Cells - the nucleus (mammalian biology)

What substructures are present in the nucleus?

• Chromatin (DNA) - in the forms of euchromatin and heterochromatin

• Nuclear envelope with pores

• Nucleoplasm

• RNA and proteins - histones and DNA-binding proteins, ribosomal proteins, mRNA and RNA-binding proteins

• Nucleolus

• Splicing speckles

• Cajal bodies

What are the structures and functions of nuclear substructures?

• Nucleoplasm - fluid that fills the nucleus

• Nucleolus - involved in synthesis of ribosomal RNA

• Splicing speckles - irregular structures that contain mRNA

• Cajal bodies - 0.2-1.0 micrometres, concentrate RNA processing factors

What is the structure of the nucleolus?

• Contains granular components and fibrillar centres

• Granular components function as ribosome assembly sites

• Fibrillar centres are sites of rRNA transcription

What is the overall function of the nucleolus?

• Produces ribosomes

• Ribosomal proteins are imported into the nucleus

• Assembled ribosome subunits are exported through the nuclear pore complexes into the cytoplasm - involves exportins and RAN-GTPase

How is the interior of the nucleus organised?

• Organised by protein fibres and the nuclear pores

• The nuclear lamina organises the nuclear pores and anchors chromosomes

• The nuclear matrix/scaffold organises DNA to support transcription by exposing particular sections of DNA

• Nuclear pore complexes anchor chromatin

How is chromatin organised within the nucleus?

• DNA associates with proteins into chromatin

• Forms heterochromatin and euchromatin

• Chromosomal DNA is thousands of times longer than the nucleus

• Is highly folded in order to fit into the nucleus

• Folding involves interaction with structural proteins - results in chromatin

What is the difference between heterochromatin and euchromatin?

• Heterochromatin remains packed after mitosis, is transcriptionally inactive and makes up ~10% of the DNA

• Euchromatin is transcriptionally active

What are the different levels of organisation and their respective sizes?

• DNA - 2nm diameter and negatively charged

• Histones - positively charged proteins made up of ~100 amino acids

• Nucleosomes - 30nm diameter

• 30nm fibre - interaction between DNA and histones

• 300nm fibre (looped domains) - DNA is packed around a scaffold containing specialised proteins

• Chromosomes - genes are always in the same position

What is the main process that occurs in the nucleus?

Transcription

What are the different types of RNA polymerase in eukaryotes?

• RNA polymerase I: ribosomal RNA

• RNA polymerase II: messenger RNA

• RNA polymerase III: transfer RNA

• RNA polymerase IV (plants only): siRNA

What is the basic principle of transcription in eukaryotic cells?

• Transcription factors bind to the TATA box in the promoter region (upstream of the gene)

• RNA polymerase binds to the template strand and synthesises an exact copy of the coding strand

• RNA is released, processes and released from the nucleus

How does gene expression differ from prokaryotes and eukaryotes?

• Prokaryotes - transcription and translation occur in the same compartment, many genes are present on one mRNA

• Eukaryotes - transcription and translation are compartmentalised, one mRNA per gene

What organelles are present in the endomembrane system?

• Nucleus

• Endoplasmic reticulum

• Golgi apparatus

• Lysosome/vacuole

• Endosomal compartment

• Transport vesicles

• Peroxisomes

What are the types of trafficking pathways through the endomembrane system?

• Secretory pathway

• Endocytic pathway

• Retrieval/recycling pathway