Section 5
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What is the difference between euchromatin and heterochromatin?
• Euchromatin: Decondensed, transcriptionally active • Heterochromatin: Condensed, transcriptionally inactive Heterochromatin is associated with the nuclear envelope or the periphery of the nucleolus.
What is epigenetic inheritance?
The transmission of information from parent to progeny that is not contained within the sequence of DNA Example: Unique histone composition allows centromeres to be stably maintained at cell division.
What are the steps in ribosome assembly?
• Ribosomal RNA genes are transcribed • Pre-rRNAs undergo processing • Ribosomal proteins are imported and assemble on pre-rRNA • Additional ribosomal proteins and 5S rRNA assemble to form pre-ribosomal particles • Final maturation occurs after export to the cytoplasm This process yields the 40S and 60S ribosomal subunits.
How are lamins associated with the inner nuclear membrane?
• Posttranslational addition of lipids (prenylation) • Binding to specific inner nuclear membrane proteins (e.g., emerin) • Connection to the cytoskeleton via LINC complexes Lamins also bind to chromatin, influencing the localization of non-transcribed heterochromatin.
What are nuclear bodies?
Discrete organelles within the nucleus that compartmentalize specific processes Unlike cytoplasmic organelles, nuclear bodies are not enclosed by membranes and are maintained by protein–protein and protein–RNA interactions.
What is the function of nuclear localization signals?
Target proteins for import into the nucleus These signals are recognized by nuclear transport receptors called importins.
True or false: Nuclear export signals are recognized by importins.
FALSE Nuclear export signals are recognized by exportins, which mediate transport to the cytoplasm.
What determines the directionality of nuclear import?
Levels of Ran/GTP High levels of Ran/GTP in the nucleus ensure importins are dislodged from their cargo upon entry.
What is the role of snRNA transport between the nucleus and cytoplasm?
• Exported from nucleus to cytoplasm by Crm1 • Associate with proteins to form snRNPs • Recognized by snurportin for transport back to the nucleus snRNPs are involved in pre-mRNA splicing.
What are Polycomb bodies?
Nuclear bodies enriched in Polycomb proteins responsible for transcriptional repression They are associated with heterochromatin and formed through liquid–liquid phase separation.
What are Polycomb bodies?
Clustered sites of gene repression Polycomb bodies are concentrated in a small number of regions rather than being distributed throughout the nucleus.
Polycomb bodies are frequently associated with what type of chromatin?
Heterochromatin This association is consistent with their role in gene repression.
The formation and maintenance of Polycomb bodies are not entirely understood, but they are indicated to be formed through what process?
Liquid–liquid phase separation This process is mediated by PRC1.
Of the four subunits that make up PRC1, one has a predominantly _______ structure that promotes phase separation.
Disordered This disordered structure helps recruit and compact PRC1-bound chromatin into a transcriptionally inactive state.
PRC1 mediates transcriptional repression through multiple mechanisms, including what two specific actions?
• H2A lysine 119 ubiquitination • Liquid–liquid phase separation These actions result in chromatin compaction.
In some cases, the chromatin within Polycomb bodies consists of a continuous DNA sequence containing a series of adjacent genes, but at least some Polycomb bodies contain clusters of repressed domains from what?
Different chromosomal regions This is similar to transcription hubs.
True or false: Polycomb bodies are distributed throughout the nucleus.
FALSE They are concentrated in a relatively small number of regions.