • histone deacetylation → tightens chromatins to turn off transcription

  • DNA methylation → gene silencing by cytosine methylation

nucleolus

non-membrane dense region inside nucleus

• functions as site of rRNA transcription by RNAPI and site of assembly of ribosomal subunits

• cells with high protein synthesis have prominent nucleoli

• disassembles during mitosis, but reappears after

cell cycle dynamics

nucleus changes shape to meet needs at different stages of cell cycle

• interphase → nuclear envelope intact, transcription and replication active

• mitosis:

  • prophase → lamins phosphorylates, envelope disassembles

  • metaphase → chromosomes align, no nucleolus

  • telophase → lamins dephosphorylated, envelope and nucleolus reassemble

nucleosomes

fundamental DNA-protein unit, with 146 bp wrapped around histone octamer

• linker DNA connects nucleosomes into “beads on a string”

• histone made of H2a, H2b, H3, H4

topologically associated domains (TADs)

neighborhoods of interacting DNA

• genes and enhancers within a TAD can loop to interact

• dynamic → change during development and differentiation for gene expression

  • some TADs turn genes on, while others turn genes off depending on cell type

• chromatin reorganization is central to differentiation

• have boundaries that prevent enhancers from “leaking” to wrong genes

lamin mutations

defective lamins disrupt chromatin organization and lead to misregulated gene expression and disease

• altered nuclear architecture is linked with aging processes