Structure of cell nucleus, nuclear receptors and intracellular

signalling

Nucleus: structure and function

Nucleus in a human cell

- diameter 5-8 m

- 10% of a total cell volume

- DNA size - 3x109 pb, length

2 m, 23 chromosome pairs

-

Important metabolic processes DNA replication transcription RNA processing formation of ribosomal subunits

Ultrathin section Freeze fracture

ne

nucleolus

np

Nuclear DNA is packaged into chromosomes

• Eucaryotic chromosome a single, long, linear DNA

molecule associated with proteins to a more compact structure. Some of the proteins are involved in DNA packaging, other take part in gene expressing, replication and repair.

Procaryotic chromosome: circular DNA lacking proteins

Mitotic chromosomes as seen in

a scanning electron microscope

Chromosome: a complex of DNA and proteins(chromatin). Basic units are nucleosomes

Nucleosomes protein core made of 8 histone molecules: 2 copies of H2A,H2B, H3 a H4 DNA

Histones:

Small proteins with a high proportion of positively charged amino acids – lysine and arginine. These positive charges help the histone bind tightly to the negatively charged sugar-phosphate backbone of DNA

Levels of chromatin packing

1. naked DNA molecule - does not occur in vivo

2. beads-on-a-string occurs teporarily during transcription

3. nucleosomes condensed into 30 nm thread due to a binding od H1 histone

4. Organization of the 300 nm thread from loops along a central axis. Non-histon proteins are involved

5. Formation od secondary loops to form a 700 nm thread

6. Metaphasic chromosome 1400 nm thick ( role of condensin)

Lidské chromosomy

Mitotic chromosomes from human lymphocytes

Chromosomes: homologous nonhomologous

Chromosome identification: banding FISH: fluorescence in situ hybridization

The three DNA sequence elements needed to produce a eucaryotic chromosome that can be replicated and then segregated at mitosis

Interphase nucleus: nuclear envelope nuclear lamina nuclear matrixnuclear pores

IF

ER

MT

Heterochromatinhighly condensed, transriptionally inactive

Euchromatindecondensed in interphasecondensed during mitosis

The nuclear pore complex

The nuclear envelope is perforated by n. pores – the gates for entering or leaving molecules of RNA or proteins

NP is composed of about 100 different proteins and forms a short channel with protruding protein fibrils

Small molecules – nucleoti-des, ATP and others enter freely.

Large molecules require an appropriate sorting signal (nuclear localization signal)

Topology of biochemical processes in the nucleusChromosomal domains - each chromosome occupies a defined sector of the nuclear space„Movement“ of chromosomes is experimentally proved – tagged centromeres or telomeres change their position to the NE

Transcription: RNA polymerase I – nucleolus RNA polymerase II and III – nucleoplasm

(small compartments 40-100 nm in diameter)Assembly of ribosomal subunits: nucleolus

DNA replication: replication foci along the chromosomes 1 replication focus contains some 5 replicons, velocity of the replication - 2kbp/min during S phase some 10 000 replication foci are operating in the nucleus

Topology of interphase chromosomes in the nucleus

Each chromosome occupies a particular region of the nucleus so that the chromosomes are not entangled with each other. The chromosomes are attached to a NE or nuclear lamina via proteins of the inner membrane NE: lamin B receptor (LBR) orlamina – associated protein(LAP2). To these proteins chromatin and lamins are bound

Nucleolus a region where the parts of different chromosomes carrying genes for ribosomal RNA cluster together. Here the genes are transcribed and ribosomal subunits are assembled (using ribosomal proteins imported from the cytoplasm

Nuclear receptors

• Common term for intracellular (mostly intranuclear) receptors, binding hormones soluble in lipids– e.g. steroids, thyroid hormones , vitamine D or retinoids

• Nuclear receptors are proteins that after binding of ligand (steroid molecules) undergo structural changes and may function as transcription factors

Nucleus and signal transduction pathway

Nuclear receptors as transcription factors

Nucleus and signal transduction pathway

Nuclear receptors

• Common term for intracellular receptors binding hormones soluble in lipids– e.g. steroids, thyroid hormones ; vitamine D or retinoids

• Nuclear receptors are proteins that after binding of ligand (e.g. steroid molecules) undergo structural changes and may function as transcription factors, binding to specific DNA sequences.

Nuclear receptors as transcription factors

Key terms 1. Overall structure and function of the human cell nucleus2. Chromosome structure as seen in the optical microscope.3. Cytochemical identification of single chromosomes 4. Three sequence elements of chromosomal DNA5. Molecular arrangement of DNA and histones in the chromosome6. Levels of chromosome condensation 7. Ultrastructure of the nuclear surface and nuclear pores 8. Nucleolus. 9. Topology of chromosomes in the nucleoplasm10. Nucleus and intracelular signalling11. Nuclear receptors as transcription factors

Vyštěpující tělískoHeterochromatin PcG tělísko

Cajalovo (svinuté) tělísko a Gem

Chromosomální teritoria

PML tělísko

Jaderné skvrny

SAM68 tělískoPerinukleární kompartment

Jadérka Periferní jadernálamina

RNA pol II transkripční faktor

OPT doména

Komplex jaderného póru

JADERNÉ DOMÉNY