The Nucleus Flashcards
Key Concepts
The nucleus is a highly-specialized organelle committed primarily to
protecting, copying, and transcribing DNA (making an RNA copy).
The interior of the nucleus is highly compartmentalized.
Most cells contain a single nucleus – exceptions include muscle and
blood cells
Nucleus
Double membrane bound organelle
Contains the hereditary information (DNA
genome)
Site of DNA replication (DNA to DNA)
Site of gene transcription (DNA to RNA)
Chromatin
DNA and associated
proteins within nucleus
Nuclear envelope
double
membrane with nuclear pores
Nucleolus
where rRNA and
proteins are assembled to form
ribosomes
Nucleoplasm:
remaining nuclear
content, similar to cytoplasm.
The nucleus contains several features that
distinguish it from other cellular structures
Envelope
Double membrane.
Separates genome from the
cytoplasm.
Functionally, separates
transcription (RNA synthesis) in
the nucleus from translation
(polypeptide synthesis) in the
cytoplasm.
Thousands of nuclear pores
perforate the nuclear envelope:
Each measuring 9nm
Regulate traffic between the two
compartments
Transcription
DNA mRNA Protein
mRNA, tRNA and rRNA
- Ribosomes consists of a few
ribosomal RNA (rRNA) molecules
and a variable number
of ribosomal proteins. - Amino acids are incorporated into
the growing polypeptide on the
ribosome according to the
sequence of codons of a mRNA. - tRNA carries appropriate amino
acid to match code.
Chromatid
one-half of
two identical copies
of a replicated
chromosome.
Telomeres
caps
on chromosomes that
offer protection
during cell division
- Each chromosome
contains DNA bound to
histones - This packaging controls
information available - DNA wrapped round
histone = nucleosome
Visibility of chromosomes through
cell cycle
Immunofluorescence images:
green dye labels microtubules
and blue dye labels DNA.
* Centrosomes form before cell
division.
* Chromosomes become visible.
* Daughter cells have DNA in the
form of chromatin
The code for life
- DNA strands held together by hydrogen
bonds between complementary bases:
Adenine (A), Thymine (T), Cytosine (C) and
Guianine (G) - Triple code system, where 3 bases is an
intstruction for one amino acid. - The genetic code is the recipe for amino
acids. - Proteins are made of amino acids.
- The code is hidden until required. How?
By wrapping around histones. - Enzymes such as Helicase and
Topoisomerase work to uncoil DNA during
replication or transcription
(Short tandem repeats –
STR)
Not all DNA codes for proteins
* ”Useless” codes that never translate are repeated
over and over again
* Number of “useless” code and repetition pattern
unique to each individual.
Euchromatin
loose
chromatin structure and active
for transcription
Heterochromatin
condensed chromatin structure
and is inactive for transcription
Heterochromatin appears as dark patches in the nucleus
during interphase (i.e. non-dividing cells).
DNA replication factories
form large
complexes in the nucleus devoted to
copying DNA with 100% accuracy
consist of DNA associated
with replication machineries, i.e.,
large protein complexes involved in
DNA replication
Replisome:
smallest functional
unit in the factories, responsible for
copying one segment of DNA
Nucleolus
The nucleolus
contains DNA that
encodes ribosomal
RNAs (rRNA)
Nucleoli is the plural
of nucleolus
Nucleoli are sites of
high transcriptional
activity for rRNA genes
Functions of Nucleolus
Not a membrane bound structure.
Transcription of ribosomal RNA from DNA
Packaging of rRNA with proteins to form
ribosomal sub-units:
Proteins translated in the cytoplasm &
transported back to the nucleus
The nuclear lamina
Beneath the inner
membrane, lies the
nuclear lamina.
* A meshwork of
intermediate filament
proteins called Lamins
and associated proteins
Lamin Proteins
form
structures known as intermediate filaments
Three types of lamin proteins: A-type, B-type
and C-type lamins
Bind to the inner nuclear membrane giving
nuclear stability, organise chromosomes and
bind nuclear pore complexes
Lamin proteins interact with chromosomes and
DNA, regulating gene expression
The nuclear lamina structure
- Nuclear Lamina fibres lie between nuclear pore complexes (NPC).
