The Nucleus

Question 1

Key Concepts

Answer 1

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

Question 2

Nucleus

Answer 2

 Double membrane bound organelle
 Contains the hereditary information (DNA
genome)
 Site of DNA replication (DNA to DNA)
 Site of gene transcription (DNA to RNA)

Question 3

Chromatin

Answer 3

DNA and associated
proteins within nucleus

Question 4

Nuclear envelope

Answer 4

double
membrane with nuclear pores

Question 5

Nucleolus

Answer 5

where rRNA and
proteins are assembled to form
ribosomes

Question 6

Nucleoplasm:

Answer 6

remaining nuclear
content, similar to cytoplasm.
The nucleus contains several features that
distinguish it from other cellular structures

Question 7

Envelope

Answer 7

 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

Question 8

Transcription

Answer 8

DNA mRNA  Protein

Question 9

mRNA, tRNA and rRNA

Answer 9

• 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.

Question 10

Chromatid

Answer 10

one-half of
two identical copies
of a replicated
chromosome.

Question 11

Telomeres

Answer 11

caps
on chromosomes that
offer protection
during cell division

Answer 12

• Each chromosome
  contains DNA bound to
  histones
• This packaging controls
  information available
• DNA wrapped round
  histone = nucleosome

Question 13

Visibility of chromosomes through
cell cycle

Answer 13

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

Question 14

The code for life

Answer 14

• 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

Question 15

(Short tandem repeats –
STR)

Answer 15

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.

Question 16

Euchromatin

Answer 16

loose
chromatin structure and active
for transcription

Question 17

Heterochromatin

Answer 17

condensed chromatin structure
and is inactive for transcription
Heterochromatin appears as dark patches in the nucleus
during interphase (i.e. non-dividing cells).

Question 18

DNA replication factories

Answer 18

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

Question 19

Replisome:

Answer 19

smallest functional
unit in the factories, responsible for
copying one segment of DNA

Question 20

Nucleolus

Answer 20

 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

Question 21

Functions of Nucleolus

Answer 21

 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

Question 22

The nuclear lamina

Answer 22

Beneath the inner
membrane, lies the
nuclear lamina.
* A meshwork of
intermediate filament
proteins called Lamins
and associated proteins

Question 23

Lamin Proteins

Answer 23

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

Question 24

The nuclear lamina structure

Answer 24

• 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

Question 25

A multitude of proteins are
needed in the nucleus

Answer 25

• 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

Question 26

NPCs

Answer 26

 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

Question 27

Nuclear Localisation Signals (NLS)

Answer 27

 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

Question 28

Import / Export through nuclear pores

Answer 28

 Regulated by two carrier proteins:
 Importin and Exportin
 Importins and exportins are members of a family
of nuclear transport receptors known as
karyopherins

Question 29

nuclear import/export system
regulates traffic through nuclear pores

Answer 29

 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.

Question 30

When NLS met importin

Answer 30

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

Question 31

journey via
the Nuclear Pore

Answer 31

 The complex translocate to the
nucleus via the NPC.
 In the nucleus, Ran-GTP
displaces the cargo protein,
which is released

Question 32

Importin-Ran-GTP then via the
NP

Answer 32

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.

Question 33

Ran-GDP together with NTF2 return to
the nucleus

Answer 33

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)

Question 34

NPC Export

Answer 34

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

Question 35

Key Concepts

Answer 35

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.