The nucleus

Question 1

What is the nucleus

Answer 1

Is principle defining characteristic of eukaryotic cells
Was first organelle to be discovered by Antonie van Leeuwenhoek in 1719

Question 2

Why do we have on

Answer 2

Organisms like this originated on planet 2 billion years ago
- Probably most varied
- Largest biomass
- Enormously successful
- Occupy a lot of habitats/niches
Use fluorescence microscopy to see processes in nucleus occurring as emits different wavelength to what it absorbs
Nucleus separates process transcription and translation
Enables alternative splicing and associated complexity
AS discovered by Richard Roberts
From one gene can generate possibly 5 or more proteins from one gene depending on how it’s spliced
This leads to diversity of cell function in metasomes

Question 3

Where do the nucleus come from evolutioinarily

Answer 3

Hypothesis 1
- Invagination of membrane around DNA
- Formed primitive nucleus
Hypothesis 2
- Endosymbiosis
- If compare yeast nuclear and mitochondrial genes with archaea or bacteria nuclear and mitochondrial genes find they are genetically similar
- Can do comparison using homology hit analysis
- Yeast mitochondrial genes more similar to bacteria but yeast nuclear genes more similar to archaea

Question 4

Nucleus enveloped

Answer 4

Both hypothesis have implications for topology (the double nuclear membrane)
Contiguity between outer nuclear envelope and ER is consistent with invagination hypothesis
Heterochromatin around edges euchromatin in centre nucleus

Question 5

Nucleus content

Answer 5

Packaging essential have entire genome within single nucleus
DNA in nucleus packaged into chromosomes
DNA wrapped around histones for effective packaging
In non-dividing cells, DNA loosely packed forming tangle strand - chromatin
Just before cell division, packaging tightens up and chromosomes become visible

Question 6

Nucleus in black and white

Answer 6

EM staining technique reveals additional chromatin organisation
Heterochromatin: Dense staining of interphase DNA
Euchromatin: Less-dense staining interphase DNA
Nucleolus: Highly dense staining of RNA

Question 7

Nucleus in colour

Answer 7

Specific chromosome staining in interphase and mitosis provides additional evidence for eu/hetero-chromatin distinction and compartmentalisation
Fluorescent In Situ Hybridisation

Question 8

Nucleus compartmentalised

Answer 8

Chromosomes occupy specific territories within nucleus that are defined
May be identified by chromosomal painting/staining where different colour used for each chromosome
Territories inherited but can change following differentiation or disease

Question 9

Nucleus dynamic

Answer 9

Location gene within nucleus and chromosomal territories changes depending on transcriptional status
If genes actively being transcribed, more likely to be in euchromatin section
If gene less actively transcribed or not at all, then more likely be in heterochromatin section

Question 10

The nucleus occupied

Answer 10

Gene “tagging” together with improvements in microscopy revealed existence and function multiple subnuclear organelles in interchromatin space
Are all non-membrane bound organelles within nucleus:
- Nucleolus ~ ribosome synthesis
- Speckles ~ pre-mRNA processing
- Cajal bodies ~ splicing
- PML bodies ~ storage depot

Question 11

The nucleus’ nucleolus

Answer 11

Not membrane bound
Site for processing ribosomal RNA to produce ribosomes
Is collection macromolecules including:
- rRNA genes
- Precursor rRNA
- Mature rRNA
- rRNA processing enzymes
- snoRNPs
- Ribosomal protein subunits
- Partly assembled ribosomes
Also produces other types RNA:
- mRNA
- tRNA

Question 12

Nucleus dynamic

Answer 12

Subnuclear organelles can be dynamic and move in non-random ways in ATP dependent manner

Question 13

Nucleus dissected

Answer 13

Nuclear envelope double unit membrane perforated with pores and supported by fibrous meshwork called lamina
Lamina responsible (in part) for ensuring asymmetric nature double membrane
Lamina attached in places to chromatins and can retain proteins in nucleus e.g. Emerin, Nesprin, LAP2/1

Question 14

Nucleus damaged

Answer 14

HCPS and other laminopathies (breaks/damages to lamina) suggest lamina plays global role in gene regulation and structural rigidity
Laminopathies encompass diverse range inherited diseases such as Hutchinson-Gilford progeria syndrome which causes body rapidly age and lead to death
LMNA mutant gene causes nucleus structurally unstable whereas wild type LMNA nucleus stable

Question 15

Nucleus perforated

Answer 15

Nuclear pore structure provided by scanning electron microscope

Question 16

Nucleus resolved

Answer 16

Xenopus laevis: useful model system for investigation nuclear components and their dynamics

Question 17

Nuclear accessed

Answer 17

Nuclear access controlled by pore in size-dependent manner
Particles MW>50,000 cannot enter nucleus by simple diffusion but can by active signal-dependent transport

Question 18

Nucleus decoded

Answer 18

Signal for entry specific peptide sequence
On signal, pore can open up to 26 nm in diameter
Tested by putting sequence into protein that hadn’t been into nucleus and then when was given sequence protein did get into nucleus
Every protein that gets into nucleus has specific sequence

Question 19

Summary

Answer 19

DNA is packaged into chromosomes. The DNA wraps around histone molecules which increases the efficiency of packaging
The nuclear envelope is composed of two membranes, perforated by pores. The outer membrane is continuous with the ER
The nucleus contains additional organelles such as the nucleolus, the site of ribosome production.
Nuclear Pores are a complex arrangement of proteins. The movement of substances into and out of the nucleus is regulated by the pore.