The cell nucleus
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THE NUCLEUS
• The largest organelle, present in eukaryotic cells
except for mammalian reticulocytes and
erythrocytes, superficial keratinocytes and lens
fibercells
• General rule: one cell-one nucleus, but exceptions
do exist!
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The nucleus – Structure
Different shapes of nuclei
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Junqueira LC Basic Histology Text and Atlas 11th Edition, 2005
The Nucleus – Ultrastructure
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The nucleus – biochemical
composition
Nucleic acids
– DNA in chromatin,
– RNA ubiquitous, mainly in nucleolus
Proteins:
– structural proteins (in chromatin and matrix),
– enzymes,
– carrier proteins.
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DNA
• DNA is a polymer, formed of nucleotides
• A nucleotide consists of a 5-carbon sugar
(deoxyribose), a nitrogen containing base
attached to the sugar, and a phosphate group.
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The nucleus – biochemical
composition
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http://www.genomebc.ca
Base pairing
Negative electric charge
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http://www.genomebc.ca
DNA Packing - Chromatin
• Chromatin – complex of double stranded DNA
with histone and non-histone proteins
• Packing of DNA relies on histone proteins:
– Core histones H2A, H2B, H3, H4
– Linker histone H1
• The nucleosome -the elementary
structural unit of chromatin
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www.shmoop.com/dna/‎
Levels of chromatin packing
fiber
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www.shmoop.com/dna/‎
The Nucleus – chromatin
• Types of chromatin:
– euchromatin – pale stained (OM),
electronoclear (EM), opened for gene
transcription
– heterochromatin – intensely stained(OM)
electronodense (EM), tightly packed, locked
for gene transcription
• constitutive – it does not serve as a template for
transcription
• facultative – silenced or condensed (ex. Barr body)
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The Nucleus – Ultrastructure
heterochromatin
euchromatin
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RNA
• A single-stranded chain of nucleotides with
ribose (instead of deoxyribose) and the bases
Adenine, Guanine, Cytosine, and Uracil
(instead of Thymine).
• Types of RNA (according to function):
– Messenger (mRNA)
– Ribosomal (rRNA)
– Transport/transfer (tRNA)
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RNA
• Types of RNA (according to its capacity to
code for protein sequences):
– Coding RNA – messenger RNA
– Non-coding RNA – cooperates with mRNA in
protein synthesis
– Long non-coding RNAs
– Short non-coding RNAs (e.g. microRNAs)
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RNA - basophylic in OM
Plasma cell in OM
Plasma cell in EM
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www.columbia.edu
The nucleus – biochemical
composition
• Proteins (other than histones)
–Enzymes (helicases, polymerases etc.)
–Structural proteins (e.g. lamins, nuclear
matrix proteins)
–Carrier proteins
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The Nucleus – Morphology
• OM: basophilic (ex: hematoxylin)
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The nucleus – Structure
One nucleus – one cell
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Junqueira LC Basic Histology Text and Atlas 11th Edition, 2005
The Nucleus – Ultrastructure
• Nuclear envelope (membrane bounded
ultrastructure)
• Hetero-/eu-chromatin
• Nuclear matrix
• Nucleolus
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The nuclear envelope
• present only in interphase
• formed by:
– inner nuclear membrane and outer nuclear
membrane
– perinuclear space (nuclear envelope lumen)
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The nuclear envelope
• outer (external) membrane is continuous with RER
cisternae and may present attached ribosomes
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Nuclear envelope
• inner membrane in
apposition with the
nuclear lamina
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course1.winona.edu/sberg/IMAGES/nuc-lam.GIF
© Pearson education, Inc. 2009
The nuclear pore
• Inner and outer
nuclear membranes
join to form an
opening for the
nuclear pore, a
protein complex
responsible for
controlled exchange
between nucleus
and cytoplasm
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The nuclear pore
1. TEM
2. Negative staining
3. Freeze-fracture deep
etching
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Nuclear matrix
• Fibrillogranular component
• Function: chromatin
support, traffic support
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NUCLEOLUS
• Non-membrane bounded organelle
• EM: heterogeneous electron dense
• 3 morphological areas:
– fibrillar center – DNA that codes for rRNA
– dense fibrillar region – transcription DNA to RNA
– granular region – ribosomal subunits in various stages of
maturation
• Function: biogenesis of ribosomal subunits
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Summary
• The nucleus is the largest organelle of eukaryotic
cells
• Biochemical composition: DNA, RNA and proteins
• OM aspect: basophilic organelle with euchromatin
and heterochromatin, nucleolus
• EM aspect: complex organization, with nuclear
envelope, nuclear lamina, nuclear matrix,
chromatin (euchromatin and heterochromatin),
nucleolus
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