Fundamentals of Cell Biology
Chapter 7: The Nucleus and DNA
Replication
Chapter Summary: The Big Picture (1)
• Chapter foci:
– Examine the anatomy of the nucleus
– Introduction to the protein complexes necessary for
building, replicating, and maintaining the structure
of DNA
– Discussion of mitosis and the importance of the
cytoskeleton in regulating mitosis
Chapter Summary: The Big Picture (2)
• Section topics:
– The nucleus contains and protects most of a
eukaryotic cell’s DNA
– DNA replication is a complex, tightly-regulated
process
– Mitosis separates replicated chromosomes
The nucleus contains and protects most of
a eukaryotic cell’s DNA
• Key Concepts:
– The nucleus is a highly-specialized organelle
committed primarily to protecting, copying, and
transcribing DNA.
– The interior of the nucleus is highly
compartmentalized.
– DNA copying, plus transcribing and splicing of RNA,
are accomplished by large, highly-specialized
molecular complexes.
The nuclear envelope is a double
membrane structure
• the nuclear envelope
encloses the nucleoplasm
• outer membrane of
the nuclear envelope is
continuous with the ER
Figure 07.01: Electron
micrograph of the
nucleus of a white blood
cell (lymphocyte).
Figure 07.03: The
nuclear envelope is
continuous with the
endoplasmic reticulum.
Nuclear pore complexes regulate
molecular traffic into and out of the nucleus
• 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
Figure 07.04: The nuclear pore complex
appears to be constructed from modular
components. Electron micrographs of these
components are shown at different stages.
The interior of the nucleus is highly
organized and contains subcompartments
• nucleolus contains
DNA that encodes
ribosomal RNAs
• nucleoli are sites of
high transcriptional
activity for rRNA
genes
Figure 07.05: Light micrograph of
a human cancer cell with two
nucleoi in its nucleus.
The nuclear matrix helps to organize
chromosomes
• chromosomes are
compartmentalized into
regions called
chromosome territories
• nuclear matrix helps
control the shape of
chromosomes and
regulate heterochromatin
and euchromatin
Figure 07.06:
Individual
chromosomes
occupy distinct areas
of the nucleus called
chromosome
territories.
Figure 07.07: The
nuclear matrix is a
network of filaments
bound to the nuclear
envelope and to DNA.
DNA replication occurs at sites called
replication factories
• DNA replication
factories form large
complexes in the
nucleus devoted to
copying DNA with 100%
accuracy and no breaks
• Replisome is the
smallest functional unit
in the factories and are
responsible for copying
one segment of DNA
Figure 07.08: DNA replication factories
appear as bright spots of fluorescently
tagged, newly synthesized DNA.
RNA polymerase complexes and
spliceosomes are distinct structures within
the nucleus
• RNA polymerase complexes are responsible for
transcribing the DNA sequence in genes into
mRNA, rRNA, tRNA, and other RNAs
• Spliceosomes are responsible for splicing the
newly synthesized RNAs into their mature form
DNA replication is a complex, tightlyregulated process
• Key Concepts (1):
– DNA replication in all organisms is performed by a
small number of highly-conserved proteins.
– Both prokaryotes and eukaryotes express different
forms of DNA polymerase, the enzyme responsible
for synthesizing DNA.
– DNA replication begins at specific sites called origins
of replication.
– During replication, double-stranded DNA is unwound
and dissociated into single strands that serve as
templates for synthesis of complementary DNA
strands.
DNA replication is a complex, tightlyregulated process
• Key Concepts (2):
– DNA synthesis occurs only in the 5’-to-3’ direction.
– DNA polymerases must bind a double-stranded portion of
a DNA molecule to begin synthesis. Most often, the double
strand consists of the template DNA strand and a short,
complementary RNA primer.
– DNA ligase connects individual pieces of newly
synthesized DNA to form a complete strand.
– The enzyme telomerase adds extra DNA to the ends of
chromosomes to protect them from degradation.