- Lamins are anchored in place by membrane components
- Several inner nuclear membrane proteins – interacts with DNA or DNA
associated proteins
helps control the shape of
chromosomes and regulate heterochromatin and
euchromatin
A multitude of proteins are
needed in the nucleus
- Proteins involved in:
- DNA replication
- Transcription
- RNA processing (splicing of exons)
- Chromosome packaging (histones)
- Ribosome construction (in the nucleolus)
- Nuclear lamina
- Envelope proteins
- Protein trafficking through nuclear pore
complexes
NPCs
Nuclear pore complexes:
Layers of rings stacked on
top of one another that span
the nuclear membranes,
linked to filamentous protein
fibrils to form a basket
structure
Structure undergoes
complex conformational
changes when it transports
material into and out of the
nucleus
Nuclear Localisation Signals (NLS)
Short peptide sequence acts like a postcode to
direct the polypeptide to the nucleus
Composed of positively charged amino acids,
e.g. Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val
Can be at the N-terminal of the polypeptide
chain or internal
This signal sequence binds to a docking protein
on the outer membrane of the nucleus, e.g.
importin
Import / Export through nuclear pores
Regulated by two carrier proteins:
Importin and Exportin
Importins and exportins are members of a family
of nuclear transport receptors known as
karyopherins
nuclear import/export system
regulates traffic through nuclear pores
Proteins transported in/out of nucleus in folded,
functional state
Nuclear localization sequences (NLS) and nuclear
export signals (NES) are amino acid sequences
recognized by NLS and NES receptors
Direction of nuclear transport is controlled by a
protein called Ran
Ran binds the nucleotide GTP (G-protein)
Ran acts like a waterwheel, moving in and out of
the nucleus in a cycle powered by GTP.
When NLS met importin
Nuclear localisation signals
(NLS) in a protein to be carried
into the nucleus (the cargo)
bind to nuclear transport
receptors called importins
The cargo/importin complex
binds to proteins in the
cytoplasmic filaments in the
nuclear pore complex and is
transported through the nuclear
pore complex
journey via
the Nuclear Pore
The complex translocate to the
nucleus via the NPC.
In the nucleus, Ran-GTP
displaces the cargo protein,
which is released
Importin-Ran-GTP then via the
NP
The importin/Ran-GTP
complex is then exported
through the nuclear pore.
GTPase-activating protein
(Ran GAP) in the cytoplasm
hydrolyses the GTP on Ran to
GDP, releasing importin in
cytoplasm.
Ran-GDP together with NTF2 return to
the nucleus
The Ran/GDP is then
transported back to the
nucleus by binding to its
own import receptor
(NTF2).
In the nucleus, a guanine
exchange protein (Ran
GEF) reforms Ran-GTP.
Ran-GTP levels high in
nucleus to support protein
cargo-importin disruption.
(part 2)
NPC Export
Proteins targeted for export
from the nucleus have a
sequence of amino acids
called a nuclear export
signal
Exportin proteins bind to
these peptides and transport
the protein back to the
cytoplasm
Exportins also bind to
Ran/GTP, but this time
enable export of the cargo
protein
Nuclear export
2. Nuclear pore complex: Export
Following transport to the
cytoplasm the GTP is
hydrolysed to GDP and
Ran/GDP is released from
the cargo.
Exportins recycled back to
the nucleus through the
nuclear pore
Key Concepts
Because DNA stores all of the information
necessary to survive, cells protect it from damage.
This is especially evident in eukaryotic cells, which
typically have much larger DNA molecules than
prokaryotes.
– In eukaryotes, the primary function of the nucleus is
to sequester DNA in its own chemically-specialized
compartment. Passage of large molecules (e.g.,
proteins) into and out of the nucleus is carefully
regulated by a structure called the nuclear pore
complex.
The nucleus carefully protects a
eukaryotic cell’s DNA
* Key Concepts (2):
– A secondary function of the nucleus is to physically
shield the DNA from damage. Nuclear lamins are
cytoskeletal proteins that form a tough, fibrous network
attached to the inner surface of the nuclear membrane.
This network protects DNA from mechanical force.