Cells adhere to one another via specialized
proteins and junctional complexes
• Key Concepts (4):
– Neural cell adhesion molecules (NCAMs) are
expressed only in neural cells and function
primarily as homotypic cell–cell adhesion and
signaling receptors.
– Selections are cell–cell adhesion receptors
expressed exclusively on cells in the
circulatory system. They arrest circulating
immune cells in blood vessels so that they
can crawl out into the surrounding tissue.
DNA polymerases are enzymes that
replicate DNA
Figure 07.09: The common organization of DNA polymerases is often compared to the
human hand.
DNA polymerases
• DNA polymerases
add
deoxyribonucleotides
to the 3’ end of DNA
strand
• DNA polymerases
proofread their work
Figure 07.10: DNA is synthesized in the 5’-to3’ direction to permit DNA polymerase to
proofread the new strand.
DNA replication is semi-discontinuous
• DNA replication begins
at sites on
chromosomes called
origins of replication
• During replication,
specialized proteins
unwind and separate
the two strands to form
a replication fork
Figure 07.11: A model of the
origin recognition complex in
yeast, a simple eukaryote.
Numerous proteins are
contained in the complex.
Figure 07.12: A
hexameric helicase
moves along one
strand of DNA.
DNA replication is semi-discontinuous
• DNA replication
requires an RNA primer
– leading/lagging
strand
– Okazaki fragments
• DNA ligases join
fragments of singlestranded DNA
Figure 07.15: Fusion of replication bubbles.
Replication of DNA at the end of
chromosomes requires additional steps
Figure 07.16: Four features of telomerase.
Cells have two main DNA repair
mechanisms
• Excision repair
systems
– Mismatch repair
– Recombination
repair
Figure 07.17: Excision repair replaces a damaged strand.
Mitosis Separates Replicated
Chromosomes
• Key Concepts:
– The function of mitosis is to safely separate
replicated chromosomes into two daughter cells.
– Mitosis is divided into five phases, based largely on
morphological changes in the location and
arrangement of chromosomes.
– The microtubule cytoskeleton, including microtubule
motor proteins, is essential for proper segregation of
chromosomes.
– The actin cytoskeleton is required for the actual
division of one cell into two daughter cells following
mitosis.
Mitosis is divided into stages
• 1879 - Walther
Flemming described
the motion of what he
saw under
microscope as
“threads” (Greek,
mitos) moving in an
actively dividing cell
Figure 07.18: The events of mitosis.
Prophase prepares the cell for division
Figure 07.19: The first frame of a video that follows the chromosomes through the initial
stages of mitosis.
Motors contribute to the formation of
the mature spindle in prophase
• Dynein motor proteins
• Kinesin-related motor
proteins
Figure 07.20: Microtubule motors help form
the mitotic spindle.
Chromosomes attach to the mitotic
spindle during prometaphase
• Kinetochores attach chromosomes to the mitotic
spindle
Figure 07.21: The structure of kinetochores.
Figure 07.23: Once attached to the
spindle, a kinetochore can exist in one
of two activity states.
Arrival of the chromosomes at the
spindle equator signals the beginning
of metaphase
• Metaphase plate = spindle equator
• Chromosome recombination takes place during
metaphase
Separation of chromatids at the metaphase
plate occurs during anaphase
• The onset of
anaphase requires
dissolving the
connections between
sister chromatids
– APC
• Anaphase is
subdivided into two
phases:
– anaphase A
– anaphase B
Figure 07.25: As the chromosomes move to the
poles (anaphase A), the poles themselves move
farther apart (anaphase B), increasing the
separation between them.
Telephase - Cytokinesis
• The structural rearrangements that occur in
prophase begin to reverse during telephase
• Cytokinesis completes mitosis by partitioning the
cytoplasm to form two new daughter cells
• Fragmentation of non-nuclear organelles
ensures their equal distribution in the daughter
cells